**Meet the editor**

Dr. Hu is currently a full professor in the Department of Mechanical Engineering, Chiba University, Japan. He has worked on a broad range of research topics including: structural and functional composites, computational solids mechanics, structural engineering. Recently, his research has been mainly focused on: impact behaviours of FRP laminated composites, structural health moni-

toring and non-destructive damage evaluation techniques, structural and functional nanocomposites, multi-scale simulations of various physical phenomena in nanocomposites. To date, he has generated over 120 papers on various journals with a high citation number, and over 80 papers on various international conferences. He is now an Editorial Board member of 4 journals. He has also delivered over 20 Keynote and Invited Talks on various conferences. He has been a reviewer for Carbon, Polymer, CST and other over 60 various journals.

Contents

**Preface IX** 

Chapter 1 **A Structural Health Monitoring of** 

Viktor Mykhas'kiv

Chapter 6 **Biocomposite Materials 113**  Khaled R. Mohamed

PeiPei Liu and Dong Liang

**for Composite Material Structures 1** 

Chapter 2 **Numerical Simulation of Wave Propagation in 3D Elastic Composites with Rigid** 

Chapter 4 **Acoustic Emission of Composite Vessel 61**  Hyun-Sup Jee and Jong-O Lee

Chapter 7 **Non-Destructive Examination of Interfacial** 

**Using Acoustic Emission 147** 

**a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 3** 

**Disk-Shaped Inclusions of Variable Mass 17** 

Chapter 3 **Structural Health Monitoring for Composite Materials 37**  Jian Cai, Lei Qiu, Shenfang Yuan, Lihua Shi,

Chapter 5 **Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 91** 

> Yaolu Liu, Alamusi, Jinhua Li, Huiming Ning, Liangke Wu, Weifeng Yuan, Bin Gu and Ning Hu

**Section 2 Bio-Medical Composites and Their Applications 111** 

**Debonding in Dental Composite Restorations** 

Haiyan Li, Jianying Li, Xiaozhou Liu and Alex Fok

K.D. Mohd Aris, F. Mustapha, S.M. Sapuan and D.L. Majid

**Section 1 Health Monitoring** 

## Contents

## **Preface XI**


Haiyan Li, Jianying Li, Xiaozhou Liu and Alex Fok

#### X Contents

**Section 3 Natural Fiber, Mineral Filler Composite Materials 169**  Chapter 8 **TEMPO-Mediated Oxidation of Lignocellulosic Fibers from Date Palm Leaves: Effect of the Oxidation on the Processing by RTM Process and Properties of Epoxy Based Composites 171**  Adil Sbiai, Abderrahim Maazouz, Etienne Fleury, Henry Sautereau and Hamid Kaddami Chapter 9 **Oil Palm Biomass Fibres and Recent Advancement in Oil Palm Biomass Fibres Based Hybrid Biocomposites 209**  H.P.S. Abdul Khalil, M. Jawaid, A. Hassan, M.T. Paridah and A. Zaidon Chapter 10 **Properties of Basalt Plastics and of Composites Reinforced by Hybrid Fibers in Operating Conditions 243**  N.M. Chikhradze, L.A. Japaridze and G.S. Abashidze **Section 4 Catalysts and Environmental Pollution Processing Composites 269**  Chapter 11 **Heterogeneous Composites on the Basis of Microbial Cells and Nanostructured Carbonized Sorbents 271**  Zulkhair Mansurov, Ilya Digel, Makhmut Biisenbaev, Irina Savitskaya, Aida Kistaubaeva, Nuraly Akimbekov and Azhar Zhubanova Chapter 12 **New Composite Materials in the Technology for Drinking Water Purification from Ionic and Colloidal Pollutants 295** 

Preface

medical materials, and batteries, etc.

sensors, etc., respectively.

on the content of each chapter.

*CFRP Panels: A Preliminary Approach* 

*Disk-Shaped Inclusions of Variable Mass*

Composites are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct within the finished structure. Basically, they can be categorized into two major types, i.e., structural composites with outstanding mechanical properties and functional composites with various outstanding physical, chemical or electrochemical properties. They have been widely used in a wide variety of products, e.g., advanced spacecraft and aircraft components, boat and scull hulls, sporting goods, sensor/actuator, catalysts and pollution processing materials, bio-

This book focuses on the properties and applications of various composites and the solutions for some encountered problems in applications, e.g., composites structural health monitoring. The book has been divided into five parts, which deal with: health or integrity monitoring techniques of composites structures, bio-medical composites and their applications as dental or tissue materials, natural fiber or mineral filler reinforced composites and their property characterization, catalysts composites and their applications, and some other potential applications of fibers or composites as

A list of chapters is given below along with short descriptions by providing a glimpse

*Chapter 1. A Structural Health Monitoring of a Pitch Catch Active Sensing of PZT Sensors on*

In this chapter, the Lamb waves based on structural health monitoring techniques by using PZT sensor network are described. The focus is put on the repaired locations

*Chapter 2. Numerical Simulation of Wave Propagation in 3D Elastic Composites with Rigid*

This chapter presents a work on the numerical simulation of wave propagation in 3D elastic composites and the interaction between the waves and embedded inclusions or damages. A novel boundary element method is proposed to carry out this analysis.

**Part 1. Health Monitoring for Composite Material Structures** 

and surface structural integrity monitoring in composites structures.


## Preface

VI Contents

**Section 3 Natural Fiber, Mineral Filler Composite Materials 169** 

**the Oxidation on the Processing by RTM Process and Properties of Epoxy Based Composites 171**  Adil Sbiai, Abderrahim Maazouz, Etienne Fleury,

**in Oil Palm Biomass Fibres Based Hybrid Biocomposites 209** 

**Reinforced by Hybrid Fibers in Operating Conditions 243** 

N.M. Chikhradze, L.A. Japaridze and G.S. Abashidze

**Cells and Nanostructured Carbonized Sorbents 271**  Zulkhair Mansurov, Ilya Digel, Makhmut Biisenbaev, Irina Savitskaya, Aida Kistaubaeva, Nuraly Akimbekov

**Water Purification from Ionic and Colloidal Pollutants 295** 

Chapter 11 **Heterogeneous Composites on the Basis of Microbial** 

Chapter 12 **New Composite Materials in the Technology for Drinking** 

**Films and Composite Photocatalyst Films 323** 

Marjan S. Ranđelović, Aleksandra R. Zarubica

Chapter 8 **TEMPO-Mediated Oxidation of Lignocellulosic Fibers from Date Palm Leaves: Effect of** 

Henry Sautereau and Hamid Kaddami

Chapter 10 **Properties of Basalt Plastics and of Composites** 

H.P.S. Abdul Khalil, M. Jawaid, A. Hassan, M.T. Paridah and A. Zaidon

**Section 4 Catalysts and Environmental Pollution Processing Composites 269** 

and Azhar Zhubanova

and Milovan M. Purenović

Chapter 13 **Mechanical Coating Technique for Composite** 

**Section 5 Other Applications of Composites 355** 

Chapter 16 **Composite Material and Optical Fibres 397** 

Yun Lu, Liang Hao and Hiroyuki Yoshida

Chapter 14 **Carbon Fibre Sensor: Theory and Application 357**  Alexander Horoschenkoff and Christian Christner

Chapter 15 **Bio-Inspired Self-Actuating Composite Materials 377**  Maria Mingallon and Sakthivel Ramaswamy

Antonio C. de Oliveira and Ligia S. de Oliveira

Chapter 9 **Oil Palm Biomass Fibres and Recent Advancement** 

Composites are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct within the finished structure. Basically, they can be categorized into two major types, i.e., structural composites with outstanding mechanical properties and functional composites with various outstanding physical, chemical or electrochemical properties. They have been widely used in a wide variety of products, e.g., advanced spacecraft and aircraft components, boat and scull hulls, sporting goods, sensor/actuator, catalysts and pollution processing materials, biomedical materials, and batteries, etc.

This book focuses on the properties and applications of various composites and the solutions for some encountered problems in applications, e.g., composites structural health monitoring. The book has been divided into five parts, which deal with: health or integrity monitoring techniques of composites structures, bio-medical composites and their applications as dental or tissue materials, natural fiber or mineral filler reinforced composites and their property characterization, catalysts composites and their applications, and some other potential applications of fibers or composites as sensors, etc., respectively.

A list of chapters is given below along with short descriptions by providing a glimpse on the content of each chapter.

#### **Part 1. Health Monitoring for Composite Material Structures**

## *Chapter 1. A Structural Health Monitoring of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach*

In this chapter, the Lamb waves based on structural health monitoring techniques by using PZT sensor network are described. The focus is put on the repaired locations and surface structural integrity monitoring in composites structures.

#### *Chapter 2. Numerical Simulation of Wave Propagation in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass*

This chapter presents a work on the numerical simulation of wave propagation in 3D elastic composites and the interaction between the waves and embedded inclusions or damages. A novel boundary element method is proposed to carry out this analysis.

#### XII Preface

For applications of waves based techniques in the structural health monitoring field, some important fundamental information is provided for deep understanding the wave propagation behaviours in composites with inclusions or damages.

Preface XI

*Chapter 9. Properties of Basalt Plastics and of Composites Reinforced by Hybrid Fibers in*

In this chapter, some new research results of a new type of composite materials based on basalt, carbon, glass and polymeric resin, are presented. In particular, a long-term resistance property of the material in corrosive media and at atmospheric action has

*Chapter 10. Heterogeneous Composites on the Basis of Microbial Cells and Nanostructured*

In this chapter, heterogeneous composite materials obtained by immobilization of microorganisms on carbonized sorbents with nanostructured surface are described with the provided evidences collected in in-vivo and in-vitro studies, which strongly suggest that the use of the nano-structured carbonized sorbents as delivery vehicles for the oral administration of probiotic microorganisms has a very big potential for

*Chapter 11. New Composite Materials in the Technology for Drinking Water Purification from* 

Due to their positive textural properties and high specific surface area, in this chapter, composite materials working as adsorbents or electrochemically active materials in water purification for deposition of some pollutants from water are described. Especially, three new/modified bentonite based composite materials are explored in detail, where bentonite is a natural and colloidal alumosilicate with particle size less than 10 μm, which is effectively used as sorbent for heavy metals and other inorganic

*Chapter 12. Mechanical Coating Technique for Composite Films and Composite Photocatalyst*

This chapter presents a newly developed mechanical coating technique (MCT). By comparing with the traditional film coating techniques such as PVD and CVD, this technique, i.e., MCT, shows many advantages including inexpensive equipments, simple process, low preparation cost and large specific area, among others. It can not only fabricate metal/alloy films but also non-metal/metal composite films such as

This chapter presents the piezoresistive carbon fiber sensor (CFS) consisting of a single carbon fiber working as a strain sensor, which is embedded in a sensor carrier (GFRP patch) for electrical isolation. It has been demonstrated that based on the integral strain measurement method, the CFS is an excellent sensor to detect delamination and

**Part 4. Catalysts and Environmental Pollution Processing Composites** 

improving functionality, safety and stability of probiotic preparations.

*Operating Conditions* 

been focused on.

*Carbonized Sorbents*

*Ionic and Colloidal Pollutants*

and organic pollutants from water.

TiO2/Ti composite photocatalyst films.

**Part 5. Other Applications of Composites** 

*Chapter 13. Carbon Fiber Sensor: Theory and Application* 

matrix cracks in multidirectional reinforced laminates.

*Films* 

#### *Chapter 3. Structural Health Monitoring for Composite Materials*

In this chapter, structural health monitoring (SHM) for composite materials is mainly focused on. The common sensors in SHM and some typical SHM methods are reviewed along with some SHM examples realized on composite structures.

#### *Chapter 4. Acoustic Emission of Composite Vessel*

In this chapter, an acoustic emission based technique to evaluate damages in a composite fuel tank is presented in detail by carrying out a massive amount of experimental studies.

#### **Part 2. Bio-medical Composites and Their Applications**

#### *Chapter 5. Biocomposite Materials*

In this chapter, the composites of ceramics with natural degradable polymers are described by using several particle composites based on degradable biopolymers as example. Their physical, chemical and biological properties and applications in bone structures and bone tissue engineering are described.

## *Chapter 6. Non-destructive Examination of Interfacial Debonding in Dental Composite Restorations Using Acoustic Emission*

This chapter is to present a study on the development of a new method to evaluate the interfacial debonding of dental composite restorations. This non-destructive method based on the acoustic emission technique is evaluated for its use to monitor *in-situ* the interfacial debonding of composite restorations during polymerization.

#### **Part 3. Natural Fiber, Mineral Filler Composite Materials**

*Chapter 7. TEMPO-mediated Oxidation of Lignocellulosic Fibers From Date Palm Leaves: Effect of the Oxidation on the Processing by RTM Process and Properties of Epoxy Based Composites* 

In this chapter, TEMPO-mediated oxidation technique for processing lignocellulosic fibers is described. Moreover, the effects of this technique on the thermal, mechanical properties of the lignocellulosic fiber based composites and the fabrication process of the composites are explored in detail.

## *Chapter 8. Oil Palm Biomass Fibres and Recent Advancement in Oil Palm Biomass Fibres based Hybrid Biocomposites*

This chapter is to give an overview on some main results of physical, mechanical, electrical, and thermal properties obtained from oil palm fibres based hybrid composites, which are promising in their applications to automotive sector, building industry etc.

### *Chapter 9. Properties of Basalt Plastics and of Composites Reinforced by Hybrid Fibers in Operating Conditions*

In this chapter, some new research results of a new type of composite materials based on basalt, carbon, glass and polymeric resin, are presented. In particular, a long-term resistance property of the material in corrosive media and at atmospheric action has been focused on.

#### **Part 4. Catalysts and Environmental Pollution Processing Composites**

X Preface

For applications of waves based techniques in the structural health monitoring field, some important fundamental information is provided for deep understanding the

In this chapter, structural health monitoring (SHM) for composite materials is mainly focused on. The common sensors in SHM and some typical SHM methods are

In this chapter, an acoustic emission based technique to evaluate damages in a composite fuel tank is presented in detail by carrying out a massive amount of

In this chapter, the composites of ceramics with natural degradable polymers are described by using several particle composites based on degradable biopolymers as example. Their physical, chemical and biological properties and applications in bone

*Chapter 6. Non-destructive Examination of Interfacial Debonding in Dental Composite* 

This chapter is to present a study on the development of a new method to evaluate the interfacial debonding of dental composite restorations. This non-destructive method based on the acoustic emission technique is evaluated for its use to monitor *in-situ* the

*Chapter 7. TEMPO-mediated Oxidation of Lignocellulosic Fibers From Date Palm Leaves: Effect of the Oxidation on the Processing by RTM Process and Properties of Epoxy Based* 

In this chapter, TEMPO-mediated oxidation technique for processing lignocellulosic fibers is described. Moreover, the effects of this technique on the thermal, mechanical properties of the lignocellulosic fiber based composites and the fabrication process of

*Chapter 8. Oil Palm Biomass Fibres and Recent Advancement in Oil Palm Biomass Fibres based* 

This chapter is to give an overview on some main results of physical, mechanical, electrical, and thermal properties obtained from oil palm fibres based hybrid composites, which are promising in their applications to automotive sector, building

interfacial debonding of composite restorations during polymerization.

**Part 3. Natural Fiber, Mineral Filler Composite Materials** 

wave propagation behaviours in composites with inclusions or damages.

reviewed along with some SHM examples realized on composite structures.

*Chapter 3. Structural Health Monitoring for Composite Materials* 

**Part 2. Bio-medical Composites and Their Applications** 

structures and bone tissue engineering are described.

*Chapter 4. Acoustic Emission of Composite Vessel* 

experimental studies.

*Composites* 

*Hybrid Biocomposites* 

industry etc.

*Chapter 5. Biocomposite Materials* 

*Restorations Using Acoustic Emission*

the composites are explored in detail.

## *Chapter 10. Heterogeneous Composites on the Basis of Microbial Cells and Nanostructured Carbonized Sorbents*

In this chapter, heterogeneous composite materials obtained by immobilization of microorganisms on carbonized sorbents with nanostructured surface are described with the provided evidences collected in in-vivo and in-vitro studies, which strongly suggest that the use of the nano-structured carbonized sorbents as delivery vehicles for the oral administration of probiotic microorganisms has a very big potential for improving functionality, safety and stability of probiotic preparations.

## *Chapter 11. New Composite Materials in the Technology for Drinking Water Purification from Ionic and Colloidal Pollutants*

Due to their positive textural properties and high specific surface area, in this chapter, composite materials working as adsorbents or electrochemically active materials in water purification for deposition of some pollutants from water are described. Especially, three new/modified bentonite based composite materials are explored in detail, where bentonite is a natural and colloidal alumosilicate with particle size less than 10 μm, which is effectively used as sorbent for heavy metals and other inorganic and organic pollutants from water.

## *Chapter 12. Mechanical Coating Technique for Composite Films and Composite Photocatalyst Films*

This chapter presents a newly developed mechanical coating technique (MCT). By comparing with the traditional film coating techniques such as PVD and CVD, this technique, i.e., MCT, shows many advantages including inexpensive equipments, simple process, low preparation cost and large specific area, among others. It can not only fabricate metal/alloy films but also non-metal/metal composite films such as TiO2/Ti composite photocatalyst films.

#### **Part 5. Other Applications of Composites**

#### *Chapter 13. Carbon Fiber Sensor: Theory and Application*

This chapter presents the piezoresistive carbon fiber sensor (CFS) consisting of a single carbon fiber working as a strain sensor, which is embedded in a sensor carrier (GFRP patch) for electrical isolation. It has been demonstrated that based on the integral strain measurement method, the CFS is an excellent sensor to detect delamination and matrix cracks in multidirectional reinforced laminates.

#### XIV Preface

#### *Chapter 14. Bio-Inspired Self-Actuating Composite Materials*

In this chapter, the research to integrate sensing and actuation functions into a fibre composite material system is described. In this system, which displays adaptive 'Integrated Functionality', fiber composites are anisotropic and heterogeneous, offering the possibility for local variations in their material properties. Embedded fiber optics are used to sense multiple parameters and shape memory alloys integrated into composite material are used for actuation.

#### *Chapter 15. Composite Material & Optical Fibres*

In this chapter, development of a special composite formed from a mixture of EPO-TEK 301-2 and some refractory material oxide in nano-particle form, cured and submitted to a customized thermal treatment is described. This material is more resistant and harder than EPO-TEK 301-2 and is found to be well suited to the fabrication of optical fiber arrays from the aspects of CTE matching, machining ability, bonding to glass and ease of polishing, etc.

#### **Acknowledgements**

I would like to express my sincere appreciation to the authors of the chapters in this book for their excellent contributions and for their efforts involved in the publication process. I do believe that the contents in this book will be helpful to many researchers in this field around the world.

> **Ning Hu, Ph.D.**  Professor, Department of Mechanical Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

**Section 1** 

**Health Monitoring** 

**for Composite Material Structures** 

**Health Monitoring for Composite Material Structures** 

XII Preface

*Chapter 14. Bio-Inspired Self-Actuating Composite Materials* 

composite material are used for actuation.

*Chapter 15. Composite Material & Optical Fibres* 

bonding to glass and ease of polishing, etc.

**Acknowledgements** 

in this field around the world.

In this chapter, the research to integrate sensing and actuation functions into a fibre composite material system is described. In this system, which displays adaptive 'Integrated Functionality', fiber composites are anisotropic and heterogeneous, offering the possibility for local variations in their material properties. Embedded fiber optics are used to sense multiple parameters and shape memory alloys integrated into

In this chapter, development of a special composite formed from a mixture of EPO-TEK 301-2 and some refractory material oxide in nano-particle form, cured and submitted to a customized thermal treatment is described. This material is more resistant and harder than EPO-TEK 301-2 and is found to be well suited to the fabrication of optical fiber arrays from the aspects of CTE matching, machining ability,

I would like to express my sincere appreciation to the authors of the chapters in this book for their excellent contributions and for their efforts involved in the publication process. I do believe that the contents in this book will be helpful to many researchers

**Ning Hu, Ph.D.** 

Chiba University,

Japan

Professor, Department of Mechanical Engineering,

1-33 Yayoi-cho, Inage-ku, Chiba 263-8522,

**Chapter 1** 

© 2012 Aris et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Aris et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

these methods prove its effectiveness and consistency in finding the anomalies.

**A Structural Health Monitoring of a Pitch Catch** 

**Active Sensing of PZT Sensors on CFRP Panels:** 

At present, the advanced composite materials have gained it acceptance in the aerospace industries. The content of these materials has increased dramatically from less than 5% in the late eighties to more than 50% at the beginning at this decade. [1] The materials offer high strength to weight ratio, high strength to weight ratio, corrosion resistance, high fatigue resistance etc. These benefits have transformed the aviation world traveling to better fuel consumption, endurance and more passengers. However, the use of these materials has posed new challenges such as impact, delamination, barely visible internal damage (BVID) etc. Before a part or component being used on the actual structure, they are being tested from small scale to the actual scale in a controlled environment either at lab or test cell. However the attributes imposed during the operation sometimes shows different behavior when the actual operations are performed due to environment factors, human factors and support availability. To ensure the safety is at the optimum level, the continuous conditional monitoring need to be carried out in order to ensure the component operate within the safety margin being placed by the aircraft manufacturers. [2] One of the areas under investigation is the structural integrity assessment through the use of non-destructive inspections (NDI). The NDI allows aircraft operator to seek information on the aircraft structure reliability by inspecting the structure without having to remove it. There are many types of inspection methods which are limited to materials, locations and accuracy depends on methodology applied. [3] Few of popular techniques are eddy current, ultrasonic, radiography, dye penetrant which have been existence in quite a time. However due to composite material applications new methods have emerged in order to improve detection to attain converging results such as tap test, laser shearography, phase array etc.. So far,

K.D. Mohd Aris, F. Mustapha, S.M. Sapuan and D.L. Majid

**A Preliminary Approach** 

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48097

**1. Introduction** 

## **A Structural Health Monitoring of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach**

K.D. Mohd Aris, F. Mustapha, S.M. Sapuan and D.L. Majid

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48097

## **1. Introduction**

At present, the advanced composite materials have gained it acceptance in the aerospace industries. The content of these materials has increased dramatically from less than 5% in the late eighties to more than 50% at the beginning at this decade. [1] The materials offer high strength to weight ratio, high strength to weight ratio, corrosion resistance, high fatigue resistance etc. These benefits have transformed the aviation world traveling to better fuel consumption, endurance and more passengers. However, the use of these materials has posed new challenges such as impact, delamination, barely visible internal damage (BVID) etc. Before a part or component being used on the actual structure, they are being tested from small scale to the actual scale in a controlled environment either at lab or test cell. However the attributes imposed during the operation sometimes shows different behavior when the actual operations are performed due to environment factors, human factors and support availability. To ensure the safety is at the optimum level, the continuous conditional monitoring need to be carried out in order to ensure the component operate within the safety margin being placed by the aircraft manufacturers. [2] One of the areas under investigation is the structural integrity assessment through the use of non-destructive inspections (NDI). The NDI allows aircraft operator to seek information on the aircraft structure reliability by inspecting the structure without having to remove it. There are many types of inspection methods which are limited to materials, locations and accuracy depends on methodology applied. [3] Few of popular techniques are eddy current, ultrasonic, radiography, dye penetrant which have been existence in quite a time. However due to composite material applications new methods have emerged in order to improve detection to attain converging results such as tap test, laser shearography, phase array etc.. So far, these methods prove its effectiveness and consistency in finding the anomalies.

© 2012 Aris et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Aris et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

However these techniques require total grounding of the aircraft and the inspection are manually intensified. The only clue where to inspect the area from the occurrence report, maintenance schedule or mandatory compliance by the authority. New inspection paradigm need to be developed as defects will arise in the non-conventional ways as the composite materials being used in the pressurized area such as in Boeing 787 and Airbus A350 aircrafts. Therefore, the available methods need to be systematically chosen depends on thin laminate, thick laminate or sandwich structure.[4] The active monitoring offers continuous monitoring either by interrogating or listen to the structure behavior. Embedded sensor and on surface sensors offers the advantages and disadvantages that yet not being explored fully and can accommodate the NDI techniques. The structure integrity will behave differently as the structure being modified and repair to ensure continuation of the aircraft operation and prolong its service life. The aircraft structural health monitoring (SHM) is one of the conditioning monitoring that has gained its usefulness. Such health monitoring of a component has been successfully being used in the aircraft avionics systems, engine management systems, rotary blade systems etc. Since the SHM is still at its infant stage, several methodology and detection methods are been explored to suite the monitoring purposes. Acoustic emission, fiber bragg grating, compact vacuum monitoring etc. are being investigated for their potential. [5] Therefore the paper is focusing on issues on the implementation of the SHM at post repair through the use of PZT sensor by using guided waves as a method of monitoring for active and passive structural surface conditions.

## **2. Theoretical background**

The use of advanced composite materials has shifted the paradigm in aircraft structure design, operation and maintenance philosophy. A simple stop drills procedure is used to prevent further propagation of crack or by removing the damage area and replacing the damage area. This procedure are well written in typical aircraft structural repair manual (SRM) under Chapter 50-xx-xx found in the ATA 100 (Air Transport Associations) [6]. The procedure above can only be applied to metallic structure since the behavior is isotropic in which properties such as damage tolerance, fracture mechanics and fatigue can be predicted although the repair has been done on the damaged structure. The composite structures are made up from various constituents that are laid up and bonded together with the assistance of pressure and temperature at predetermine times. During operations, the aircraft structures are subjected to damages due to impact, environmental, residual imperfections, delaminations that reduces the structural integrity of the aircraft [7]. Typically, there are four types of repair applied to the composite structures. There are external bonded patches, flush or scarf bonded repair, bolted patch and bonded patches [8]. This operation requires the strength to be returned back to the original strength [9]. Due to the orientation, number of plies and materials used the level of recovery of the operating strain is much dependent on the stiffness of the laminates. The governing equation for the actual load to be transmitted to the new repaired laminates are given by the equation below [10] & [11]

A Structural Health Monitoring

of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 5

Where, P, ea, Ex, and t are actual load, ultimate design strain, modulus in the primary loading direction and the laminate thickness respectively. A simple calculation of the strength of materials can be applied to scrutinized the scarf join for the maximum allowable

max sin cos

1:30 ratio in order to attained minimum ultimate stress for the repair structure strength to be

Studies have shown the use of PZT sensors on experimental aircraft component such as flaps and wings are promising [12] and [13]. For this experiment, an aircraft spoiler was used as the experimental subject by mounting the sensor arbitrarily on the spoiler's surface. The sensor can also be used to detect the surface condition of normal, damaged and repaired structures.

Most of the structural damage diagnoses were predicted by using analytical or finite element modeling [14], [15] and [16]. Although the results were accepted but it requires a powerful computing hardware, labor intensive interaction and modeling errors before a solution can be converged. Another method is to utilize the statistic to evaluate the captured data. However large amount of data are required to achieve higher reliability and probability to converge to the intended solution. The statistical approach utilizes supervised and unsupervised learning in order to process the data. [17] and [18] The supervised learning uses data as its references and the unsupervised learning uses to cluster the data and group them for selective conditions. The approach can be achieved by using the

Outlier Analysis is one of the method applied in SPR. The OA is used as the detection of cluster, which deviates from other normal trend cluster. One of the most common

> *i i*

where zi is the outlier index for univariate data, di is the potential outlier and *d* and σ are the mean sample and standard deviation. The multivariate discordance test was known as

*z*

*d d*

(3)

*u*

*P t*

*p*

*t*

p is the maximum load, ultimate stress, thickness, shear stress and

(2)

*,* the scarf angle is found to be at 20 or at

stress [10] & [11]. The equation is given by

scarf angle respectively. By solving the value of

Statistical Pattern Recognition [19]. The principles in SPR are:-

discordance tests is based on the deviation statistic [19] given by

Where *Pmax,* 

*u, t,*  and 

similar with the parent structure.

1. Operational Evaluation, 2. Data Acquisition & Cleansing,

3. Feature Extraction & Data Reduction and 4. Statistical Model Development or Prognosis

Only no 1 and 2 were concerned in this paper.

Mahalanobis square distance given by

Where, P, ea, Ex, and t are actual load, ultimate design strain, modulus in the primary loading direction and the laminate thickness respectively. A simple calculation of the strength of materials can be applied to scrutinized the scarf join for the maximum allowable stress [10] & [11]. The equation is given by

$$P\_{\text{max}} = \sigma\_{\mu} t \le \frac{\tau\_p t}{\sin \theta \cos \theta} \tag{2}$$

Where *Pmax, u, t,*  and p is the maximum load, ultimate stress, thickness, shear stress and scarf angle respectively. By solving the value of *,* the scarf angle is found to be at 20 or at 1:30 ratio in order to attained minimum ultimate stress for the repair structure strength to be similar with the parent structure.

Studies have shown the use of PZT sensors on experimental aircraft component such as flaps and wings are promising [12] and [13]. For this experiment, an aircraft spoiler was used as the experimental subject by mounting the sensor arbitrarily on the spoiler's surface. The sensor can also be used to detect the surface condition of normal, damaged and repaired structures.

Most of the structural damage diagnoses were predicted by using analytical or finite element modeling [14], [15] and [16]. Although the results were accepted but it requires a powerful computing hardware, labor intensive interaction and modeling errors before a solution can be converged. Another method is to utilize the statistic to evaluate the captured data. However large amount of data are required to achieve higher reliability and probability to converge to the intended solution. The statistical approach utilizes supervised and unsupervised learning in order to process the data. [17] and [18] The supervised learning uses data as its references and the unsupervised learning uses to cluster the data and group them for selective conditions. The approach can be achieved by using the Statistical Pattern Recognition [19]. The principles in SPR are:-

1. Operational Evaluation,

4 Composites and Their Applications

**2. Theoretical background** 

However these techniques require total grounding of the aircraft and the inspection are manually intensified. The only clue where to inspect the area from the occurrence report, maintenance schedule or mandatory compliance by the authority. New inspection paradigm need to be developed as defects will arise in the non-conventional ways as the composite materials being used in the pressurized area such as in Boeing 787 and Airbus A350 aircrafts. Therefore, the available methods need to be systematically chosen depends on thin laminate, thick laminate or sandwich structure.[4] The active monitoring offers continuous monitoring either by interrogating or listen to the structure behavior. Embedded sensor and on surface sensors offers the advantages and disadvantages that yet not being explored fully and can accommodate the NDI techniques. The structure integrity will behave differently as the structure being modified and repair to ensure continuation of the aircraft operation and prolong its service life. The aircraft structural health monitoring (SHM) is one of the conditioning monitoring that has gained its usefulness. Such health monitoring of a component has been successfully being used in the aircraft avionics systems, engine management systems, rotary blade systems etc. Since the SHM is still at its infant stage, several methodology and detection methods are been explored to suite the monitoring purposes. Acoustic emission, fiber bragg grating, compact vacuum monitoring etc. are being investigated for their potential. [5] Therefore the paper is focusing on issues on the implementation of the SHM at post repair through the use of PZT sensor by using guided

waves as a method of monitoring for active and passive structural surface conditions.

The use of advanced composite materials has shifted the paradigm in aircraft structure design, operation and maintenance philosophy. A simple stop drills procedure is used to prevent further propagation of crack or by removing the damage area and replacing the damage area. This procedure are well written in typical aircraft structural repair manual (SRM) under Chapter 50-xx-xx found in the ATA 100 (Air Transport Associations) [6]. The procedure above can only be applied to metallic structure since the behavior is isotropic in which properties such as damage tolerance, fracture mechanics and fatigue can be predicted although the repair has been done on the damaged structure. The composite structures are made up from various constituents that are laid up and bonded together with the assistance of pressure and temperature at predetermine times. During operations, the aircraft structures are subjected to damages due to impact, environmental, residual imperfections, delaminations that reduces the structural integrity of the aircraft [7]. Typically, there are four types of repair applied to the composite structures. There are external bonded patches, flush or scarf bonded repair, bolted patch and bonded patches [8]. This operation requires the strength to be returned back to the original strength [9]. Due to the orientation, number of plies and materials used the level of recovery of the operating strain is much dependent on the stiffness of the laminates. The governing equation for the actual load to be transmitted to the new repaired laminates are given by the equation below [10] & [11]

 *P=eaExt* (1)


Only no 1 and 2 were concerned in this paper.

Outlier Analysis is one of the method applied in SPR. The OA is used as the detection of cluster, which deviates from other normal trend cluster. One of the most common discordance tests is based on the deviation statistic [19] given by

$$z\_i = \frac{d\_i - \overline{d}}{\sigma} \tag{3}$$

where zi is the outlier index for univariate data, di is the potential outlier and *d* and σ are the mean sample and standard deviation. The multivariate discordance test was known as Mahalanobis square distance given by

$$Z\_i = (\{\mathbf{x}\_i\} - \{\mathbf{\bar{x}}\})^T [S]^{-1} (\{\mathbf{x}\_i\} - \{\mathbf{\bar{x}}\}) \tag{4}$$

A Structural Health Monitoring

of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 7

damage plies were removed and replaced in accordance with the SRM [23]. The damaged

The repair was conducted by using hot bonder from Heatcon Inc. The Hexply® M10/38%/UD300/CHS/460mm CFRP pre-preg system from Hexcel Corp was used for the repair process. Care and take were observed to ensure similar procedures as per SRM recommendation. All plies were cut according to the sizes required and laid up accordingly. The affected area were vacuum bag as per Figure 2 and cured at 1200C at atmospheric pressure for 120 minutes. All vacuum bag materials were removed once the cycle ended.

**Figure 1.** Locations of the structural conditions and PZT sensor placements

**Figure 2.** Hot bonder materials sequence for repairing aircraft composite parts. [24]

The PZT sensors were placed at 100mm apart for the three studied conditions. For damaged condition, the sensors were placed in between the damage area and for the repaired area, the sensors were replace across the actual and the repair doubler surface. This is to ensure

area was repaired by scarfing method.

where *Zi* is the outlier index for multivariate data, *xi* is the potential outlier vector and *x* is the sample mean vector and e is the sample co-variance matrix [20] and [21]. The result of the above equation is congregated when the distance of a data vector is higher than a preset threshold level.

## **3. Experimental setup**

There were two experimental procedures were taken place. The first was the study of the wavelet through an aircraft part at normal, damaged and repaired conditions. The second is to observed the guided Lamb wave behavior when subjected to tensile loading for the three conditions stated above.

The APC 850 PZT sensor from APC International Inc. was used for both experiments. The properties of the sensors are shown in Table 1 below. Two sensors were used as an actuator and receiver with a diameter of 10mm and thickness of 0.5mm. The pitch catch active sensing was used to obtain the data at the receiving sensors. The sensors were placed at 100mm apart due to the optimum wave attenuation from the actuator to the receiver. The actuator was connected to a function generator where a selected input variable were set and the receiver were connected to the oscilloscope for data mining and further processing.


**Table 1.** APC-850 properties [22]

## **3.1. Aircraft component analysis**

An aircraft spoiler was used for this research. The use of the structure is only arbitrary at this stage. It is use to seek the workability of the sensor upon trial on several flat panels. Three conditions were introduced to the panel which is the undamaged/ parent, damaged and repaired area. The undamaged/ parent was the area free from any defects. The undamaged area is the original conditions or controlled area. The damaged area was damage caused by impact that removes the top laminate. It was made by impacting the faced planes with a blunt object and creating damage less than 40mm diameter fracture. The level of impact is not an interest in this particular testing due to the studied conditions is only applicable to small surface damage due to impact. The repaired area was where the damage plies were removed and replaced in accordance with the SRM [23]. The damaged area was repaired by scarfing method.

**Figure 1.** Locations of the structural conditions and PZT sensor placements

6 Composites and Their Applications

**3. Experimental setup** 

conditions stated above.

**Table 1.** APC-850 properties [22]

**3.1. Aircraft component analysis** 

threshold level.

<sup>1</sup> ({ } { }) [ ] ({ } { }) *<sup>T</sup> Z x xS x x i i <sup>i</sup>*

where *Zi* is the outlier index for multivariate data, *xi* is the potential outlier vector and *x* is the sample mean vector and e is the sample co-variance matrix [20] and [21]. The result of the above equation is congregated when the distance of a data vector is higher than a preset

There were two experimental procedures were taken place. The first was the study of the wavelet through an aircraft part at normal, damaged and repaired conditions. The second is to observed the guided Lamb wave behavior when subjected to tensile loading for the three

The APC 850 PZT sensor from APC International Inc. was used for both experiments. The properties of the sensors are shown in Table 1 below. Two sensors were used as an actuator and receiver with a diameter of 10mm and thickness of 0.5mm. The pitch catch active sensing was used to obtain the data at the receiving sensors. The sensors were placed at 100mm apart due to the optimum wave attenuation from the actuator to the receiver. The actuator was connected to a function generator where a selected input variable were set and the receiver were connected to the oscilloscope for data mining and further processing.

> Description Value Voltage limit AC/DC 8/ 15 V

Relative dielectric constant 1750 Dielectric loss 1.4% Curie Temperature 3600C

Output Power 20 watts/ inch

Density 7.7 X 103 kg/m3 Young's Modulus 6.3 X 1010 N/m2

An aircraft spoiler was used for this research. The use of the structure is only arbitrary at this stage. It is use to seek the workability of the sensor upon trial on several flat panels. Three conditions were introduced to the panel which is the undamaged/ parent, damaged and repaired area. The undamaged/ parent was the area free from any defects. The undamaged area is the original conditions or controlled area. The damaged area was damage caused by impact that removes the top laminate. It was made by impacting the faced planes with a blunt object and creating damage less than 40mm diameter fracture. The level of impact is not an interest in this particular testing due to the studied conditions is only applicable to small surface damage due to impact. The repaired area was where the

(4)

The repair was conducted by using hot bonder from Heatcon Inc. The Hexply® M10/38%/UD300/CHS/460mm CFRP pre-preg system from Hexcel Corp was used for the repair process. Care and take were observed to ensure similar procedures as per SRM recommendation. All plies were cut according to the sizes required and laid up accordingly. The affected area were vacuum bag as per Figure 2 and cured at 1200C at atmospheric pressure for 120 minutes. All vacuum bag materials were removed once the cycle ended.

**Figure 2.** Hot bonder materials sequence for repairing aircraft composite parts. [24]

The PZT sensors were placed at 100mm apart for the three studied conditions. For damaged condition, the sensors were placed in between the damage area and for the repaired area, the sensors were replace across the actual and the repair doubler surface. This is to ensure

distance consistency of 100mm between the sensors. One of the sensors acted as an actuator. The actuator controls the surface guided in the form of elastic perturbation through the surface guided wave across the panel. The wave was controlled by a function generator with the setup as per Table 2.

A Structural Health Monitoring

of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 9

bonding was cured in accordance with Aircraft Structural Repair Manual (SRM). A scarf cutting technique was used to remove the damage and replaced the affected its areas. Curing was achieved by using the Heatcon HCS4000 hot bonder with assisted consolidation from vacuum bag as per Figure 2. The parameters were ramp rate at 30C/min, dwell time at 120 minutes, dwell temperature at 1210C, cooling rate at 30C/min and vacuum pressure

Once cured, the panels were cut into specimen size according to ASTM D638 standard with five specimens prepared for each conditions by using Shimadzu AGx-50kN Universal Testing Machine as per Figure 5. The specimens were clamped on both ends. The data for mechanical properties were collected by using the Trapezium-X software came with the UTM machine. For the wavelet pitch-catch analysis, two APC 850 PZT smart sensors were affixed at 100mm apart and symmetrical to each other. Similar connection with the spoiler's test was applied to the relevant apparatus for data mining and post processing. The data from the sensor was interrogated and collected at three stages which were at the beginning of the test, within the

The results for both experiments are being presented into two sections. The initial test was evaluated upon the Vpp from the wavelet analysis. Further overlaying pattern are also

elastic range, after the detection of the first ply failure and prior to separation.

**Figure 4.** Tensile test with PZT sensor on specimens set-up.

**4. Results and discussion** 

attained at 22 in mg/ 1 bar.


**Table 2.** Actuating setting parameter.

The receiving sensor modulated as the guided wave reached and transmit the energy to electrical signal. The received signals were saved for post processing by using oscilloscope. The arrangement of the equipment is shown in Figure 4.

**Figure 3.** Aircraft spoiler with PZT sensor on specimens set-up.

### **3.2. Tensile testing**

Further investigation was conducted by using 2 sets of tensile testing specimens with condition of normal and repair attached with pair of PZT sensors.Tthree composite plate of 300mm by 300mm were fabricated by using the Hexply® M10/38%/UD300/CHS/460mm from Hexcel Corps. The ply orientation was set to [0/90]S2 orientation to produce a balanced symmetrical flat monolithic structure. The parent specimen was subjected to one time curing. However the repaired specimens undergone for secondary curing once the damage area was removed and new replacement plies were laid up. Both initial and secondary bonding was cured in accordance with Aircraft Structural Repair Manual (SRM). A scarf cutting technique was used to remove the damage and replaced the affected its areas. Curing was achieved by using the Heatcon HCS4000 hot bonder with assisted consolidation from vacuum bag as per Figure 2. The parameters were ramp rate at 30C/min, dwell time at 120 minutes, dwell temperature at 1210C, cooling rate at 30C/min and vacuum pressure attained at 22 in mg/ 1 bar.

Once cured, the panels were cut into specimen size according to ASTM D638 standard with five specimens prepared for each conditions by using Shimadzu AGx-50kN Universal Testing Machine as per Figure 5. The specimens were clamped on both ends. The data for mechanical properties were collected by using the Trapezium-X software came with the UTM machine. For the wavelet pitch-catch analysis, two APC 850 PZT smart sensors were affixed at 100mm apart and symmetrical to each other. Similar connection with the spoiler's test was applied to the relevant apparatus for data mining and post processing. The data from the sensor was interrogated and collected at three stages which were at the beginning of the test, within the elastic range, after the detection of the first ply failure and prior to separation.

**Figure 4.** Tensile test with PZT sensor on specimens set-up.

## **4. Results and discussion**

8 Composites and Their Applications

with the setup as per Table 2.

Phase 00

**3.2. Tensile testing** 

**Table 2.** Actuating setting parameter.

distance consistency of 100mm between the sensors. One of the sensors acted as an actuator. The actuator controls the surface guided in the form of elastic perturbation through the surface guided wave across the panel. The wave was controlled by a function generator

Parameter Unit Parameter Unit Frequency 250kHz Symmetrical 50%

Vpp 50 Burst Count 5

The arrangement of the equipment is shown in Figure 4.

**Figure 3.** Aircraft spoiler with PZT sensor on specimens set-up.

Voltage +10V Time Generation 3ms at 333.3 Hz

The receiving sensor modulated as the guided wave reached and transmit the energy to electrical signal. The received signals were saved for post processing by using oscilloscope.

Further investigation was conducted by using 2 sets of tensile testing specimens with condition of normal and repair attached with pair of PZT sensors.Tthree composite plate of 300mm by 300mm were fabricated by using the Hexply® M10/38%/UD300/CHS/460mm from Hexcel Corps. The ply orientation was set to [0/90]S2 orientation to produce a balanced symmetrical flat monolithic structure. The parent specimen was subjected to one time curing. However the repaired specimens undergone for secondary curing once the damage area was removed and new replacement plies were laid up. Both initial and secondary

The results for both experiments are being presented into two sections. The initial test was evaluated upon the Vpp from the wavelet analysis. Further overlaying pattern are also being presented. The latter testing involved with the tensile testing and only the results from wavelet analysis are shown accordingly.

A Structural Health Monitoring

of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 11

**Figure 6.** Wave packet samples from a) undamaged, b) damaged and c) repaired structural conditions

The Vpp showed similar values for the undamaged and damaged structure condition. It was assumed that the wave travel without discontinuity due to partial damage at that particular area. However several other types of damage need to be examined before any conclusive

**Voltage peak to peak distribution**

A further post processing was carried out by overlapping the three conditions in one graph with similar time-domain comparison. The most common interest point lies within points 12250 ~ 12750. This was the first spike seen in the wave packet. In the earlier Vpp comparison, the distribution data between the damage and undamaged were identical. Therefore it was difficult to interpret the data for the latter machine learning process.

0 10 20 30 40 50

**Numbers of reading**

Repair Normal Damage

after synthesized.

0

20

40

60

80

**Voltage Peak to Peak, (mV)**

100

120

140

evidence can be finalized.

**Figure 7.** Vpp distribution among the structural conditions

## **4.1. Aircraft component analysis**

Statistical pattern recognition was used to analyze the lamb wave generated by the PZT actuator. [15] A total of 100 wave packets were taken for each conditions stated. Each wave packets consisted of 25000 points by default from the oscilloscope. From the 25000 points, it was then grouped to 1000 intervals data set for analysis. There were two significant spike occurred each at point 12000 ~ 13000 and 18000 ~19000 as shown in Figure 4. The reduction of data intervals were applied in order to assist the further analyze the distributions. By judgment, the first group of the spike was concerned and the data packet was zoomed again in 500 data intervals. Figure 6 shows the actuating signals for each of the testing. Consistence settings are required to ensure the wavelet generates similar wave perturbation throughout the experiment.

**Figure 5.** Actuating signals

Figure 7 shows the results of the receiving wave packet upon synthesized by the points for each condition. Different behavior from the voltage (Vpp) and complete time of flight cycle are shown which characterized the evaluated conditions.

Then, each of the receiving structural conditions wavelet data were compared between to ensure the signals was homogeneous to each other on the timeline basis. Since this is the unsupervised learning process the clusters were assigned to separate three conditions as stated in the methodology. The Vpp or voltage peak to peak is the attribute to distinguish the conditions. More than 50 Vpp values were collected and tabulated. The scattering of the Vpp for the three conditions were examined and is shown in Figure 8.

A Structural Health Monitoring of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 11

10 Composites and Their Applications

throughout the experiment.

Voltage, mV


are shown which characterized the evaluated conditions.

for the three conditions were examined and is shown in Figure 8.

**Figure 5.** Actuating signals




0

20

40

60

wavelet analysis are shown accordingly.

**4.1. Aircraft component analysis** 

being presented. The latter testing involved with the tensile testing and only the results from

Statistical pattern recognition was used to analyze the lamb wave generated by the PZT actuator. [15] A total of 100 wave packets were taken for each conditions stated. Each wave packets consisted of 25000 points by default from the oscilloscope. From the 25000 points, it was then grouped to 1000 intervals data set for analysis. There were two significant spike occurred each at point 12000 ~ 13000 and 18000 ~19000 as shown in Figure 4. The reduction of data intervals were applied in order to assist the further analyze the distributions. By judgment, the first group of the spike was concerned and the data packet was zoomed again in 500 data intervals. Figure 6 shows the actuating signals for each of the testing. Consistence settings are required to ensure the wavelet generates similar wave perturbation

> Time, n-sec 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000 18000 19000

Figure 7 shows the results of the receiving wave packet upon synthesized by the points for each condition. Different behavior from the voltage (Vpp) and complete time of flight cycle

Then, each of the receiving structural conditions wavelet data were compared between to ensure the signals was homogeneous to each other on the timeline basis. Since this is the unsupervised learning process the clusters were assigned to separate three conditions as stated in the methodology. The Vpp or voltage peak to peak is the attribute to distinguish the conditions. More than 50 Vpp values were collected and tabulated. The scattering of the Vpp

**Figure 6.** Wave packet samples from a) undamaged, b) damaged and c) repaired structural conditions after synthesized.

The Vpp showed similar values for the undamaged and damaged structure condition. It was assumed that the wave travel without discontinuity due to partial damage at that particular area. However several other types of damage need to be examined before any conclusive evidence can be finalized.

**Figure 7.** Vpp distribution among the structural conditions

A further post processing was carried out by overlapping the three conditions in one graph with similar time-domain comparison. The most common interest point lies within points 12250 ~ 12750. This was the first spike seen in the wave packet. In the earlier Vpp comparison, the distribution data between the damage and undamaged were identical. Therefore it was difficult to interpret the data for the latter machine learning process.

However, when all three data was overlapped, a significant different can be seen as shown in Figure 9. The undamaged signals appear at the initial time frame indicated that there was a clean surface wave traveling from the actuator to the receiver. However, once the partial damage was introduced, the spike appear later about 200nsec due to the discontinuity of the spoiler surface. The unaffected wave bifurcated to the receiver with delay. For the repair condition, since the surface integrity has been restored by the flush repair, the continuity of the surface wave was preserved again with delay about 50 nsec .

A Structural Health Monitoring

of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 13

**Figure 9.** CFRP result before and after tensile test for parent specimens for a) normal and b) repaired

(b)

(a)

**Figure 10.** Receiving signals for parent panel with a) at elastic range, b) after first ply failure and c) at failure.

Figure 11 shows the behavior of the full repair panel throughout the testing. The degradation of the signal indicates the lamb wave attenuation has lost due to separation of the repair plies. This can be seen by 50% reduction of the Vpp at the initial of the testing. The significant reduction of the signal strength correlates with the structural integrity lost as the test reached the total fractured. Towards the end of the testing, all the specimen failed at the

**Figure 11.** Receiving signals for parent panel with a) at elastic range, b) after first ply failure and c) at

Both results from the experiments shows a promising indicator on the usage of PZT sensors to monitor structure integrity of the aerospace components and controlled testing specimen.

center and disintegration of the sensor due to the failure of the panels.

specimens

failure.

**5. Discussion** 

Results from the spoiler shows:-

**Figure 8.** Overlay Outlier Pattern for different structural condition

## **4.2. Tensile testing**

The tensile test results showed a similar behavior towards a brittle stress-strain curve. Below are the results of the tensile test on normal and repaired specimens. All specimens were found to have breakage at center. The fiber breakage for parent specimens occurred at arbitrary layer. However, for the repaired specimens, the breakage occurred at the intermediate bonding layer between the original layers and repair plies. This is due to the fiber discontinuity and the matrix is the only medium to transfer the stress from the parent surface to the repair surface.

Based of the wavelet analysis from the Sigmaplot software, both conditions showed a significant changes at the investigated stages. At the elastic range, the wavelet shows a significant solid wave at the respective time of flight. However a slight change appeared after the first ply failure occurred. The Vpp values were found to be higher and there are also a distinct echo developed after the main wave packets. At the end of the testing, the signal lost its signature due to damage upon breakage of the panels. An online monitoring during the course of the test shown a good unique characteristics for anomalies to be identified.

A Structural Health Monitoring

of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 13

**Figure 9.** CFRP result before and after tensile test for parent specimens for a) normal and b) repaired specimens

**Figure 10.** Receiving signals for parent panel with a) at elastic range, b) after first ply failure and c) at failure.

Figure 11 shows the behavior of the full repair panel throughout the testing. The degradation of the signal indicates the lamb wave attenuation has lost due to separation of the repair plies. This can be seen by 50% reduction of the Vpp at the initial of the testing. The significant reduction of the signal strength correlates with the structural integrity lost as the test reached the total fractured. Towards the end of the testing, all the specimen failed at the center and disintegration of the sensor due to the failure of the panels.

**Figure 11.** Receiving signals for parent panel with a) at elastic range, b) after first ply failure and c) at failure.

## **5. Discussion**

12 Composites and Their Applications

However, when all three data was overlapped, a significant different can be seen as shown in Figure 9. The undamaged signals appear at the initial time frame indicated that there was a clean surface wave traveling from the actuator to the receiver. However, once the partial damage was introduced, the spike appear later about 200nsec due to the discontinuity of the spoiler surface. The unaffected wave bifurcated to the receiver with delay. For the repair condition, since the surface integrity has been restored by the flush repair, the continuity of

The tensile test results showed a similar behavior towards a brittle stress-strain curve. Below are the results of the tensile test on normal and repaired specimens. All specimens were found to have breakage at center. The fiber breakage for parent specimens occurred at arbitrary layer. However, for the repaired specimens, the breakage occurred at the intermediate bonding layer between the original layers and repair plies. This is due to the fiber discontinuity and the matrix is the only medium to transfer the stress from the parent surface to the repair surface.

Based of the wavelet analysis from the Sigmaplot software, both conditions showed a significant changes at the investigated stages. At the elastic range, the wavelet shows a significant solid wave at the respective time of flight. However a slight change appeared after the first ply failure occurred. The Vpp values were found to be higher and there are also a distinct echo developed after the main wave packets. At the end of the testing, the signal lost its signature due to damage upon breakage of the panels. An online monitoring during the course of the test shown a good unique characteristics for anomalies to be identified.

the surface wave was preserved again with delay about 50 nsec .

**Figure 8.** Overlay Outlier Pattern for different structural condition

**4.2. Tensile testing** 

Both results from the experiments shows a promising indicator on the usage of PZT sensors to monitor structure integrity of the aerospace components and controlled testing specimen. Results from the spoiler shows:-

1. The Vpp value for all tested condition showed a significant different due to the signal intensity once it passed the tested conditions. Although the undamaged and damaged signals are almost identical, the repair area shows a higher Vpp values. This might due to the additional plies of the repair and the bouncing of the intensification of the signal to travel the tested structure. The time of flight for the repaired reading was found to be delayed from the two conditions. The additional ply or doubler may contribute to the delay. This can promote detection of hidden repair area. The outlier behavior of the Vpp is an initial indications that PZT sensors can be used to detect interested conditions.

A Structural Health Monitoring

of a Pitch Catch Active Sensing of PZT Sensors on CFRP Panels: A Preliminary Approach 15

This research is part of Ministry of Science and Technology, Malaysia (MOSTI) SF000064 Escience Fund grant and Spirit Aerosystem (Malaysia) Inc. for donating the aircraft spoiler.

[1] The World Wide Composite Industry Structure, Trends and Innovation: New 2010

[2] Soutis C. and Diamanti K., Structural Health Monitoring Techniques for Aircraft

[3] Baker A., Dutton S. and Kelly D., Joining of Composite Structure, Composite Matrials

[4] Baker A., Dutton S. and Kelly D., Repair Technology, Materials for Aircraft Structure,

[5] Mujica L.E. et al., Impact Damage Detection in Aircraft Composites Using Knowledge-

[6] Xie Jian, Lu Yao, Study on Airworthiness Requirements of Composite Aircraft Structure for Transport Category Aircraft in FAA, Procedia Engineering, Volume 17 (2011), 270-

[7] Kim I. and Park C.Y., Prediction of Impact Forces on an Aircraft Composite Wing, J. of

[8] S.M. Sapuan, F. Mustapha, D.L. Majid, Z. Leman, A.H.M. Ariff, M.K.A. Ariffin, M.Y.M. Zuhri, M.R. Ishak and J. Sahari, Fiber Reinforced Composite Structure with Bolted Joint

[9] G. Goulios, Z. Marioli-Riga, Composite patch repairs for commercial aircraft:

[10] S.B. Kumar, S. Sivashanker, Asim Bag, I. Sridhar, Failure of aerospace composite scarfjoints subjected to uniaxial compression, Materials Science and Engineering: A, Volume

[11] Dan He, Toshiyuki Sawa, Takeshi Iwamoto, Yuya Hirayama, Stress analysis and strength evaluation of scarf adhesive joints subjected to static tensile loadings, International Journal of Adhesion and Adhesives, Volume 30, Issue 6, (2010), 387-392 [12] Chang F. K and Ihn J. B., Pitch Catch Active Sensing Methods in Strucural Health Monitoring for Aircraft Structures, Structural Health Monitoring (2008) Vol 7, 5~ 19. [13] Inman D.J. et al, Damage Prognosis for Aerospace, civil and Mechanical Systems, John

[14] Ostachowicz W. M., Damage Detection of Structures Using Spectral Finite Element

[15] Kesavan A., John S and Herszberg, Structural Health Monitoring of Composite Structures Using Artificial Intelligence Protocol, Journal of Intelligent Material Systems

Composite Structures, Progress in Aerospace Sciences 46(2010), 342 -353)

Based Reasoning, Structural Health Monitoring (2008); 7; 215~230,

Intelligent Materials Systems and Structures, Vol 19, (2008), 319 ~ 324,

COMPRES, Air & Space Europe, Volume 3, Issues 3–4, (2001), 143-147

– A Review, Key Engineering Materials, 471-472, pg 939-944

Method, Computers and Structures 86 (2008), 454 ~ 462.

**Acknowledgement** 

release, JEC Composite, 2010

AIAA Inc., (2004), 290 ~295.

412, Issues 1–2, (2005), 117-122

Wiley and Sons Ltd., 2005

and Structures; 19;63, 63 ~ 72

for Aircraft Structure, AIAA Inc., 2004. 290 ~295.

**7. References** 

278


## **6. Conclusion**

As conclusion, both experiments has proved that PZT sensors can be used to detect anomalies of the CFRP structure either passive or active sensing. In passive sensing, the data received data is very stable and shows a significant consistence reading at any duration. The latter experiment shows that the ability of the sensors to sense structural integrity of a normal and repaired specimens. However, further investigations are required to this robust detection system in order to ensure the results are established. This can be done by comparing the results through various techniques, statistical methods and analytical analysis.

## **Author details**

K.D. Mohd Aris *Universiti Kuala Lumpur, Malaysian Institute of Aviation Technology, Jalan Jenderam Hulu, Selangor, Malaysia* 

F. Mustapha, S.M. Sapuan, D.L. Majid *Universiti Putra Malaysia, Serdang, Selangor, Malaysia* 

## **Acknowledgement**

This research is part of Ministry of Science and Technology, Malaysia (MOSTI) SF000064 Escience Fund grant and Spirit Aerosystem (Malaysia) Inc. for donating the aircraft spoiler.

## **7. References**

14 Composites and Their Applications

conditions.

**6. Conclusion** 

analysis.

**Author details** 

K.D. Mohd Aris

*Selangor, Malaysia* 

F. Mustapha, S.M. Sapuan, D.L. Majid

*Universiti Putra Malaysia, Serdang, Selangor, Malaysia* 

1. The Vpp value for all tested condition showed a significant different due to the signal intensity once it passed the tested conditions. Although the undamaged and damaged signals are almost identical, the repair area shows a higher Vpp values. This might due to the additional plies of the repair and the bouncing of the intensification of the signal to travel the tested structure. The time of flight for the repaired reading was found to be delayed from the two conditions. The additional ply or doubler may contribute to the delay. This can promote detection of hidden repair area. The outlier behavior of the Vpp is an initial indications that PZT sensors can be used to detect interested

2. The overlay patter analysis show a promising results as it can bifurcate each of the conditions. The outlier analysis can be applied to differentiate the condition of the surface integrity. Due to separation of the fibers, the time for the Lamb wave to travel from the actuator to the receiver has been delayed and takes a longer time with the

3. The tensile test indicates that the materials behave in accordance with typical brittle materials. The breakage at the center indicates a good distribution of the load during the testing. The wavelet signals at the elastic area, within the first ply failure and total failure shows good indications of how the Lamb wave behaves before the specimen fails. However, the method of taking the PZT sensor receiver reading need to be

4. Further post processing techniques need to be used in order to further scrutinize the behavior of the data. A more confident result can be further utilize for more converging result such as by generic algorithm, neural network etc. in order to enhance the

As conclusion, both experiments has proved that PZT sensors can be used to detect anomalies of the CFRP structure either passive or active sensing. In passive sensing, the data received data is very stable and shows a significant consistence reading at any duration. The latter experiment shows that the ability of the sensors to sense structural integrity of a normal and repaired specimens. However, further investigations are required to this robust detection system in order to ensure the results are established. This can be done by comparing the results through various techniques, statistical methods and analytical

*Universiti Kuala Lumpur, Malaysian Institute of Aviation Technology, Jalan Jenderam Hulu,* 

reduction of the Vpp value when compared to the repaired area.

improved as the fluctuation of the signals is unbearable.

prognosis of the structure at later stages.


	- [16] Worden K, Manson G and Filler N. R. J. , Damage Detection Using Outlier Analysis, Journal of Sound and Vibration 229(3) (2000), 647 ~ 667),

**Chapter 2** 

© 2012 Mykhas'kiv, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Mykhas'kiv, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Numerical Simulation of Wave Propagation** 

In many practical situations elastic composites are subjected to dynamic loadings of different physical nature, which origin the wave propagation in such structures. Then overall dynamic response of composite materials is characterized by the wave attenuation and dispersion due to the multiple wave scattering, in the local sense these materials are exhibited also by the dynamic stress intensification due to the wave interaction with the composite fillers. Essential influences on the mentioned phenomena have the shapes and the space distributions of inclusions, i.e. composite architecture, as well as matrix-inclusion materials characteristics. In this respect the numerical investigation of elastic wave propagation in the composite materials with inclusions of non-classical shape and contrast rigidity in comparison with the matrix material is highly demanded. A deep insight into their dynamic behavior, especially on the microscale, is extremely helpful to the design, optimization and manufacturing of composite materials with desired mechanical qualities, fracture and damage analysis, ultrasonic non-destructive testing of composites, and

The macroscopic dynamic properties of particulate elastic composites can be described by effective dynamic parameters of the equivalent homogeneous effective medium via a suitable homogenization procedure. Generally speaking, the homogenization procedure to determine the effective dynamic properties of particulate elastic composites is much more complicated than its static counterpart because of the inclusion interactions and multiple wave scattering effects. For small inclusion concentration or dilute inclusion distribution, their mutual interactions and the multiple wave scattering effects can be neglected approximately. In this case, the theory of Foldy [1], the quasi-crystalline approximation of

**Disk-Shaped Inclusions of Variable Mass** 

**in 3D Elastic Composites with Rigid** 

Additional information is available at the end of the chapter

modeling of seismic processes in complex geological media.

Viktor Mykhas'kiv

**1. Introduction** 

http://dx.doi.org/10.5772/48113


## **Numerical Simulation of Wave Propagation in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass**

Viktor Mykhas'kiv

16 Composites and Their Applications

[16] Worden K, Manson G and Filler N. R. J. , Damage Detection Using Outlier Analysis,

[18] F. Mustapha, G. Manson, K. Worden, S.G. Pierce, Damage location in an isotropic plate using a vector of novelty indices, Mechanical Systems and Signal Processing, Volume

[19] Charles R Farrar and K. Worden, An introduction to structural health monitoring, Phil.

[20] Ihn J and Chang F. K., Pitch Catch Active Sensing Methods in Structural Health Monitoring for Aircraft Structures, Structural Health Monitoring 2008 Vol. 7, 1 ~ 19. [21] Webb A.R, Statistical Pattern Recognition, John Wiley and Sons Ltd, 2002) (Park et al, An Outlier Analysis Framework for Impedance Based Structural Health Monitoring

[22] Piezoelectric Ceramics: Principles and Applications, APC International Ltd, (2008)

[23] Boeing 737-300 Structural Repair Manual, The Boeing Company Inc., 1996 [24] Heatcon Composite Systems: Composite Repair Solutions, Product Catalog, 2011

Journal of Sound and Vibration 229(3) (2000), 647 ~ 667),

Trans. R. Soc. A 15 February 2007 vol. 365 no. 1851 303-315

System, Journal of Sound and Vibration 286 (2005), 229 ~ 250

21, Issue 4, May 2007, Pages 1885-1906

[17] Webb A.R, Statistical Pattern Recognition, John Wiley and Sons Ltd, 2002

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48113

## **1. Introduction**

In many practical situations elastic composites are subjected to dynamic loadings of different physical nature, which origin the wave propagation in such structures. Then overall dynamic response of composite materials is characterized by the wave attenuation and dispersion due to the multiple wave scattering, in the local sense these materials are exhibited also by the dynamic stress intensification due to the wave interaction with the composite fillers. Essential influences on the mentioned phenomena have the shapes and the space distributions of inclusions, i.e. composite architecture, as well as matrix-inclusion materials characteristics. In this respect the numerical investigation of elastic wave propagation in the composite materials with inclusions of non-classical shape and contrast rigidity in comparison with the matrix material is highly demanded. A deep insight into their dynamic behavior, especially on the microscale, is extremely helpful to the design, optimization and manufacturing of composite materials with desired mechanical qualities, fracture and damage analysis, ultrasonic non-destructive testing of composites, and modeling of seismic processes in complex geological media.

The macroscopic dynamic properties of particulate elastic composites can be described by effective dynamic parameters of the equivalent homogeneous effective medium via a suitable homogenization procedure. Generally speaking, the homogenization procedure to determine the effective dynamic properties of particulate elastic composites is much more complicated than its static counterpart because of the inclusion interactions and multiple wave scattering effects. For small inclusion concentration or dilute inclusion distribution, their mutual interactions and the multiple wave scattering effects can be neglected approximately. In this case, the theory of Foldy [1], the quasi-crystalline approximation of

Lax [2] and their generalizations to the elastic wave propagation [3-5] can be applied to determine the effective wave (phase) velocities and the attenuation coefficients in the composite materials with randomly distributed inclusions. In these models, wave scattering by a single inclusion has to be considered in the first step. Most previous publications on the subject have been focused on 3D elastic wave propagation analysis in composite materials consisting of an elastic matrix and spherical elastic inclusions (for example, see [6,7]). Aligned and randomly oriented ellipsoidal elastic inclusions have been considered in [8-10] under the assumption that the wavelength is sufficiently long compared to the dimensions of the individual inclusions (quasi-static limit). As special cases, the results for a random distribution of cracks and penny-shaped inclusions can be derived from those for ellipsoidal inclusions. In the long wavelength approximation, analytical solutions for a single inclusion as a series of the wave number have been presented in these works. However, this approach is applicable only for low frequencies or small wave numbers. For moderate and high frequencies, numerical methods such as the finite element method or the boundary element method can be applied. By using the boundary integral equation method (BIEM) or the boundary element method (BEM) in conjunction with Foldy's theory the effective wave velocities and the wave attenuations in linear elastic materials with open and fluid-filled penny-shaped cracks as well as soft thin-walled circular inclusions have been calculated in [11,12]. Both aligned and randomly oriented defect configurations have been studied, where a macroscopic anisotropy for aligned cracks and non-spherical inclusions appears. Previous results have shown that distributed crack-like defects may cause a decrease in the phase velocity and an increase in the wave attenuation. The efficiency and the applicability ranges of 2D homogenization analysis of elastic wave propagation through a random array of scatters of different shapes and dilute concentrations based on the BEM and Foldy-type dispersion relations were demonstrated also by many authors, for instance, in the papers [13,14]. In 3D case this approach was applied for the numerical simulation of the average dynamic response of composite material containing rigid disk-shaped inclusions of equal mass only [15]. Dynamic stresses near single inclusion of such type under time-harmonic and impulse elastic waves incidence where also investigated [16-18].

Numerical Simulation of Wave Propagation

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 19

effective wave numbers for a dilute concentration of inclusions, where their interactions and multiple wave scattering can be neglected. The averages of the forward scattering amplitudes over 3D inclusion orientations or directions of the wave incidence and over inclusions masses are included into the resulting homogenization formula (dispersion relations). Finally, the effective wave velocity and the attenuation coefficient are obtained by taking the real and the imaginary parts of the effective wave numbers. To investigate the influence of the wave frequency on the effective dynamic parameters, representative numerical examples for longitudinal and transverse elastic waves in infinite elastic composite materials containing rigid disk-shaped inclusions with aligned and random orientation, as well as aligned, normal and uniform mass distribution are presented and discussed. Besides the global dynamic parameters, the mixed-mode dynamic stress intensity factors in the inclusion vicinities are

calculated. They can be used for the fracture or cracking analysis of a composite.

**single massive inclusion** 

displacement vector in in in in

the *T*-wave.

written in the form

**2. Boundary integral formulation of 3D wave scattering problem for a** 

Let us consider an elastic solid consisting of an infinite, homogeneous, isotropic and linearly elastic matrix specified by the mass density , the shear modulus G and Poisson's ratio , and a rigid disk-shaped inclusion with the mass *M*, which thickness is much smaller than the characteristic size of its middle-surface *S*. The center of the Cartesian coordinate system Ox x x coincides with the mass center of the in <sup>123</sup> clusion (see Figure 1), within the described geometrical assumptions the limit values 3 x 0 correspond to the opposite interfaces between the matrix and the inclusion, where a welded contact is assumed. The stress-strain state in the solid is induced by harmonic in the time t plane longitudinal *L*-wave or transverse *T*-wave with the frequency , the constant amplitude U , the phase velocities 0 <sup>L</sup> c and T c , and the wave numbers L L c and T T c , respectively. The

<sup>123</sup> **u** (u ,u ,u ) of the incident wave is given by the relation

Here and hereafter the common factor exp i t is omitted, is the wave number of the incident wave, 0 0 **n** (sin ,0,cos ) is the direction of propagation of the incident wave, 0 is the angle characterizing the direction of the wave incidence, and **U** is the polarization vector with *<sup>L</sup>* and U0 **U n** for the *L*-wave and T χ = χ , U0 **U e** and 0 **n e** for

By using the superposition principle, the total displacement field tot **u** in the solid can be

where 123 **u** (u ,u ,u ) is the unknown displacement vector of the scattered wave, which satisfies the equations of motion and the radiation conditions at infinity (these well-known

governing relations of elastodynamic theory can be found in [19]).

in( ) exp i <sup>χ</sup> **u x U nx** (1)

tot in **u x u x ux** ( ) ( ) ( ), (2)

In this Chapter the effective medium concept is extended to the time-harmonic plane elastic wave propagation in an infinite linear elastic matrix with rigid disk-shaped movable inclusions of variable mass. Both time-harmonic plane longitudinal and transverse waves are considered in the analysis. The solution procedure consists of three steps. In the first step, the wave scattering problem is formulated as a system of boundary integral equations (BIEs) for the stress jumps across the inclusion surfaces. A BEM is developed to solve the BIEs numerically, where the kinetics of the inclusion and the "square-root" singularity of the stress jumps at the inclusion edge are taken into account properly. The improved regularization procedure for the obtained BIEs involving the analytical evaluation of regularizing integrals and results of mapping theory is elaborated to ensure the stable and correct numerical solution of the BIEs. The far-field scattering amplitudes of elastic waves induced by a single inclusion are calculated from the numerically computed stress jumps. In the second step, the simple Foldy-type approximation [1] is utilized to calculate the complex effective wave numbers for a dilute concentration of inclusions, where their interactions and multiple wave scattering can be neglected. The averages of the forward scattering amplitudes over 3D inclusion orientations or directions of the wave incidence and over inclusions masses are included into the resulting homogenization formula (dispersion relations). Finally, the effective wave velocity and the attenuation coefficient are obtained by taking the real and the imaginary parts of the effective wave numbers. To investigate the influence of the wave frequency on the effective dynamic parameters, representative numerical examples for longitudinal and transverse elastic waves in infinite elastic composite materials containing rigid disk-shaped inclusions with aligned and random orientation, as well as aligned, normal and uniform mass distribution are presented and discussed. Besides the global dynamic parameters, the mixed-mode dynamic stress intensity factors in the inclusion vicinities are calculated. They can be used for the fracture or cracking analysis of a composite.

18 Composites and Their Applications

Lax [2] and their generalizations to the elastic wave propagation [3-5] can be applied to determine the effective wave (phase) velocities and the attenuation coefficients in the composite materials with randomly distributed inclusions. In these models, wave scattering by a single inclusion has to be considered in the first step. Most previous publications on the subject have been focused on 3D elastic wave propagation analysis in composite materials consisting of an elastic matrix and spherical elastic inclusions (for example, see [6,7]). Aligned and randomly oriented ellipsoidal elastic inclusions have been considered in [8-10] under the assumption that the wavelength is sufficiently long compared to the dimensions of the individual inclusions (quasi-static limit). As special cases, the results for a random distribution of cracks and penny-shaped inclusions can be derived from those for ellipsoidal inclusions. In the long wavelength approximation, analytical solutions for a single inclusion as a series of the wave number have been presented in these works. However, this approach is applicable only for low frequencies or small wave numbers. For moderate and high frequencies, numerical methods such as the finite element method or the boundary element method can be applied. By using the boundary integral equation method (BIEM) or the boundary element method (BEM) in conjunction with Foldy's theory the effective wave velocities and the wave attenuations in linear elastic materials with open and fluid-filled penny-shaped cracks as well as soft thin-walled circular inclusions have been calculated in [11,12]. Both aligned and randomly oriented defect configurations have been studied, where a macroscopic anisotropy for aligned cracks and non-spherical inclusions appears. Previous results have shown that distributed crack-like defects may cause a decrease in the phase velocity and an increase in the wave attenuation. The efficiency and the applicability ranges of 2D homogenization analysis of elastic wave propagation through a random array of scatters of different shapes and dilute concentrations based on the BEM and Foldy-type dispersion relations were demonstrated also by many authors, for instance, in the papers [13,14]. In 3D case this approach was applied for the numerical simulation of the average dynamic response of composite material containing rigid disk-shaped inclusions of equal mass only [15]. Dynamic stresses near single inclusion of such type under time-harmonic

and impulse elastic waves incidence where also investigated [16-18].

In this Chapter the effective medium concept is extended to the time-harmonic plane elastic wave propagation in an infinite linear elastic matrix with rigid disk-shaped movable inclusions of variable mass. Both time-harmonic plane longitudinal and transverse waves are considered in the analysis. The solution procedure consists of three steps. In the first step, the wave scattering problem is formulated as a system of boundary integral equations (BIEs) for the stress jumps across the inclusion surfaces. A BEM is developed to solve the BIEs numerically, where the kinetics of the inclusion and the "square-root" singularity of the stress jumps at the inclusion edge are taken into account properly. The improved regularization procedure for the obtained BIEs involving the analytical evaluation of regularizing integrals and results of mapping theory is elaborated to ensure the stable and correct numerical solution of the BIEs. The far-field scattering amplitudes of elastic waves induced by a single inclusion are calculated from the numerically computed stress jumps. In the second step, the simple Foldy-type approximation [1] is utilized to calculate the complex

## **2. Boundary integral formulation of 3D wave scattering problem for a single massive inclusion**

Let us consider an elastic solid consisting of an infinite, homogeneous, isotropic and linearly elastic matrix specified by the mass density , the shear modulus G and Poisson's ratio , and a rigid disk-shaped inclusion with the mass *M*, which thickness is much smaller than the characteristic size of its middle-surface *S*. The center of the Cartesian coordinate system Ox x x coincides with the mass center of the in <sup>123</sup> clusion (see Figure 1), within the described geometrical assumptions the limit values 3 x 0 correspond to the opposite interfaces between the matrix and the inclusion, where a welded contact is assumed. The stress-strain state in the solid is induced by harmonic in the time t plane longitudinal *L*-wave or transverse *T*-wave with the frequency , the constant amplitude U , the phase velocities 0 <sup>L</sup> c and T c , and the wave numbers L L c and T T c , respectively. The displacement vector in in in in <sup>123</sup> **u** (u ,u ,u ) of the incident wave is given by the relation

$$\mathbf{u}^{\text{in}}(\mathbf{x}) = \mathbf{U} \exp\left[\mathbf{i}\chi\left(\mathbf{n}\cdot\mathbf{x}\right)\right] \tag{1}$$

Here and hereafter the common factor exp i t is omitted, is the wave number of the incident wave, 0 0 **n** (sin ,0,cos ) is the direction of propagation of the incident wave, 0 is the angle characterizing the direction of the wave incidence, and **U** is the polarization vector with *<sup>L</sup>* and U0 **U n** for the *L*-wave and T χ = χ , U0 **U e** and 0 **n e** for the *T*-wave.

By using the superposition principle, the total displacement field tot **u** in the solid can be written in the form

$$\mathbf{u}^{\text{tot}}(\mathbf{x}) = \mathbf{u}^{\text{in}}(\mathbf{x}) + \mathbf{u}(\mathbf{x}),\tag{2}$$

where 123 **u** (u ,u ,u ) is the unknown displacement vector of the scattered wave, which satisfies the equations of motion and the radiation conditions at infinity (these well-known governing relations of elastodynamic theory can be found in [19]).

**Figure 1.** Single disk-shaped inclusion subjected to an incident elastic wave.

The inclusion is regarded as a rigid unit and its motion is described by the translation 0 000 <sup>123</sup> **u** (u ,u ,u ) and the rotation with respect to the coordinate axes with the angles <sup>1</sup> , <sup>2</sup> and <sup>3</sup> , respectively. Then the displacement components in the domain *S* can be represented by

$$
\mathbf{u}\_{\rangle}(\mathbf{x}) = -\mathbf{u}\_{\rangle}^{\mathrm{in}}(\mathbf{x}) + \mathbf{u}\_{\rangle}^{\mathrm{0}} + (-1)^{\mathrm{i}} \Omega\_{3} \mathbf{x}\_{3-\mathrm{i}}, \qquad \mathbf{j} = \mathrm{1}, \mathbf{2},
$$

$$
\mathbf{u}\_{\rangle}(\mathbf{x}) = -\mathbf{u}\_{\rangle}^{\mathrm{in}}(\mathbf{x}) + \mathbf{u}\_{\rangle}^{\mathrm{0}} + \Omega\_{1} \mathbf{x}\_{2} - \Omega\_{2} \mathbf{x}\_{1}, \quad \mathbf{x}(\mathbf{x}\_{1}, \mathbf{x}\_{2}, \pm \mathbf{0}) \in \mathrm{S}. \tag{3}
$$

Numerical Simulation of Wave Propagation

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 21

<sup>j</sup> 1,2 , (6)

 <sup>0</sup> j j 2 S 1

j 1 j 2 2 3j 3 S j

3

where j i is the radius of inertia of the inclusion with respect to the j x -axis.

stress jumps as

1

<sup>u</sup> dS , <sup>M</sup> 

dS , Mi

<sup>3</sup> 12 21 2 2 <sup>S</sup>

The displacement components in the matrix and the kinematical parameters of the inclusion are related to the stress jumps across the inclusion by the relations (4) and (6). Substitution of Eqs. (4) and (6) into Eqs. (3) results in three boundary integral equations (BIEs) for the

> 2 in 3 3 T 3 S

2 in

2

R, L <sup>L</sup> 1 , <sup>M</sup> <sup>i</sup> 

j 1 2 2 2

 112 2 k j kj 2 2 2

R , <sup>L</sup> , <sup>M</sup> <sup>i</sup> 

In Eq. (7), the kernels Rj , R <sup>12</sup> and R <sup>21</sup> have the form

 **x x x** 

2 2

**x** 

**xx x**

R , dS 4 G u , **x x** <sup>S</sup> **<sup>x</sup>** ,

j j 3 j j3 j T j <sup>S</sup> R , R , dS 4 G u , **xx x** <sup>j</sup> 1,2 , S **<sup>x</sup>** (7)

j j 3j 3j

M i i

<sup>12</sup> T T l d 1 i d d,

x 4 x

j 1,2 ,

**x x** ,

 11 22 3 1 2 2

<sup>4</sup> x x R, L <sup>1</sup>

x x 4 x

 L T j j1 3 3 j 2 exp i d exp i d Ld l d ld , d d

<sup>11</sup> <sup>L</sup> l d 1 i d, 2 2

<sup>21</sup> L L l d 3 3i d d , 2 2

The problem governed by the BIEs (7) can be divided into an antisymmetric problem and a symmetric problem. The antisymmetric problem corresponding to the transverse motion of

3

2 1

3

<sup>22</sup> T T l d 3 3i d d .

k, j 1,2 , k j , (8)

j 1,2 ,

<sup>j</sup> 1,2,3 ,

<sup>1</sup> dS Mi ,

In order to obtain the integral representations for the displacement components we apply the Betty-Rayleigh reciprocity theorem in conjunction with the properties of the elastodynamic fundamental solutions. As a result, the displacement components of the scattered waves can be written in the form [18]:

 <sup>T</sup> j j 2 1 2 S T j1 2 1 1 exp i u 4 G xx x **<sup>x</sup> x x** L T <sup>3</sup> 3 exp i exp i dS , x **x x x x** j 1,2,3 , (4)

where the displacement continuity conditions across the inclusion are used, **x** is the distance between the field point 123 **x** (x ,x ,x ) and integration point 1 2 ( , ,0) , and <sup>j</sup> (j 1,2,3) are the jumps of the interfacial stresses across the inclusion, which are defined by

$$
\Delta \sigma\_{\rangle}(\mathbf{x}) = \sigma\_{\rangle^{\circ}}^{-}(\mathbf{x}) - \sigma\_{\rangle^{\circ}}^{+}(\mathbf{x}), \qquad \mathbf{j} = 1, 2, 3, \quad \mathbf{x} \in \mathbf{S}, \qquad \sigma\_{\rangle^{\circ}}^{\pm}(\mathbf{x}) = \lim\_{\chi\_{\geq} \to \pm 0} \sigma\_{\rangle^{\circ}}(\mathbf{x}). \tag{5}
$$

Eqs. (5) together with the equations of motion of the inclusion as a rigid unit yields the following relations between the translations and the rotations of the inclusion and the stress jumps <sup>j</sup> :

Numerical Simulation of Wave Propagation

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 21

$$\begin{aligned} \mathbf{u}\_{\rangle}^{0} &= \frac{1}{\mathrm{co}^{2}\mathrm{M}} \iint\_{\mathrm{S}} \Delta \sigma\_{\rangle} \left(\mathbf{\hat{g}}\right) \mathrm{d}\mathbf{S}\_{\mathbf{g}} \, \mathrm{d}\mathbf{s} \\\\ \boldsymbol{\Omega}\_{\rangle} &= \frac{(-1)^{l+1}}{\mathrm{co}^{2}\mathrm{M}^{2}\_{\mathrm{j}}} \iint\_{\mathrm{S}} \underline{\mathbb{E}}\_{3-\mathrm{j}} \Delta \sigma\_{\mathrm{3}} \left(\mathbf{\hat{g}}\right) \mathrm{d}\mathbf{S}\_{\mathbf{g}} \, \mathrm{d}\mathbf{s} \end{aligned} \tag{6}$$

$$\boldsymbol{\Omega}\_{\boldsymbol{\mathfrak{z}}} = \frac{1}{\boldsymbol{\alpha}^{2}\boldsymbol{\mathsf{M}}\boldsymbol{\mathfrak{i}}\_{\boldsymbol{\mathfrak{z}}}^{2}} \iint\_{\boldsymbol{\mathfrak{z}}} \Big[ \mathop{\mathsf{f}}\_{\mathfrak{z}} \, \boldsymbol{\Delta}\boldsymbol{\sigma}\_{\boldsymbol{\mathfrak{z}}} \Big( \boldsymbol{\mathfrak{g}} \Big) - \mathop{\mathsf{f}}\_{\mathfrak{z}} \, \boldsymbol{\Delta}\boldsymbol{\sigma}\_{\boldsymbol{\mathfrak{i}}} \Big( \boldsymbol{\mathfrak{g}} \Big) \Big] d\boldsymbol{S}\_{\boldsymbol{\mathfrak{g}}\boldsymbol{\mathfrak{z}}} $$

where j i is the radius of inertia of the inclusion with respect to the j x -axis.

The displacement components in the matrix and the kinematical parameters of the inclusion are related to the stress jumps across the inclusion by the relations (4) and (6). Substitution of Eqs. (4) and (6) into Eqs. (3) results in three boundary integral equations (BIEs) for the stress jumps as

$$\iint\_{\mathbf{S}} \Lambda \boldsymbol{\sigma}\_{\boldsymbol{\upbeta}} \left( \mathbf{f} \right) \mathbf{R}\_{\boldsymbol{\upbeta}} \left( \mathbf{x}, \mathbf{f} \right) d\mathbf{S}\_{\boldsymbol{\upbeta}} = -4\pi G \chi\_{\boldsymbol{\upalpha}}^2 \mathbf{u}\_{\boldsymbol{\upbeta}}^{\text{in}} \left( \mathbf{x} \right), \quad \mathbf{x} \in \mathbf{S} \tag{7}$$
 
$$\iint\_{\mathbf{S}} \left[ \Delta \boldsymbol{\sigma}\_{\boldsymbol{\upbeta}} \left( \mathbf{f} \right) \mathbf{R}\_{\boldsymbol{\upbeta}} \left( \mathbf{x}, \mathbf{f} \right) + \Delta \boldsymbol{\sigma}\_{\boldsymbol{\upbeta} - \boldsymbol{\upbeta}} \left( \mathbf{f} \right) \mathbf{R}\_{\boldsymbol{\upbeta}(3-\boldsymbol{\upbeta})} \left( \mathbf{x}, \mathbf{f} \right) \right] d\mathbf{S}\_{\boldsymbol{\upalpha}} = -4\pi G \chi\_{\boldsymbol{\upalpha}}^2 \mathbf{u}\_{\boldsymbol{\upalpha}}^{\text{in}} \left( \mathbf{x} \right), \quad \mathbf{j} = 1, 2, \quad \mathbf{x} \in \mathbf{S} \tag{7}$$

In Eq. (7), the kernels Rj , R <sup>12</sup> and R <sup>21</sup> have the form

20 Composites and Their Applications

represented by

u

jumps <sup>j</sup> :

**x**

**Figure 1.** Single disk-shaped inclusion subjected to an incident elastic wave.

in 0

1 1 exp i

**x**

L T <sup>3</sup>

**<sup>x</sup>**

scattered waves can be written in the form [18]:

3

x

The inclusion is regarded as a rigid unit and its motion is described by the translation 0 000 <sup>123</sup> **u** (u ,u ,u ) and the rotation with respect to the coordinate axes with the angles <sup>1</sup> , <sup>2</sup> and <sup>3</sup> , respectively. Then the displacement components in the domain *S* can be

In order to obtain the integral representations for the displacement components we apply the Betty-Rayleigh reciprocity theorem in conjunction with the properties of the elastodynamic fundamental solutions. As a result, the displacement components of the

> <sup>T</sup> j j 2 1 2 S

4 G xx x

exp i exp i dS ,

**x x**

**x x**

where the displacement continuity conditions across the inclusion are used, **x** is the distance between the field point 123 **x** (x ,x ,x ) and integration point 1 2 ( , ,0) , and <sup>j</sup> (j 1,2,3) are the jumps of the interfacial stresses across the inclusion, which are defined by

> <sup>j</sup> j3 j3 j3 j3 x 0 ( ) ( ) ( ), j 1,2,3, S, ( ) lim ( ).

Eqs. (5) together with the equations of motion of the inclusion as a rigid unit yields the following relations between the translations and the rotations of the inclusion and the stress

 

**xxx x x x** (5)

j 1,2,3 , (4)

<sup>3</sup> 3 3 12 21 1 2 u ( ) u ( ) u x x , (x ,x , 0) S. **x x x** (3)

T j1 2

3

in 0 j <sup>j</sup> j j 3 3j u ( ) u ( ) u ( 1) x , j 1,2, **x x**

 2 j j 3j 3j j 1 2 2 2 3 x 4 x R, L <sup>L</sup> 1 , <sup>M</sup> <sup>i</sup> **xx x x** j 1,2 , 11 22 3 1 2 2 2 1 <sup>4</sup> x x R, L <sup>1</sup> M i i **x x** , 112 2 k j kj 2 2 2 3 x x 4 x R , <sup>L</sup> , <sup>M</sup> <sup>i</sup> **x x x** k, j 1,2 , k j , (8) L T j j1 3 3 j 2 exp i d exp i d Ld l d ld , d d j 1,2 , <sup>11</sup> <sup>L</sup> l d 1 i d, 2 2 <sup>12</sup> T T l d 1 i d d, 2 2 <sup>21</sup> L L l d 3 3i d d , 2 2 <sup>22</sup> T T l d 3 3i d d .

The problem governed by the BIEs (7) can be divided into an antisymmetric problem and a symmetric problem. The antisymmetric problem corresponding to the transverse motion of

the inclusion is described by first equation of the BIEs (7) for the stress jump <sup>3</sup> . After the solution of this equation the displacement <sup>0</sup> <sup>3</sup> u and the rotations 1 and 2 can be obtained by using the relations (6). The symmetric problem corresponds to the motion of the inclusion in its own plane, which is governed by the last two equations of the BIEs (7) for the stress jumps 1 and <sup>2</sup> . After these quantities have been computed by solving these equations, the kinematical parameters 0 0 1 2 u ,u and 3 can be obtained by using the relations (6).

Numerical Simulation of Wave Propagation

(12)

(14)

(13)

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 23

Substitution of Eq. (10) into Eq. (9) results in a system of BIEs for the functions j f( ) **x** . These BIEs have a weak singularity | **x** at the source point **x** and a "square-root" singularity at the edge of the inclusion. To regularize the singular BIEs, the following

> 2 S S 222 222 1 2 1 2

3 3 S S 222 222 1 2 1 2

**<sup>x</sup>**

3 3 S S 222 222 1 2 1 2

**<sup>x</sup>**

S 222 1 2

a ||

Here the special integral identities, taken from [20], are used, namely:

k j 2

1 12 2 12 x asin y cos y , x asin y sin y ,

By applying Eqs. (11)-(13) to the BIEs (9) we obtain their regularized version as

 0 0 y y 2 1 in 3 2 33 1 3 S S sin <sup>1</sup> Af dS f R , sin dS 4 Gu R , **<sup>y</sup> y y**

0 0 y y

j j 1 S S sin f A dS B , sin dS R , <sup>2</sup>

**y** 

Next we perform the following transformation of the variables:

**x**

 

11 22 S 222 3 1 2

of the inclusion corresponding to 1 / 2 .

**y**

2 1

a a | | | |

a a | | 2 | |

**x**

 **<sup>x</sup> x x x**

a || a ||

0 x

0 x 2 2 <sup>2</sup> j j j j

**x x**

**x x**

2

1 12 2 12

, **y**y ,y S 1 2 ,

2

**y y**

 0 0 y y 3 j 1 2 j j S S <sup>1</sup> Bf , sin dS f R , **y y <sup>y</sup>** 

asin cos , asin sin , 

(11)

f( ) f( ) f( ) dS f( ) dS ,

(x ) f( ) f( ) (x ) f( ) dS f( ) dS ,

0 x 1 22 1 12 2

f( ) (x )(x ) f( ) f( ) (x )(x ) dS dS .

dS ,

**x**

(x ) (x ) / 2, when k = 2, j = 0 or k = 0, j = 2, dS a || 0, when k 1, j 1.

where 1 2 **y** (y ,y ) and 1 2 (,) are new variables in the rectangular domain S : 0 y , / 2;0 y , 2 1 1 2 2 . Equation (13) transforms the circular integration domain to a rectangular integration domain and eliminates the "square-root" singularity at the front

integral relations for the elastostatic kernels are utilized when S **x** :

The kernels of the BIEs (7) contain weakly singular integrals only. To isolate these singularities explicitly we rewrite the BIEs (7) as

 <sup>3</sup> in 3 3 2 3 S S T 1 A A dS R , dS 4 Gu , **x x x x** S **x** , <sup>2</sup> j j j 1122 2 3 3 j S S x x x A B dS B dS **<sup>x</sup> x x** 2 j j j 2 j 3 2 3 j j3 j S T T <sup>x</sup> 1 A <sup>1</sup> R, B R , **x x x x** (9) <sup>1122</sup> in 3 j x x B dS 4 Gu , **x x** j 1,2 , S **x** ,

where

$$\mathbf{A} = \left(\mathbf{3} - 4\mathbf{v}\right) \Big/ \left[\mathbf{4}\left(\mathbf{1} - \mathbf{v}\right)\right], \quad \mathbf{B} = \mathbf{1} \Big/ \left[\mathbf{4}\left(\mathbf{1} - \mathbf{v}\right)\right].$$

In Eq. (9), the last integrals on the left-hand sides exist in the ordinary sense. This fact follows from an analysis of the integrand in the limit **x** . Therefore, in the numerical evaluation of these integrals it is sufficient to perform the integration over <sup>0</sup> <sup>x</sup> S by excluding a small region (the neighborhood of the **x** -point) around **x** from *S*.

The singularities of the BIEs (9) are identical to those of the corresponding BIEs for the static inclusion problems, which have been investigated in [20] both for the antisymmetric and symmetric cases. The local behavior of the stress jumps at the front of the inclusion is also the same as in the static case. For a circular disk-shaped inclusion, the stress jumps have a "square-root" singularity, which can be expressed as

$$\Delta \sigma\_{\parallel}(\mathbf{x}) = \frac{\mathbf{f}\_{\parallel}(\mathbf{x})}{\sqrt{\mathbf{a}^2 - \mathbf{x}\_1^2 - \mathbf{x}\_2^2}}, \quad \mathbf{j} = 1, 2, 3, \qquad \mathbf{x} \in \mathbf{S}\_{\prime} \tag{10}$$

where j f( ) **x** are unknown smooth functions, and a is the radius of the inclusion.

Substitution of Eq. (10) into Eq. (9) results in a system of BIEs for the functions j f( ) **x** . These BIEs have a weak singularity | **x** at the source point **x** and a "square-root" singularity at the edge of the inclusion. To regularize the singular BIEs, the following integral relations for the elastostatic kernels are utilized when S **x** :

0 x 2 S S 222 222 1 2 1 2 f( ) f( ) f( ) dS f( ) dS , a || a || **<sup>x</sup> x x x** 0 x 2 2 <sup>2</sup> j j j j 3 3 S S 222 222 1 2 1 2 (x ) f( ) f( ) (x ) f( ) dS f( ) dS , a a | | 2 | | **<sup>x</sup> x x x** (11) 0 x 1 22 1 12 2 3 3 S S 222 222 1 2 1 2 f( ) (x )(x ) f( ) f( ) (x )(x ) dS dS . a a | | | | **<sup>x</sup> x x** 

Here the special integral identities, taken from [20], are used, namely:

22 Composites and Their Applications

the kinematical parameters 0 0

solution of this equation the displacement <sup>0</sup>

singularities explicitly we rewrite the BIEs (7) as

x x

**x**

"square-root" singularity, which can be expressed as

where

the inclusion is described by first equation of the BIEs (7) for the stress jump <sup>3</sup> . After the

by using the relations (6). The symmetric problem corresponds to the motion of the inclusion in its own plane, which is governed by the last two equations of the BIEs (7) for the stress jumps 1 and <sup>2</sup> . After these quantities have been computed by solving these equations,

The kernels of the BIEs (7) contain weakly singular integrals only. To isolate these

 <sup>3</sup> in 3 3 2 3 S S T

 <sup>2</sup> j j j 1122 2 3 3 j S S

x x x A B dS B dS 

<sup>x</sup> 1 A <sup>1</sup> R, B R ,

2 j j

**x**

j 1,2 , S **x** ,

1 A A dS R , dS 4 Gu ,

 **x x x x** 

j 2 j 3 2 3 j j3 j S T T

 **x x x x**

A 3 4 41 , B 1 41

In Eq. (9), the last integrals on the left-hand sides exist in the ordinary sense. This fact follows from an analysis of the integrand in the limit **x** . Therefore, in the numerical

The singularities of the BIEs (9) are identical to those of the corresponding BIEs for the static inclusion problems, which have been investigated in [20] both for the antisymmetric and symmetric cases. The local behavior of the stress jumps at the front of the inclusion is also the same as in the static case. For a circular disk-shaped inclusion, the stress jumps have a

f( ) ( ) , j 1,2,3, S,

**x x** (10)

**<sup>x</sup> x x**

 <sup>1122</sup> in 3 j

evaluation of these integrals it is sufficient to perform the integration over <sup>0</sup>

j <sup>j</sup> <sup>222</sup>

axx

where j f( ) **x** are unknown smooth functions, and a is the radius of the inclusion.

 **x**

1 2

a small region (the neighborhood of the **x** -point) around **x** from *S*.

B dS 4 Gu ,

1 2 u ,u and 3 can be obtained by using the relations (6).

<sup>3</sup> u and the rotations 1 and 2 can be obtained

S **x** ,

(9)

<sup>x</sup> S by excluding

$$\iint\_{\mathbf{S}} \frac{\mathbf{dS}\_{\mathbf{S}}}{\sqrt{\mathbf{a}^2 - \xi\_1^2 - \xi\_2^2} \mid \mathbf{x} - \mathbf{S}} = \pi^2 \,\mu$$

$$\iint\_{S} \frac{(\mathbf{x}\_{1} - \overline{\mathbf{y}}\_{1})^{k} (\mathbf{x}\_{2} - \overline{\mathbf{y}}\_{2})^{l}}{\sqrt{\mathbf{a}^{2} - \overline{\mathbf{y}}\_{1}^{2} - \overline{\mathbf{y}}\_{2}^{2}} \, |\mathbf{x} - \overline{\mathbf{g}}|^{3}} \, \mathrm{d}\mathbf{S}\_{\mathbf{g}} = \begin{cases} \pi^{2} / 2, & \text{when } \mathbf{k} = 2, \text{ j} = 0 \text{ or } \ \mathbf{k} = 0, \text{ j} = 2, \\\ 0, & \text{when } \ \mathbf{k} = 1, \quad \text{ j} = 1. \end{cases} \tag{12}$$

Next we perform the following transformation of the variables:

$$\begin{cases} \mathbf{x}\_1 = \mathbf{a}\sin\mathbf{y}\_1 \cos\mathbf{y}\_{2'} & \begin{cases} \sharp\_1 = \mathbf{a}\sin\eta\_1 \cos\eta\_{2'}\\ \sharp\_2 = \mathbf{a}\sin\mathbf{y}\_1 \sin\mathbf{y}\_{2'} & \begin{cases} \sharp\_2 = \mathbf{a}\sin\eta\_1 \sin\eta\_{2'} \end{cases} \end{cases} \end{cases} \tag{13}$$

where 1 2 **y** (y ,y ) and 1 2 (,) are new variables in the rectangular domain S : 0 y , / 2;0 y , 2 1 1 2 2 . Equation (13) transforms the circular integration domain to a rectangular integration domain and eliminates the "square-root" singularity at the front of the inclusion corresponding to 1 / 2 .

By applying Eqs. (11)-(13) to the BIEs (9) we obtain their regularized version as

 0 0 y y 2 1 in 3 2 33 1 3 S S sin <sup>1</sup> Af dS f R , sin dS 4 Gu R , **<sup>y</sup> y y y** , **y**y ,y S 1 2 , 0 0 y y 2 2 1 j j 1 S S sin f A dS B , sin dS R , <sup>2</sup> **y y y** (14) 0 0 y y 3 j 1 2 j j S S <sup>1</sup> Bf , sin dS f R , **y y <sup>y</sup>** 

$$\left[ + \tilde{\mathbf{f}}\_{\text{-}(\text{)}} \left( \mathbf{\eta} \right) \tilde{\mathbf{R}}\_{\text{j}(\text{2}-)} \left( \mathbf{y}, \mathbf{\eta} \right) \right] \text{sin } \eta\_{\text{l}} \text{dS}\_{\mathbf{\eta}} = -4 \pi \text{G} \, \tilde{\mathbf{u}}\_{\text{l}}^{\text{in}} \left( \mathbf{y} \right), \text{ j} = 1, 2, \, \text{ y} \left( \text{y}\_{1}, \text{y}\_{2} \right) \in \tilde{\mathbf{S}}\_{\text{-}}.$$

where

$$
\tilde{\mathbf{f}}\_{\uparrow}(\mathbf{y}) = \mathbf{f}\_{\downarrow}(\mathbf{x}), \quad \tilde{\mathbf{u}}\_{\uparrow}^{\text{in}}(\mathbf{y}) = \mathbf{u}\_{\downarrow}^{\text{in}}(\mathbf{x}), \quad \tilde{\mathbf{R}}\_{\uparrow}(\mathbf{y}, \boldsymbol{\eta}) = \mathbf{a}^3 \mathbf{R}\_{\downarrow}(\mathbf{x}, \mathbf{f}\_{\mathbf{g}}), \qquad \mathbf{j} = 1, 2, 3,
$$

$$
\tilde{\mathbf{R}}\_{\mathbf{k}\downarrow}(\mathbf{y}, \boldsymbol{\eta}) = \mathbf{a}^3 \mathbf{R}\_{\mathbf{k}\downarrow}(\mathbf{x}, \mathbf{f}\_{\mathbf{g}}), \quad \mathbf{k}, \mathbf{j} = 1, 2, \quad \mathbf{k} \neq \mathbf{j}. \tag{15}
$$

Numerical Simulation of Wave Propagation

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 25

R

**<sup>p</sup>** , (19)

<sup>3</sup> 2 2 <sup>2</sup>

<sup>3</sup> 2 2 <sup>2</sup>

3 2

<sup>T</sup>

Here, L F , TV F , and TH F are the longitudinal, vertically polarized transverse, and horizontally polarized transverse wave scattering amplitudes, respectively, which are

> <sup>3</sup> L 0 jj L <sup>S</sup> j 1 1 2 F ,, p exp i dS 2(1- )G **<sup>p</sup>** ,

> > <sup>3</sup>

<sup>3</sup>

where jj j p , r , (j 1,2,3) are the coordinates of the spherical unit vectors **p** (sin cos , sin sin , cos ) , **r** (cos cos , cos sin , -sin ) and (-sin , cos , 0) .

The forward scattering amplitudes are defined as the values of F , , (Z L,TV,TH) Z 0

Thus, the scattering problem in the far-field is reduced to the numerical solution of the BIEs (14) and the subsequent computation of the scattering amplitudes by using Eq. (19), where

For the convenient description of the wave parameters in the inclusion vicinity let us introduce the local coordinate system Otqz with the center in the inclusion contour point, so that the value z 0 corresponds to the inclusion plane, the axes Ot and Oq lie in the normal and tangential directions relative to the inclusion contour line, respectively, as depicted in Figure 1. Then the corresponding displacement and stress components at the

arbitrary point *P* near the inclusion in the plane q 0 can be approximated as [21]:

z I II

t I II

q III

2r 2r u (r, , ) sin 1 sin K ( ) cos 2 4 cos K ( ) O(r ) 2G 2 2 2G 2 <sup>2</sup> ,

2r 2r u (r, , ) cos 4 4 cos K ( ) sin 1 sin K ( ) O(r ) 2G 2 2 2G 2 2 ,

> 2r u (r, , ) cos K ( ) O(r ) 2G 2 ,

zz I 1 3 (r, , ) cos 1 2 sin sin K ( ) 2r 2 22

TV <sup>0</sup> j j <sup>T</sup> <sup>S</sup> j 1 <sup>1</sup> F , , r exp i dS <sup>G</sup>

TH 0 j j <sup>T</sup> <sup>S</sup> j 1 <sup>1</sup> F ,, exp i dS <sup>G</sup> **<sup>p</sup>** ,

in the direction of the wave incidence, i.e., F ,0, Z0 0 .

the transformation or mapping relations (13) have to be considered.

exp i R u R, , F ,, , 4 R 

related to the inclusion of normalized mass <sup>3</sup> <sup>0</sup> M( a) . They are given by

TH 0

In Eq. (15), **x** and are defined by Eq. (13), T a is the normalized wave number of the *T*-wave, <sup>0</sup> S**<sup>y</sup>** is the mapping of the domain <sup>0</sup> <sup>S</sup>**x** due to the transformation (13) (in the domain 0 y S the points **y** and do not coincide), and

$$\begin{aligned} \mathrm{R}\begin{pmatrix} \mathbf{y}, \boldsymbol{\mathfrak{n}} \end{pmatrix} &= \left[ \sin^2 \mathbf{y}\_1 + \sin^2 \boldsymbol{\eta}\_1 - 2 \sin \mathbf{y}\_1 \sin \boldsymbol{\eta}\_1 \cos \left( \mathbf{y}\_2 - \boldsymbol{\eta}\_2 \right) \right]^{12}, \\\\ \boldsymbol{\Phi}\_1 \begin{pmatrix} \mathbf{y}, \boldsymbol{\mathfrak{n}} \end{pmatrix} &= \left( \sin \mathbf{y}\_1 \cos \mathbf{y}\_2 - \sin \boldsymbol{\eta}\_1 \cos \boldsymbol{\eta}\_2 \right)^2 \left[ \mathrm{R} \begin{pmatrix} \mathbf{y}, \boldsymbol{\mathfrak{n}} \end{pmatrix} \right]^{-3}, \\\\ \boldsymbol{\Phi}\_2 \begin{pmatrix} \mathbf{y}, \boldsymbol{\mathfrak{n}} \end{pmatrix} &= \left( \sin \mathbf{y}\_1 \sin \mathbf{y}\_2 - \sin \boldsymbol{\eta}\_1 \sin \boldsymbol{\eta}\_2 \right)^2 \left[ \mathrm{R} \begin{pmatrix} \mathbf{y}, \boldsymbol{\mathfrak{n}} \end{pmatrix} \right]^{-3}, \end{aligned} \tag{16}$$

$$\begin{aligned} \mathrm{R}\begin{pmatrix} \mathbf{y}, \boldsymbol{\mathfrak{n}} \end{pmatrix} &= \left( \sin \mathbf{y}\_1 \cos \mathbf{y}\_2 - \sin \boldsymbol{\eta}\_1 \sin \boldsymbol{\eta}\_2 \right) \left[ \mathrm{S} \begin{pmatrix} \mathbf{y}, \boldsymbol{\mathfrak{n}} \end{pmatrix} \right]^{-3}, \end{aligned} $$

For the discretization of the domain S , a boundary element mesh with equal-sized rectangular elements is used. For simplicity, constant elements are adopted in this analysis. By collocating the BIEs (14) at discrete points coinciding with the centroids of each element, a system of linear algebraic equations for discrete values of j f is obtained. After solving the system of linear algebraic equations numerically, the stress jumps <sup>j</sup> across the inclusion can be obtained by the relations (10) and (15).

The far-field quantities of the scattered elastic waves can be computed from the stress jumps <sup>j</sup> . For this purpose we use the asymptotic relations for an observation point far away from the inclusion, namely **x xx x** and 1 1 **x x** , when **<sup>x</sup>** . By substituting of these relations into the integral representation formula (4) and introducing the spherical coordinate system with the origin at the center of the inclusion as

$$\mathbf{x}\_1 = \mathbf{R}\sin\theta\cos\varphi, \quad \mathbf{x}\_2 = \mathbf{R}\sin\theta\sin\varphi, \quad \mathbf{x}\_3 = \mathbf{R}\cos\theta, \; 0 \le \theta, \varphi \le 2\pi,\tag{17}$$

the asymptotic expressions for the scattered radial R u and tangential u ,u displacements in the far-field are obtained in the form

$$
\mathbf{u}\_{\text{R}}\left(\mathbf{R},\theta,\boldsymbol{\uprho}\right) = \frac{\exp\left(\mathbf{i}\mathbf{y}\_{\text{L}}\mathbf{R}\right)}{4\pi\mathbf{R}}\mathbf{F}\_{\text{L}}\left(\theta,\boldsymbol{\uprho},\boldsymbol{\uprho}\_{0}\right), \; \mathbf{R} \to \infty,
$$

$$
\mathbf{u}\_{\text{e}}\left(\mathbf{R},\theta,\boldsymbol{\uprho}\right) = \frac{\exp\left(\mathbf{i}\mathbf{y}\_{\text{r}}\mathbf{R}\right)}{4\pi\mathbf{R}}\mathbf{F}\_{\text{TV}}\left(\theta,\boldsymbol{\uprho},\boldsymbol{\uprho}\_{0}\right), \; \mathbf{R} \to \infty,\tag{18}
$$

Numerical Simulation of Wave Propagation

#### in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 25

$$\mathbf{u}\_{\boldsymbol{\phi}}\left(\mathbf{R}, \boldsymbol{\theta}, \boldsymbol{\phi}\right) = \frac{\exp\left(\mathbf{i}\boldsymbol{\chi}\_{\mathrm{T}}\mathbf{R}\right)}{4\pi\mathbf{R}}\mathbf{F}\_{\mathrm{TH}}\left(\boldsymbol{\theta}, \boldsymbol{\upphi}, \boldsymbol{\upchi}\_{0}\right), \; \mathbf{R} \to \infty$$

24 Composites and Their Applications

where

*T*-wave, <sup>0</sup> S**<sup>y</sup>**

y S

in

f f, j j **y x** in in

the points **y** and do not coincide), and

For the discretization of the domain S

can be obtained by the relations (10) and (15).

in the far-field are obtained in the form

a system of linear algebraic equations for discrete values of j

3 j j3 j <sup>1</sup> <sup>j</sup> f R , sin dS 4 Gu , **y y** <sup>j</sup> 1,2 , **<sup>y</sup>**y ,y S 1 2 ,

In Eq. (15), **x** and are defined by Eq. (13), T a is the normalized wave number of the

is the mapping of the domain <sup>0</sup> <sup>S</sup>**x** due to the transformation (13) (in the domain 0

1 2 2 2 R , sin y sin 2sin y sin cos y 1 1 1 1 22 **<sup>y</sup>** ,

 <sup>2</sup> <sup>3</sup> <sup>1</sup> 12 12 , sin y cos y sin cos R ,

<sup>2</sup> 12 12 , sin y sin y sin sin R , **y y** ,

 <sup>3</sup> 1 2 1 2 12 12 , sin y cos y sin cos sin y sin y sin sin R , **y y**

rectangular elements is used. For simplicity, constant elements are adopted in this analysis. By collocating the BIEs (14) at discrete points coinciding with the centroids of each element,

system of linear algebraic equations numerically, the stress jumps <sup>j</sup> across the inclusion

The far-field quantities of the scattered elastic waves can be computed from the stress jumps <sup>j</sup> . For this purpose we use the asymptotic relations for an observation point far away from the inclusion, namely **x xx x** and 1 1 **x x** , when **<sup>x</sup>** . By substituting of these relations into the integral representation formula (4) and introducing

the asymptotic expressions for the scattered radial R u and tangential u ,u displacements

 <sup>L</sup> R L 0 exp i R u R, , F ,, , 4 R 

<sup>T</sup>

exp i R u R, , F ,, , 4 R 

<sup>1</sup> x R sin cos , <sup>2</sup> x R sin sin , <sup>3</sup> x R cos , 0,2 , (17)

TV 0

the spherical coordinate system with the origin at the center of the inclusion as

j j u u, **y x** <sup>3</sup> R , a R , , j 1,2,3, j j **y x**

<sup>3</sup> R , aR , kj kj **y x** , k, j 1,2 , k j . (15)

**y y** , (16)

2 3

f

R ,

, a boundary element mesh with equal-sized

is obtained. After solving the

R , (18)

Here, L F , TV F , and TH F are the longitudinal, vertically polarized transverse, and horizontally polarized transverse wave scattering amplitudes, respectively, which are related to the inclusion of normalized mass <sup>3</sup> <sup>0</sup> M( a) . They are given by

$$\begin{aligned} \mathrm{F}\_{\mathsf{L}}\left(\boldsymbol{\theta},\boldsymbol{\phi},\boldsymbol{\gamma}\_{0}\right) &= \frac{1-2\nu}{2\left(1\cdot\nu\right)\mathrm{G}}\sum\_{|\cdot|=1}^{3} \mathrm{p}\_{\mathsf{f}}\left[\int\_{\mathsf{S}} \Delta\boldsymbol{\sigma}\_{\boldsymbol{\flat}}\left(\boldsymbol{\mathsf{\dot{\mathsf{g}}}}\right) \exp\left[-\mathrm{i}\boldsymbol{\chi}\_{\mathsf{L}}\left(\boldsymbol{\mathsf{p}}\cdot\boldsymbol{\mathsf{\dot{\mathsf{g}}}}\right)\right] \mathrm{d}\mathbf{S}\_{\mathsf{g}}\right] \\\\ \mathrm{F}\_{\mathsf{TV}}\left(\boldsymbol{\Theta},\boldsymbol{\uprho},\boldsymbol{\chi}\_{0}\right) &= \frac{1}{\mathrm{G}}\sum\_{|\cdot|=1}^{3} \mathrm{r}\_{\mathsf{f}}\left[\int\_{\mathsf{S}} \Delta\boldsymbol{\sigma}\_{\boldsymbol{\flat}}\left(\boldsymbol{\mathsf{\dot{\mathsf{g}}}}\right) \exp\left[-\mathrm{i}\boldsymbol{\chi}\_{\mathsf{T}}\left(\boldsymbol{\mathsf{p}}\cdot\boldsymbol{\mathsf{\dot{\mathsf{g}}}}\right)\right] \mathrm{d}\mathbf{S}\_{\mathsf{g}}\right] \\\\ \mathrm{F}\_{\mathsf{T}\mathsf{H}}\left(\boldsymbol{\theta},\boldsymbol{\uprho},\boldsymbol{\chi}\_{0}\right) &= \frac{1}{\mathrm{G}}\sum\_{|\cdot|=1}^{3} \mathrm{r}\_{\mathsf{f}}\left[\int\_{\mathsf{S}} \Delta\boldsymbol{\sigma}\_{\boldsymbol{\flat}}\left(\boldsymbol{\mathsf{g}}\right) \exp\left[-\mathrm{i}\boldsymbol{\chi}\_{\mathsf{T}}\left(\boldsymbol{\mathsf{p}}\cdot\boldsymbol{\mathsf{g}}\right)\right] \mathrm{d}\mathbf{S}\_{\mathsf{g}}\right]. \end{aligned} \tag{19}$$

where jj j p , r , (j 1,2,3) are the coordinates of the spherical unit vectors **p** (sin cos , sin sin , cos ) , **r** (cos cos , cos sin , -sin ) and (-sin , cos , 0) .

The forward scattering amplitudes are defined as the values of F , , (Z L,TV,TH) Z 0 in the direction of the wave incidence, i.e., F ,0, Z0 0 .

Thus, the scattering problem in the far-field is reduced to the numerical solution of the BIEs (14) and the subsequent computation of the scattering amplitudes by using Eq. (19), where the transformation or mapping relations (13) have to be considered.

For the convenient description of the wave parameters in the inclusion vicinity let us introduce the local coordinate system Otqz with the center in the inclusion contour point, so that the value z 0 corresponds to the inclusion plane, the axes Ot and Oq lie in the normal and tangential directions relative to the inclusion contour line, respectively, as depicted in Figure 1. Then the corresponding displacement and stress components at the arbitrary point *P* near the inclusion in the plane q 0 can be approximated as [21]:

<sup>3</sup> 2 2 <sup>2</sup> z I II 2r 2r u (r, , ) sin 1 sin K ( ) cos 2 4 cos K ( ) O(r ) 2G 2 2 2G 2 <sup>2</sup> , <sup>3</sup> 2 2 <sup>2</sup> t I II 2r 2r u (r, , ) cos 4 4 cos K ( ) sin 1 sin K ( ) O(r ) 2G 2 2 2G 2 2 , 3 2 q III 2r u (r, , ) cos K ( ) O(r ) 2G 2 , zz I 1 3 (r, , ) cos 1 2 sin sin K ( ) 2r 2 22 

II 1 3 sin 2 2 cos cos K ( ) O(1) 2r 2 22 , tt I II <sup>1</sup> 31 3 (r, , ) cos 3 2 sin sin K ( ) sin 2 cos cos K ( ) O(1) 2r 2 22 2r 2 22 , qq <sup>I</sup> II 2 2 (r, , ) cos K ( ) sin K ( ) O(1) 2r 2 2 2r , (20) zq III <sup>1</sup> (r, , ) sin K ( ) O(1) 2r 2 , tq III <sup>1</sup> (r, , ) cos K ( ) O(1) 2r 2 , zt I 1 3 (r, , ) sin 2 2 cos cos K ( ) 2r 2 22 II 1 3 cos 1 2 sin sin K ( ) O(1) 2r 2 22 

Numerical Simulation of Wave Propagation

, (22)

(24)

ZF is

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 27

orientation is either completely random or aligned, see Figure 2. In the case of aligned inclusions, it is postulated that the inclusions are parallel to the 1 2 x x -plane. The radius a of the inclusions is assumed to be equal, while their masses can be variable due to the

**Figure 2.** Multiple disk-shaped inclusions in the elastic wave field: parallel inclusions with the angle of

The average response of the composite materials to the wave propagation is characterized by the geometrical dispersion and attenuation of waves due to the wave scattering process. To describe these phenomena within the coherent wave field, the dynamic properties of the composite can be modeled by a complex and frequency-dependent wave number K as

> K i <sup>c</sup>

where c is the effective phase velocity and is the attenuation coefficient for the wave of corresponding mode. With Eq. (22), the amplitude of a plane time-harmonic elastic

For low concentration of inclusions or small number density, the interaction or multiple scattering effects among the inclusions can be neglected. Under these assumptions the complex effective wave numbers K (Z L,T) <sup>Z</sup> of plane L - and T -waves can be calculated by using the Foldy-type dispersion relation, which was extended to elastic waves in [3] and

2 2 3\* K a F , Z L,T ZZ Z

In Eq. (24), is the density parameter of the inclusions, <sup>3</sup> a corresponds to the number density of inclusions of the same radius, i.e. the number of inclusions per unit volume, \*

the average forward scattering amplitude of the corresponding wave mode by a single

\* ( , ) exp iK exp ( ) exp i c ( ) **ux U n x U n x n x** (23)

wave incidence (left); randomly oriented inclusions (right).

wave propagating in the **n**-direction can be expressed as

may be stated as

different material properties of inclusions and their geometric aspect ratios.

Here r and are the polar coordinates of the point *P*, is the angular coordinate of the inclusion contour point (see Figure 1), K , K and I II K are the mode-I, II, and III dynamic III stress intensity factors in the inclusion vicinity.

By using the Eq. (20) the K -factors can be defined directly from the stress jumps I,II,III j or the solutions of BIEs (7) by the following relations:

 <sup>1</sup> 2 222 I 0 12 1 <sup>2</sup> x a cos ; x a sin <sup>1</sup> K( , ) a x x ( )cos ( )sin , 4(1 ) a **x x** 1 2 222 II 0 <sup>123</sup> x a cos ; x a sin <sup>1</sup> K(, ) a x x () , 4(1 ) a **x** (21) <sup>1</sup> 2 222 III 0 12 1 <sup>2</sup> x a cos ; x a sin <sup>1</sup> K ( , ) a x x (x)sin ( )cos 4 a **x** ,

where the dependence of K -factors on the inclusion mass al I,II,III so is fixed by the variable 0 .

#### **3. Dispersion relations for distributed inclusions of variable mass**

We consider now a statistical distribution of rigid disk-shaped micro-inclusions in the matrix. The location of the micro-inclusions is assumed to be random, while their orientation is either completely random or aligned, see Figure 2. In the case of aligned inclusions, it is postulated that the inclusions are parallel to the 1 2 x x -plane. The radius a of the inclusions is assumed to be equal, while their masses can be variable due to the different material properties of inclusions and their geometric aspect ratios.

26 Composites and Their Applications

II

, (20)

1 3 sin 2 2 cos cos K ( ) O(1)

<sup>1</sup> 31 3 (r, , ) cos 3 2 sin sin K ( ) sin 2 cos cos K ( ) O(1) 2r 2 22 2r 2 22 ,

> 2 2 (r, , ) cos K ( ) sin K ( ) O(1) 2r 2 2 2r

> > <sup>1</sup> (r, , ) sin K ( ) O(1) 2r 2 ,

<sup>1</sup> (r, , ) cos K ( ) O(1) 2r 2 ,

<sup>1</sup>

**x** ,

1 2

x a sin

2

x a sin

**x x**

4(1 ) a

222

4(1 ) a

<sup>1</sup>

4 a

2

x a sin

**x** (21)

II

 ,

2r 2 22

tt I II

zq III

tq III

2r 2 22

222

222

<sup>1</sup> K ( , ) a x x (x)sin ( )cos

**3. Dispersion relations for distributed inclusions of variable mass** 

zt I 1 3 (r, , ) sin 2 2 cos cos K ( ) 2r 2 22

 

1 3 cos 1 2 sin sin K ( ) O(1)

Here r and are the polar coordinates of the point *P*, is the angular coordinate of the inclusion contour point (see Figure 1), K , K and I II K are the mode-I, II, and III dynamic III

By using the Eq. (20) the K -factors can be defined directly from the stress jumps I,II,III j or

I 0 12 1 <sup>2</sup> x a cos ;

II 0 <sup>123</sup> x a cos ;

III 0 12 1 <sup>2</sup> x a cos ;

where the dependence of K -factors on the inclusion mass al I,II,III so is fixed by the variable 0 .

We consider now a statistical distribution of rigid disk-shaped micro-inclusions in the matrix. The location of the micro-inclusions is assumed to be random, while their

<sup>1</sup> K(, ) a x x () ,

<sup>1</sup> K( , ) a x x ( )cos ( )sin ,

qq <sup>I</sup> II

stress intensity factors in the inclusion vicinity.

the solutions of BIEs (7) by the following relations:

**Figure 2.** Multiple disk-shaped inclusions in the elastic wave field: parallel inclusions with the angle of wave incidence (left); randomly oriented inclusions (right).

The average response of the composite materials to the wave propagation is characterized by the geometrical dispersion and attenuation of waves due to the wave scattering process. To describe these phenomena within the coherent wave field, the dynamic properties of the composite can be modeled by a complex and frequency-dependent wave number K as

$$\mathbf{K}\left(\alpha\right) = \frac{\alpha}{\mathbf{c}^\*\left(\alpha\right)} + \mathrm{i}\alpha\left(\alpha\right),\tag{22}$$

where c is the effective phase velocity and is the attenuation coefficient for the wave of corresponding mode. With Eq. (22), the amplitude of a plane time-harmonic elastic wave propagating in the **n**-direction can be expressed as

$$\mathbf{u}(\mathbf{x},\alpha) = \mathbf{U} \exp\left[\mathbf{i}\mathbf{K}\left(\mathbf{n}\cdot\mathbf{x}\right)\right] = \mathbf{U} \exp\left[-\alpha(\alpha)\left(\mathbf{n}\cdot\mathbf{x}\right)\right] \exp\left[\mathbf{i}\alpha\left(\mathbf{n}\cdot\mathbf{x}\right)\right] \mathbf{c}^\*(\alpha) \tag{23}$$

For low concentration of inclusions or small number density, the interaction or multiple scattering effects among the inclusions can be neglected. Under these assumptions the complex effective wave numbers K (Z L,T) <sup>Z</sup> of plane L - and T -waves can be calculated by using the Foldy-type dispersion relation, which was extended to elastic waves in [3] and may be stated as

$$\mathbf{K}\_{\mathbf{Z}}^{2} = \chi\_{\mathbf{Z}}^{2} + \varepsilon \mathbf{a}^{-3} \mathbf{F}\_{\mathbf{Z}}^{\*} \qquad \mathbf{Z} = \mathbf{L}\_{\prime} \mathbf{T} \tag{24}$$

In Eq. (24), is the density parameter of the inclusions, <sup>3</sup> a corresponds to the number density of inclusions of the same radius, i.e. the number of inclusions per unit volume, \* ZF is the average forward scattering amplitude of the corresponding wave mode by a single

inclusion. For randomly oriented inclusions of variable mass the averages should be taken both over all possible inclusion orientations and masses. It should be noted here that the average over all inclusion orientations is the same as the average over all directions of the wave incidence (to avoid the additional average over all wave polarizations for an incoming *T*-wave, we assume that the normal to the inclusions lie in the plane of incidence of the incoming *TV*- or *TH*-wave). Hence, in the case of parallel inclusions the expressions for the average forward scattering amplitudes of corresponding wave mode are

$$\mathbf{F}\_{\mathbf{Z}}^{\*}\left(\boldsymbol{\Theta}\_{0}\right) = \int\_{\mathbf{z}}^{\mathbb{H}} \mathbf{F}\_{\mathbf{Z}}\left(\boldsymbol{\Theta}\_{0}, \mathbf{0}, \boldsymbol{\gamma}\right) \mathbf{g}(\boldsymbol{\gamma}) d\boldsymbol{\gamma}, \qquad \mathbf{Z} = \mathbf{L}\_{\prime} \mathbf{T} \mathbf{V}\_{\prime} \mathbf{T} \mathbf{H}\_{\prime} \tag{25}$$

and in the case of randomly oriented inclusion they become the form

$$\mathbf{F}\_{\mathbf{Z}}^{\prime} = \frac{1}{2} \int\_{0}^{\pi} \int\_{\mathbf{r}}^{\emptyset} \mathbf{F}\_{\mathbf{Z}}^{\prime} \left(\theta\_{0}, 0, \gamma\right) \mathbf{g}(\gamma) \sin \theta\_{0} d\gamma d\theta\_{0}, \qquad \mathbf{Z} = \mathbf{L}\_{\prime} \mathbf{T} \mathbf{V}\_{\prime} \mathbf{T} \mathbf{H}. \tag{26}$$

Here 0 is the angle characterizing the direction of the wave incidence, the parameters and characterize the minimal and maximal masses, respectively, in the system of distributed inclusions with the density function g of inclusion mass, and Z 0 F ( ,0, ) are the forward scattering amplitudes given by Eq. (19). The density distribution function of inclusion mass should satisfy the normalization condition

$$\int\_{\varepsilon}^{\beta} \mathbf{g}(\gamma) d\gamma = 1 \tag{27}$$

Numerical Simulation of Wave Propagation

is divided into

<sup>0</sup> ( 0) of *L*-

is the boundary element with the point

<sup>0</sup> ( 90 ) of *L*- wave. This choice provides the

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 29

Z Z Im K , Z L,TV,TH (28)

<sup>Z</sup>

is chosen as <sup>0</sup> S S\ S **y y**

of low concentration of inclusions in a matrix.

and *T*-waves and grazing incidence <sup>o</sup>

stands for *L*- and *TV*-waves, respectively.

the domain <sup>0</sup> S**<sup>y</sup>**

c , Re K 

Z

It should be remarked here that Foldy's theory was derived for isotropic wave scattering, which is appropriate macroscopically for the configuration of randomly oriented inclusions. A composite solid with aligned (parallel) disk-shaped inclusions exhibits a macroscopic anisotropy, namely a transversal anisotropy. When an incident plane wave propagates in an arbitrary direction, this gives rise to a coupling between the *L*- and *T*-waves, and thus a change in the effective polarization vector. However, it is reasonable to apply Foldy's theory, when the wave propagation is along the principal axes because of the decoupling of the *L*and *T*-waves. In this special case, wave propagation can be treated like in the isotropic case.

**4. Numerical analysis of global dynamic parameters of a composite** 

waves. For numerical discretization of the inclusion surface, the domain S

, where S **<sup>y</sup>**

The method presented in the previous sections is used to calculate the effective dynamic parameters of a composite elastic solid with both parallel and randomly oriented rigid diskshaped inclusions of variable mass for the propagation of time-harmonic plane *L*- or *TV*-

264 rectangular elements of length 1 y 22 and 2 y 12 in the 1 y - and 2 y -directions,

**y** in the center of the element. Poisson's ratio is selected as 0.3, the radii of inertia of the circular inclusion are defined as 12 3 i i a 2,i a 2 . In the numerical examples, the radius a of the inclusions is assumed to be equal, while their masses are varied by the distribution laws defined in Table 1 from the minimal value 5 to the maximal value 20 . The inclusion density parameter is fixed as 0.01 in accordance to the assumption

For comparison purpose, normalized effective wave velocities and normalized attenuation coefficients are introduced as Z ZZ c cc , Z Z 2a , where the subscript Z L,TV

For parallel or aligned disk-shaped inclusions, the macroscopic dynamic behavior of the composite materials is transversely isotropic. Thus, the effective wave velocities and the attenuation coefficient are dependent on the direction of the wave incidence. In this analysis, only two wave incidence directions are considered, namely normal incidence <sup>o</sup>

For normal incidence of a plane *L*-wave, the normalized attenuation coefficient L and the normalized effective wave velocity Lc versus the dimensionless wave number are presented in Figure 3. Three separate parametrical studies are performed to show the effects of different inclusion mass distributions: aligned with 0 20 , normal and uniform. To check the accuracy of the implemented BEM, the present numerical results are compared with the analytical low-frequency solutions given in [9]. Here and hereafter a very good

vanishing of the dynamic torque on the inclusions and, therefore, their zero-rotations.

agreement between both results is observed in the frequency range 0 0.3 .

A suitable set of inclusion mass variations, which corresponds to aligned, normal and uniform distributions, is defined in Table 1.


**Table 1.** Various types of density distribution functions of inclusion mass ( is the Dirac delta function).

The approximation for the complex wave number (24) can be considered as a special case of the multiple wave scattering models of higher orders [4,5], and it involves only the first order in the inclusion density and is thus only valid for a dilute or small inclusion density. In the case of a large density or high concentration of inclusions, more sophisticated models such as the self-consistent approach or the multiple scattering models should be applied, to take the mutual dynamic interactions between individual inclusions into account.

Once the complex effective wave numbers K have been determined via Eq. (24), the Z effective wave velocities Z c and the attenuation coefficients Z of the plane L - and T waves can be obtained by considering the definition (22). This results in

$$\mathbf{c}\_{\mathbf{Z}}^{\*}\left(\boldsymbol{\alpha}\right) = \frac{\boldsymbol{\alpha}}{\mathrm{Re}\left[\mathbf{K}\_{\mathbf{Z}}\left(\boldsymbol{\alpha}\right)\right]}, \quad \mathbf{a}\_{\mathbf{Z}}\left(\boldsymbol{\alpha}\right) = \mathrm{Im}\left[\mathbf{K}\_{\mathbf{Z}}\left(\boldsymbol{\alpha}\right)\right], \; \mathbf{Z} = \mathrm{L}, \mathrm{TV}, \mathrm{TH} \tag{28}$$

It should be remarked here that Foldy's theory was derived for isotropic wave scattering, which is appropriate macroscopically for the configuration of randomly oriented inclusions. A composite solid with aligned (parallel) disk-shaped inclusions exhibits a macroscopic anisotropy, namely a transversal anisotropy. When an incident plane wave propagates in an arbitrary direction, this gives rise to a coupling between the *L*- and *T*-waves, and thus a change in the effective polarization vector. However, it is reasonable to apply Foldy's theory, when the wave propagation is along the principal axes because of the decoupling of the *L*and *T*-waves. In this special case, wave propagation can be treated like in the isotropic case.

## **4. Numerical analysis of global dynamic parameters of a composite**

28 Composites and Their Applications

inclusion. For randomly oriented inclusions of variable mass the averages should be taken both over all possible inclusion orientations and masses. It should be noted here that the average over all inclusion orientations is the same as the average over all directions of the wave incidence (to avoid the additional average over all wave polarizations for an incoming *T*-wave, we assume that the normal to the inclusions lie in the plane of incidence of the incoming *TV*- or *TH*-wave). Hence, in the case of parallel inclusions the expressions for the

Z0 Z0 F F ,0, g( )d , Z L,TV,TH,

<sup>1</sup> F F ,0, g( )sin d d , Z L,TV,TH. <sup>2</sup>

Here 0 is the angle characterizing the direction of the wave incidence, the parameters and characterize the minimal and maximal masses, respectively, in the system of distributed inclusions with the density function g of inclusion mass, and Z 0 F ( ,0, ) are the forward scattering amplitudes given by Eq. (19). The density distribution function of

g( )d 1

A suitable set of inclusion mass variations, which corresponds to aligned, normal and

**Table 1.** Various types of density distribution functions of inclusion mass ( is the Dirac delta

take the mutual dynamic interactions between individual inclusions into account.

waves can be obtained by considering the definition (22). This results in

The approximation for the complex wave number (24) can be considered as a special case of the multiple wave scattering models of higher orders [4,5], and it involves only the first order in the inclusion density and is thus only valid for a dilute or small inclusion density. In the case of a large density or high concentration of inclusions, more sophisticated models such as the self-consistent approach or the multiple scattering models should be applied, to

Once the complex effective wave numbers K have been determined via Eq. (24), the Z effective wave velocities Z c and the attenuation coefficients Z of the plane L - and T -

(25)

(26)

(27)

average forward scattering amplitudes of corresponding wave mode are

\*

and in the case of randomly oriented inclusion they become the form

Z Z 0 0 0 0

\*

inclusion mass should satisfy the normalization condition

Distribution type g( )

Aligned 0 ( ) Normal <sup>233</sup> 3( ) Uniform 1( )

uniform distributions, is defined in Table 1.

function).

The method presented in the previous sections is used to calculate the effective dynamic parameters of a composite elastic solid with both parallel and randomly oriented rigid diskshaped inclusions of variable mass for the propagation of time-harmonic plane *L*- or *TV*waves. For numerical discretization of the inclusion surface, the domain S is divided into 264 rectangular elements of length 1 y 22 and 2 y 12 in the 1 y - and 2 y -directions, the domain <sup>0</sup> S**<sup>y</sup>** is chosen as <sup>0</sup> S S\ S **y y** , where S **<sup>y</sup>** is the boundary element with the point **y** in the center of the element. Poisson's ratio is selected as 0.3, the radii of inertia of the circular inclusion are defined as 12 3 i i a 2,i a 2 . In the numerical examples, the radius a of the inclusions is assumed to be equal, while their masses are varied by the distribution laws defined in Table 1 from the minimal value 5 to the maximal value 20 . The inclusion density parameter is fixed as 0.01 in accordance to the assumption of low concentration of inclusions in a matrix.

For comparison purpose, normalized effective wave velocities and normalized attenuation coefficients are introduced as Z ZZ c cc , Z Z 2a , where the subscript Z L,TV stands for *L*- and *TV*-waves, respectively.

For parallel or aligned disk-shaped inclusions, the macroscopic dynamic behavior of the composite materials is transversely isotropic. Thus, the effective wave velocities and the attenuation coefficient are dependent on the direction of the wave incidence. In this analysis, only two wave incidence directions are considered, namely normal incidence <sup>o</sup> <sup>0</sup> ( 0) of *L*and *T*-waves and grazing incidence <sup>o</sup> <sup>0</sup> ( 90 ) of *L*- wave. This choice provides the vanishing of the dynamic torque on the inclusions and, therefore, their zero-rotations.

For normal incidence of a plane *L*-wave, the normalized attenuation coefficient L and the normalized effective wave velocity Lc versus the dimensionless wave number are presented in Figure 3. Three separate parametrical studies are performed to show the effects of different inclusion mass distributions: aligned with 0 20 , normal and uniform. To check the accuracy of the implemented BEM, the present numerical results are compared with the analytical low-frequency solutions given in [9]. Here and hereafter a very good agreement between both results is observed in the frequency range 0 0.3 .

Numerical Simulation of Wave Propagation

<sup>0</sup> 0 . The normalized

<sup>0</sup> 90 ) of plane *L*-wave

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 31

<sup>0</sup> 90 is smaller than that in the case with <sup>o</sup>

effective wave velocity Lc in Figure 4(b) for grazing incidence of a plane *L*-wave is also very similar to Figure 3(b). Compared to the normalized effective wave velocity Lc for a normal incidence of plane *L*-wave, the maximum effective wave velocity Lc is increased while its minimum value is decreased. Also in contrast to Figure 3(b), the normalized effective wave velocity Lc for all considered functions g is larger than that of the matrix material at high frequencies. In general, the quite tangled effects of the inclusions' stiffness and mass on the

 **Figure 4.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for parallel

For normal incidence of a plane *T*-wave (then *TV*- and *TH*-wave are the same), the numerical results for the normalized attenuation coefficient T and the normalized effective wave velocity Tc are presented in Figure 5 versus the dimensionless wave number . The global behavior of the normalized attenuation coefficient T and the normalized effective

as presented in Figure 4. In comparison to the peak values of L for normal and grazing incidence of a plane *L*-wave, the peak values of T for an normal incidence of a plane *T*wave are increased, what follows from Figures 3(a), 4(a) and 5(a). Comparison of Figures 3(b), 4(b) and 5(b) shows, that the minimum values of the normalized effective wave velocity Tc are reduced compared to Lc for normal and grazing incidence of a plane *L*wave for the same inclusion mass distribution, while the maximum values of Tc lie between

Next numerical examples concern the randomly oriented micro-inclusions, when the macroscopic dynamic behavior of the composite material is isotropic. It means that the effective wave velocity and the attenuation coefficient do not depend on the direction of the wave incidence. Both the translations and the rotations of the inclusions are exhibited in this

wave velocity Tc is very similar to that for grazing incidence (i.e. <sup>o</sup>

the Lc -values for normal and grazing incidence of a plane *L*-wave.

average phase velocity are observed at different frequencies.

in the case with <sup>o</sup>

inclusions and grazing *L*-wave incidence.

case.

**Figure 3.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for parallel inclusions and normal *L*-wave incidence.

In the low frequency range, the normalized attenuation coefficient L increases rapidly with increasing , after reaching a maximum it then decreases and approaches its high frequency limit (see Figure 3(a)). The peak value of L increases and is shifted to a smaller value of the dimensionless frequency with changing of inclusion mass distribution from uniform to normal and then to aligned. The normalized attenuation coefficient L in the high-frequency or shortwave limit does not depend on the frequency and the inclusion mass. Also, the inclusions are unmovable in the high-frequency limit and the normalized attenuation coefficient L can be obtained by using the Kirchhoff approximation for short waves [19]. At low frequencies, the normalized effective wave velocity Lc in the composite is smaller than that in the homogeneous matrix material (see Figure 3(b)). Then the normal inclusion mass distribution is characterized by the bigger (smaller) value of Lc in comparison with the aligned (uniform) situation. An opposite tendency is observed in the range of higher frequencies. In addition, the normalized effective wave velocity Lc in the composite can be bigger than that in the homogeneous matrix material, for instance for the considered inclusions of aligned mass and 1 . The high-frequency limit Lc 1 at means that the velocity of the L -wave in the short-wave limit coincides with that in the matrix. The explanation of this high-frequency limit follows from the geometrical optical interpretation of the wave field. The wave field at high frequencies may be considered as a set of independent beams propagating through the medium. Because of the existing continuous matrix material, the effective wave velocity should coincide with the wave velocity in the matrix in the high-frequency limit.

The corresponding numerical results for grazing incidence of a plane *L*-wave are presented in Figure 4. As followed from Figure 4(a), the normalized attenuation coefficient L shows a similar dependence on the dimensionless wave number and the inclusion mass distribution. For the same density function g of inclusion mass, the peaks of L are larger than that for normal incidence as depicted in Figure 3(a), but the high-frequency limit of <sup>L</sup>

in the case with <sup>o</sup> <sup>0</sup> 90 is smaller than that in the case with <sup>o</sup> <sup>0</sup> 0 . The normalized effective wave velocity Lc in Figure 4(b) for grazing incidence of a plane *L*-wave is also very similar to Figure 3(b). Compared to the normalized effective wave velocity Lc for a normal incidence of plane *L*-wave, the maximum effective wave velocity Lc is increased while its minimum value is decreased. Also in contrast to Figure 3(b), the normalized effective wave velocity Lc for all considered functions g is larger than that of the matrix material at high frequencies. In general, the quite tangled effects of the inclusions' stiffness and mass on the average phase velocity are observed at different frequencies.

30 Composites and Their Applications

inclusions and normal *L*-wave incidence.

 **Figure 3.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for parallel

In the low frequency range, the normalized attenuation coefficient L increases rapidly with increasing , after reaching a maximum it then decreases and approaches its high frequency limit (see Figure 3(a)). The peak value of L increases and is shifted to a smaller value of the dimensionless frequency with changing of inclusion mass distribution from uniform to normal and then to aligned. The normalized attenuation coefficient L in the high-frequency or shortwave limit does not depend on the frequency and the inclusion mass. Also, the inclusions are unmovable in the high-frequency limit and the normalized attenuation coefficient L can be obtained by using the Kirchhoff approximation for short waves [19]. At low frequencies, the normalized effective wave velocity Lc in the composite is smaller than that in the homogeneous matrix material (see Figure 3(b)). Then the normal inclusion mass distribution is characterized by the bigger (smaller) value of Lc in comparison with the aligned (uniform) situation. An opposite tendency is observed in the range of higher frequencies. In addition, the normalized effective wave velocity Lc in the composite can be bigger than that in the homogeneous matrix material, for instance for the considered inclusions of aligned mass and 1 . The high-frequency limit Lc 1 at means that the velocity of the L -wave in the short-wave limit coincides with that in the matrix. The explanation of this high-frequency limit follows from the geometrical optical interpretation of the wave field. The wave field at high frequencies may be considered as a set of independent beams propagating through the medium. Because of the existing continuous matrix material, the effective wave velocity

should coincide with the wave velocity in the matrix in the high-frequency limit.

The corresponding numerical results for grazing incidence of a plane *L*-wave are presented in Figure 4. As followed from Figure 4(a), the normalized attenuation coefficient L shows a similar dependence on the dimensionless wave number and the inclusion mass distribution. For the same density function g of inclusion mass, the peaks of L are larger than that for normal incidence as depicted in Figure 3(a), but the high-frequency limit of <sup>L</sup>

**Figure 4.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for parallel inclusions and grazing *L*-wave incidence.

For normal incidence of a plane *T*-wave (then *TV*- and *TH*-wave are the same), the numerical results for the normalized attenuation coefficient T and the normalized effective wave velocity Tc are presented in Figure 5 versus the dimensionless wave number . The global behavior of the normalized attenuation coefficient T and the normalized effective wave velocity Tc is very similar to that for grazing incidence (i.e. <sup>o</sup> <sup>0</sup> 90 ) of plane *L*-wave as presented in Figure 4. In comparison to the peak values of L for normal and grazing incidence of a plane *L*-wave, the peak values of T for an normal incidence of a plane *T*wave are increased, what follows from Figures 3(a), 4(a) and 5(a). Comparison of Figures 3(b), 4(b) and 5(b) shows, that the minimum values of the normalized effective wave velocity Tc are reduced compared to Lc for normal and grazing incidence of a plane *L*wave for the same inclusion mass distribution, while the maximum values of Tc lie between the Lc -values for normal and grazing incidence of a plane *L*-wave.

Next numerical examples concern the randomly oriented micro-inclusions, when the macroscopic dynamic behavior of the composite material is isotropic. It means that the effective wave velocity and the attenuation coefficient do not depend on the direction of the wave incidence. Both the translations and the rotations of the inclusions are exhibited in this case.

Numerical Simulation of Wave Propagation

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 33

<sup>0</sup> 0 , while the normalized effective wave velocity Tc could

Figure 7 demonstrates the corresponding results for the normalized attenuation coefficient T and the normalized effective wave velocity Tc for an incident plane *TV*-wave. In contrast to parallel inclusions (see Figure 5), now the variations of T and Tc with the dimensionless wave number are rather complicated at low frequencies. For instance, the normalized attenuation coefficient T for aligned distribution of inclusion mass in Figure 7(a) shows two distinct peaks, which are not observed in the case of parallel inclusions. The normalized attenuation coefficient T for randomly oriented inclusions is smaller than that

be larger or smaller than that for parallel inclusions depending on the dimensionless wave

 **Figure 7.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for randomly

Description of macroscopic dynamic response of a composite to elastic wave propagation by Eqs. (23) and (28) allows us the extension of analysis on the near-field quantities connected with each inclusion. Special attention should be paid to the dynamic stress intensity factors as the most important fracture parameters. Taking in mind the assumptions of neglecting the inclusions interaction, the relations (21) are applied for the estimation of the mode-I, II, and III dynamic stress intensity factors K , K and I II K in the vicinity of separate inclusion. III We suppose the impinge on the inclusion of plane longitudinal *L*-wave with constant amplitude U , which should be corrected for each inclusion in accordance to its localization 0 in a composite with the considering of attenuation and dispersion law (23). The value K U G 4(1 ) a \* 0 is chosen as the normalizing factor for the amplitudes of dynamic stress intensity factors, so that K KK I,II,III I,II,III \* . All material and discretization parameters are the same as it is fixed in the previous Section. Different inclusion masses are involved

At normal *L-*wave incidence on the inclusion (antisymmetric problem) K K 0, I III and the mode-II dynamic stress intensity factor K does not vary along the inclusion contour (see II

**5. Numerical analysis of local dynamic parameters of a composite** 

number and the density function g of inclusion mass (see Figure 7(b)).

for parallel inclusions with <sup>o</sup>

oriented inclusions and *TV*-wave incidence.

into analysis by the changing of parameter 0 .

**Figure 5.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for parallel inclusions and normal *T*-wave incidence.

For an incident plane *L*-wave, the normalized attenuation coefficient L and the normalized effective wave velocity Lc are presented in Figure 6 versus the dimensionless wave number . A comparison of Figure 6 with Figures 3 and 4 for parallel inclusions shows a similar dependence of the L and Lc on the dimensionless wave number and the inclusion mass distribution. As expected, the peak values of L and Lc for randomly oriented pennyshaped inclusions are larger than that for parallel inclusions with <sup>o</sup> <sup>0</sup> 0 but smaller than that for parallel inclusions with <sup>o</sup> <sup>0</sup> 90 .

**Figure 6.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for randomly oriented inclusions and *L*-wave incidence.

Figure 7 demonstrates the corresponding results for the normalized attenuation coefficient T and the normalized effective wave velocity Tc for an incident plane *TV*-wave. In contrast to parallel inclusions (see Figure 5), now the variations of T and Tc with the dimensionless wave number are rather complicated at low frequencies. For instance, the normalized attenuation coefficient T for aligned distribution of inclusion mass in Figure 7(a) shows two distinct peaks, which are not observed in the case of parallel inclusions. The normalized attenuation coefficient T for randomly oriented inclusions is smaller than that for parallel inclusions with <sup>o</sup> <sup>0</sup> 0 , while the normalized effective wave velocity Tc could be larger or smaller than that for parallel inclusions depending on the dimensionless wave number and the density function g of inclusion mass (see Figure 7(b)).

32 Composites and Their Applications

inclusions and normal *T*-wave incidence.

that for parallel inclusions with <sup>o</sup>

oriented inclusions and *L*-wave incidence.

 **Figure 5.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for parallel

For an incident plane *L*-wave, the normalized attenuation coefficient L and the normalized effective wave velocity Lc are presented in Figure 6 versus the dimensionless wave number . A comparison of Figure 6 with Figures 3 and 4 for parallel inclusions shows a similar dependence of the L and Lc on the dimensionless wave number and the inclusion mass distribution. As expected, the peak values of L and Lc for randomly oriented penny-

 **Figure 6.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for randomly

<sup>0</sup> 0 but smaller than

shaped inclusions are larger than that for parallel inclusions with <sup>o</sup>

<sup>0</sup> 90 .

**Figure 7.** Effects of the inclusion mass distribution on the normalized attenuation coefficient (a) and the normalized effective wave velocity (b) as the functions of dimensionless wave number for randomly oriented inclusions and *TV*-wave incidence.

## **5. Numerical analysis of local dynamic parameters of a composite**

Description of macroscopic dynamic response of a composite to elastic wave propagation by Eqs. (23) and (28) allows us the extension of analysis on the near-field quantities connected with each inclusion. Special attention should be paid to the dynamic stress intensity factors as the most important fracture parameters. Taking in mind the assumptions of neglecting the inclusions interaction, the relations (21) are applied for the estimation of the mode-I, II, and III dynamic stress intensity factors K , K and I II K in the vicinity of separate inclusion. III We suppose the impinge on the inclusion of plane longitudinal *L*-wave with constant amplitude U , which should be corrected for each inclusion in accordance to its localization 0 in a composite with the considering of attenuation and dispersion law (23). The value K U G 4(1 ) a \* 0 is chosen as the normalizing factor for the amplitudes of dynamic stress intensity factors, so that K KK I,II,III I,II,III \* . All material and discretization parameters are the same as it is fixed in the previous Section. Different inclusion masses are involved into analysis by the changing of parameter 0 .

At normal *L-*wave incidence on the inclusion (antisymmetric problem) K K 0, I III and the mode-II dynamic stress intensity factor K does not vary along the inclusion contour (see II Figure 8). It follows from the Figure 8 that in the initial range of wave numbers K -factor II rapidly increases from a zero value, what is more pronounced for the inclusions of large mass. A further increase in in the case of an inclusion with the mass characteristic 0 20 leads to a local maximum of K . For higher wave numbers, a II regularity is the approaching of K -factors for the inclusions of different mass, in addition, this approaching is from II above as the mass of inclusion increases. Subsequently, a linear relationship between K II and is reached in accordance to the increasing order of stresses in an incident wave.

Numerical Simulation of Wave Propagation

in 3D Elastic Composites with Rigid Disk-Shaped Inclusions of Variable Mass 35

similar frequency behaviour (with a difference in the quantitative values) as in the previous antisymmetric problem is observed here for K -factor at the point, where the wave runs on I the inclusion. It should be mentioned that K -factor at this point ex <sup>I</sup> ceed one at the point, where the wave runs down from the inclusion. In the range of high wave numbers K - <sup>I</sup> factors for the inclusions of different mass approach also, but now from below as the mass of

Attenuation and dispersion of time-harmonic elastic waves, as well as dynamic stress concentration in 3D composite materials consisting of a linear elastic matrix and rigid diskshaped inclusions of variable mass is simulated numerically. Translations and rotations of the inclusions in the matrix are taken into account in the analysis. Wave scattering by a single disk-shaped inclusion is investigated by a boundary element method to obtain the stress jumps across the inclusion surfaces. Then, far-field scattering amplitudes of elastic waves are computed by using the stress jumps. To describe the average macroscopic dynamic properties of the composite materials with a random distribution of disk-shaped micro-inclusions, complex wave numbers are computed by the Foldy-type dispersion relations, from which the effective wave velocities and the wave attenuation can be obtained. The present analysis concerns a dilute distribution of micro-inclusions, when the mutual inclusion interactions and the multiple scattering effects are approximately neglected. Numerical examples involve:

both longitudinal and transversal waves propagation in a composite material;

frequency-domain analysis of global dynamic parameters, such as the wave attenuation

frequency-domain analysis of local dynamic parameters, such as the dynamic stress

As shown, particular dynamic properties of composite materials can be varied by controlled

*Pidstryhach Institute for Applied Problems of Mechanics and Mathematics NASU, Lviv, Ukraine* 

This work is sponsored by the State Foundation for Fundamental Researches of Ukraine

[1] Foldy LL (1945) Multiple Scattering Theory of Waves. *Physical Review* 67: 107-119.

 parallel and randomly oriented rigid disk-shaped inclusions; aligned, normal and uniform distributions of inclusion mass;

coefficients and effective wave velocities;

intensity factors in the inclusion vicinities.

*Department of Computational Mechanics of Deformable Systems,* 

(Project No. 40.1/018), which is gratefully acknowledged.

changes in the microstructure.

**Author details** 

Viktor Mykhas'kiv

**7. References** 

**Acknowledgement** 

inclusion increases.

**6. Conclusion** 

**Figure 8.** Effects of the inclusion mass on the normalized amplitude of the mode-II dynamic stress intensity factor as the function of dimensionless wave number for normal *L*-wave incidence on an inclusion.

**Figure 9.** Effects of the inclusion mass on the normalized amplitude of the mode-I dynamic stress intensity factor as the function of dimensionless wave number for grazing *L*-wave incidence on an inclusion: solid lines correspond to the contour point where the wave runs on the inclusion; marked lines correspond to the contour point where the wave runs down from the inclusion.

At grazing *L-*wave incidence on the inclusion (symmetric problem) K 0, II and the mode-I and mode-III dynamic stress intensity factor K and I K depend on the angular coordinate III of the inclusion contour point. Hence, in Figure 9 K -factor at the most representative I points of the inclusion contour is showed, where K 0 III due to the symmetry conditions. A similar frequency behaviour (with a difference in the quantitative values) as in the previous antisymmetric problem is observed here for K -factor at the point, where the wave runs on I the inclusion. It should be mentioned that K -factor at this point ex <sup>I</sup> ceed one at the point, where the wave runs down from the inclusion. In the range of high wave numbers K - <sup>I</sup> factors for the inclusions of different mass approach also, but now from below as the mass of inclusion increases.

## **6. Conclusion**

34 Composites and Their Applications

Figure 8). It follows from the Figure 8 that in the initial range of wave numbers K -factor II rapidly increases from a zero value, what is more pronounced for the inclusions of large mass. A further increase in in the case of an inclusion with the mass characteristic 0 20 leads to a local maximum of K . For higher wave numbers, a II regularity is the approaching of K -factors for the inclusions of different mass, in addition, this approaching is from II above as the mass of inclusion increases. Subsequently, a linear relationship between K II and is reached in accordance to the increasing order of stresses in an incident wave.

**Figure 8.** Effects of the inclusion mass on the normalized amplitude of the mode-II dynamic stress intensity

factor as the function of dimensionless wave number for normal *L*-wave incidence on an inclusion.

**Figure 9.** Effects of the inclusion mass on the normalized amplitude of the mode-I dynamic stress intensity factor as the function of dimensionless wave number for grazing *L*-wave incidence on an inclusion: solid lines correspond to the contour point where the wave runs on the inclusion; marked

At grazing *L-*wave incidence on the inclusion (symmetric problem) K 0, II and the mode-I and mode-III dynamic stress intensity factor K and I K depend on the angular coordinate III of the inclusion contour point. Hence, in Figure 9 K -factor at the most representative I points of the inclusion contour is showed, where K 0 III due to the symmetry conditions. A

lines correspond to the contour point where the wave runs down from the inclusion.

Attenuation and dispersion of time-harmonic elastic waves, as well as dynamic stress concentration in 3D composite materials consisting of a linear elastic matrix and rigid diskshaped inclusions of variable mass is simulated numerically. Translations and rotations of the inclusions in the matrix are taken into account in the analysis. Wave scattering by a single disk-shaped inclusion is investigated by a boundary element method to obtain the stress jumps across the inclusion surfaces. Then, far-field scattering amplitudes of elastic waves are computed by using the stress jumps. To describe the average macroscopic dynamic properties of the composite materials with a random distribution of disk-shaped micro-inclusions, complex wave numbers are computed by the Foldy-type dispersion relations, from which the effective wave velocities and the wave attenuation can be obtained. The present analysis concerns a dilute distribution of micro-inclusions, when the mutual inclusion interactions and the multiple scattering effects are approximately neglected. Numerical examples involve:


As shown, particular dynamic properties of composite materials can be varied by controlled changes in the microstructure.

## **Author details**

Viktor Mykhas'kiv *Department of Computational Mechanics of Deformable Systems, Pidstryhach Institute for Applied Problems of Mechanics and Mathematics NASU, Lviv, Ukraine* 

## **Acknowledgement**

This work is sponsored by the State Foundation for Fundamental Researches of Ukraine (Project No. 40.1/018), which is gratefully acknowledged.

## **7. References**

[1] Foldy LL (1945) Multiple Scattering Theory of Waves. *Physical Review* 67: 107-119.

	- [2] Lax M (1952) Multiple Scattering of Waves. *Physical Review* 85: 621-629.
	- [3] Gubernatis JE, Domany E (1984) Effects of Microstructure on the Speed and Attenuation of Elastic Waves in Porous Materials. *Wave Motion* 6: 579-589.

**Chapter 3** 

© 2012 Yuan et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Yuan et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Structural Health Monitoring** 

Jian Cai, Lei Qiu, Shenfang Yuan, Lihua Shi, PeiPei Liu and Dong Liang

A composite material can be defined as a combination of two or more distinct materials at a macroscopic level to attain new properties that can't be achieved by those of individual components acting alone. Different from metallic alloys, each material keeps its own chemical, physical and mechanical properties [1]. Composite materials have reinforcing and matrix phases. The reinforcing phase with higher strength and stiffness is usually fibers, flakes or particles while the matrix phase can be polymers, ceramics or metals. Composite materials are commonly classified into four types, i.e., fibrous composite materials,

Compared with traditional metallic materials, the main advantages of composites are: a) low destiny and high specific strength and stiffness, which are help for weight savings; b) good vibration damping ability, long fatigue life and high wear, creep, corrosion and temperature resistances; b) strong tailor ability in both microstructures and properties make them easily designed to satisfy different application needs; c) since detail accessories can be combined into a single cured assembly, the number of required fasteners and the amount of assembly

The above advantages make composite materials wildly used in various fields. In aeronautic structures, composite materials are increasingly utilized to decrease weight for payload and radius purposes. The percentages by weight of composites in USA fighters rise from 2% in F-15E to 35.2% in F-35/CV. The overall structure of Eurofighter Typhoon is composed of 40% carbon-fiber composite materials. For commercial aircrafts, the usage percentages of fiber-reinforced composite materials in latest Boeing B787 and newly-designed Airbus A350- XWB reach 50% and 52%, respectively. To meet the performance and fuel efficiency

laminated composite materials, particulate composite materials and the others [2].

**for Composite Materials** 

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48215

**1.1. Composite materials** 

labor can be significantly reduced [1].

**1. Introduction** 


## **Structural Health Monitoring for Composite Materials**

Jian Cai, Lei Qiu, Shenfang Yuan, Lihua Shi, PeiPei Liu and Dong Liang

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48215

## **1. Introduction**

36 Composites and Their Applications

*Motion* 47: 183-197.

[2] Lax M (1952) Multiple Scattering of Waves. *Physical Review* 85: 621-629.

of Elastic Waves in Porous Materials. *Wave Motion* 6: 579-589.

Cambridge: Cambridge University Press 437 p.

*International Journal of Engineering Science* 30: 169-186.

*Mechanics and Physics of Solids* 41: 1573-1588.

Cracked Solid. *Wave Motion* 22: 297-310.

*Random and Complex Media* 20: 491-510.

*Solids and Structures* 46: 602-616.

Publishing Company 425 p.

[3] Gubernatis JE, Domany E (1984) Effects of Microstructure on the Speed and Attenuation

[4] Martin PA (2006) *Multiple Scattering: Interaction of Time-harmonic Waves with N Obstacles.*

[5] Conoir JM, Norris AN (2010) Effective Wavenumbers and Reflection Coefficients for an Elastic Medium Containing Random Configurations of Cylindrical Scatterers. *Wave* 

[6] Kerr FH (1992) The Scattering of a Plane Elastic Wave by Spherical Elastic Inclusions.

[7] Kanaun SK, Levin VM (2007) Propagation of Longitudinal Elastic waves in Composites with a Random Set of Spherical Inclusions. *Archive of Applied Mechanics* 77: 627-651. [8] Sabina FJ, Smyshlaev VP, Willis JR (1993) Self Consistent Analysis of Waves in a Matrix-Inclusion Composite I. Randomly Oriented Spheroidal Inclusions. *Journal of the* 

[9] Kanaun SK, Levin VM (2008) *Self-Consistent Methods for Composites. Volume 2 ― Wave* 

[10] LevinV, Markov M, Kanaun S (2008) Propagation of Long Elastic Waves in Porous Rocks with Crack-Like Inclusions. *International Journal of Engineering Science* 46: 620-638. [11] Eriksson AS, Boström A, Datta SK (1995) Ultrasonic Wave Propagation through a

[12] Zhang Ch, Gross D (1998) *On Wave Propagation in Elastic Solids with Cracks.*

[13] Sato H, Shindo Y (2002) Influence of Microstructure on Scattering of Plane Elastic Waves by a Distribution of Partially Debonded Elliptical Inclusions. *Mechanics of Materials* 34: 401-409. [14] Maurel A, Mercier JF, Lund F (2004) Elastic Wave Propagation through a Random

[15] Mykhas'kiv VV, Khay OM, Zhang Ch, Boström A (2010) Effective Dynamic Properties of 3D Composite Materials Containing Rigid Penny-Shaped Inclusions. *Waves in* 

[16] Kit HS, Mykhas'skiv VV, Khay OM (2002) Analysis of the Steady Oscillations of a Plane Absolutely Rigid Inclusion in a Three-Dimensional Elastic Body by the Boundary

[17] Mykhas'kiv VV (2005) Transient Response of a Plane Rigid Inclusion to an Incident

[18] Mykhas'kiv VV, Khay OM (2009) Interaction between Rigid-Disc Inclusion and Penny-Shaped Crack under Elastic Time-Harmonic Wave Incidence. *International Journal of* 

[19] Achenbach JD (1973) *Wave Propagation in Elastic Solids.* Amsterdam: North-Holland

[20] Khaj MV (1993) *Two-Dimensional Integral Equations of Newton Potential Type and Their* 

[21] Kassir MK, Sih GC (1968) Some Three-Dimensional Inclusion Problems in Elasticity.

Element Method. *Journal of Applied Mathematics and Mechanics* 66: 817-824.

*Propagation in Heterogeneous Materials.* Heidelberg: Springer 294 p.

Southampton: Computational Mechanics Publications 248 p.

Array of Dislocations. *Physical Review B* 70: 024303 (1-15).

Wave in an Elastic Solid. *Wave Motion* 41: 133-144.

*Applications.* Kyiv: Naukova Dumka 253 p. (in Russian).

*International Journal of Solids and Structures* 4: 225-241.

## **1.1. Composite materials**

A composite material can be defined as a combination of two or more distinct materials at a macroscopic level to attain new properties that can't be achieved by those of individual components acting alone. Different from metallic alloys, each material keeps its own chemical, physical and mechanical properties [1]. Composite materials have reinforcing and matrix phases. The reinforcing phase with higher strength and stiffness is usually fibers, flakes or particles while the matrix phase can be polymers, ceramics or metals. Composite materials are commonly classified into four types, i.e., fibrous composite materials, laminated composite materials, particulate composite materials and the others [2].

Compared with traditional metallic materials, the main advantages of composites are: a) low destiny and high specific strength and stiffness, which are help for weight savings; b) good vibration damping ability, long fatigue life and high wear, creep, corrosion and temperature resistances; b) strong tailor ability in both microstructures and properties make them easily designed to satisfy different application needs; c) since detail accessories can be combined into a single cured assembly, the number of required fasteners and the amount of assembly labor can be significantly reduced [1].

The above advantages make composite materials wildly used in various fields. In aeronautic structures, composite materials are increasingly utilized to decrease weight for payload and radius purposes. The percentages by weight of composites in USA fighters rise from 2% in F-15E to 35.2% in F-35/CV. The overall structure of Eurofighter Typhoon is composed of 40% carbon-fiber composite materials. For commercial aircrafts, the usage percentages of fiber-reinforced composite materials in latest Boeing B787 and newly-designed Airbus A350- XWB reach 50% and 52%, respectively. To meet the performance and fuel efficiency

© 2012 Yuan et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Yuan et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

requirements, the consumption of composites in automobile industry is growing. The blades of wind turbines are normally made of composites to improve electrical energy harvest efficiency [1]. In ships or infrastructures, the composite materials with high corrosion resistance have received wild acceptance. The brake and engine parts working in high temperature are often fabricated from metal or ceramic matrix composites. In addition, the sports and recreation market is also one of the primary consumers of composites [3].

Structural Health Monitoring for Composite Materials 39

structural 'health' condition [8]. Different from NDT, the real-time and on-line damage detection via in-situ sensors can be achieved in SHM. Now, the needs of SHM for composites have been continuously increased. The potential benefits of SHM include improving reliability and safety, reducing lifecycle costs and helping design of composite

In the following, after the sensors commonly applied in SHM are presented, some typical SHM methods for composites are reviewed. Hereafter arranged are the two SHM examples

In SHM, various sensors are integrated with target structures to obtain different structural information, such as temperature, stress, strain, vibration and so on. The familiar SHM sensors are resistance strain gages, fibre optic sensors, piezoelectric sensors, eddy current

Resistance strain gage is a traditional strain sensor element. The gage mainly consists of a resistance grid of thin wire or foil, connector and encapsulation layer, as shown in figure 1(a). With the strain-resistance effect, the grid senses the structure's strain as its resistance value, which can be finally converted to the voltage signal with Wheatstone bridge circuit (seen in figure 1(b)). Resistance strain gage, of very small thickness but high sensitivity, can be easily bonded onto the structures and applicable in high temperature or pressure

> Resistance grid

Connector

on composite structures. Summary and conclusions are given at last.

sensors, and microelectromechanical systems (MEMS) sensors.

**Figure 1.** (a) Configuration of a resistance strain gage (b) Wheatstone bridge circuit

Fibre optic sensors (FOSs) are competitive candidates for SHM applications because of their unique advantages of light weight, high stability and reliability, long life cycle, low power utilization, EMI immunity, high bandwidth, compatibility with optical date transmission and processing, etc. According to the sensing range, FOSs can be categorized into local,

(a) (b)

materials.

conditions.

**2. Sensors of SHM** 

**2.1. Resistance strain gages** 

**2.2. Fibre optic sensors** 

Encapsulation layer

## **1.2. Problems of composite materials**

Despite having the great advantages and applicability, composite materials are not exempt from some problems. As multiphase materials, composites exhibit distinct anisotropic properties. Their material capabilities, largely relating to manufacturing processes, are dispersive. Furthermore, the mechanisms of flaw initiating, spreading over the composite volume and leading to the ultimate failure are very complicated. So far, clearly description for the damage evolution and fracture behavior in composites remains a challenge work. Both the complex mechanical and damage characteristics can also make the optimization design for composites very difficult. Because of lacking enough data cumulation and available standards, the composite design efficiency usually depends on the designer's experiences and the final structures are easily prone to be over-designed [4].

Another important problem for composites is that they are susceptible to impact damages due to the lack of reinforcement in the out-of-plane direction. In a high energy impact, only small total penetration appears in composites. While in the low or medium energy impact, matrix crack will occur and interact, inducing delamination process. Fibre breakage would also happen on the opposite side to the impact [5]. Moreover, damages can be induced in composites by incorrect operations during manufacture and assembly, aging or service condition.

## **1.3. Requirements of SHM for composite materials**

The common damages in composites are fibre breakage, matrix cracking, fibre-matrix debonding and delamination between plies, most of which occur beneath the top surfaces and are barely visible. They can severely degrade the performance of composites and should be identified in time to avoid catastrophic structural failures.

The conventional non-destructive testing (NDT) methods, such as ultrasonic, X-ray, thermography and eddy current methods can be adopted for detecting damages in composites. However, these NDT methods, merely allowing the off-line testing in a local manner with complicated and heavy equipments, are labor-extensive and time-consuming especially for large-scale structures. Meanwhile, disassembling the tested structures may be required to ensure the inspection area accessible, which can increase the maintenance costs [6-7].

Structural health monitoring (SHM), an emerging technique developed from NDT, combines advanced sensor technology with intelligent algorithms to interrogate the structural 'health' condition [8]. Different from NDT, the real-time and on-line damage detection via in-situ sensors can be achieved in SHM. Now, the needs of SHM for composites have been continuously increased. The potential benefits of SHM include improving reliability and safety, reducing lifecycle costs and helping design of composite materials.

In the following, after the sensors commonly applied in SHM are presented, some typical SHM methods for composites are reviewed. Hereafter arranged are the two SHM examples on composite structures. Summary and conclusions are given at last.

## **2. Sensors of SHM**

38 Composites and Their Applications

condition.

[6-7].

**1.2. Problems of composite materials** 

requirements, the consumption of composites in automobile industry is growing. The blades of wind turbines are normally made of composites to improve electrical energy harvest efficiency [1]. In ships or infrastructures, the composite materials with high corrosion resistance have received wild acceptance. The brake and engine parts working in high temperature are often fabricated from metal or ceramic matrix composites. In addition, the

Despite having the great advantages and applicability, composite materials are not exempt from some problems. As multiphase materials, composites exhibit distinct anisotropic properties. Their material capabilities, largely relating to manufacturing processes, are dispersive. Furthermore, the mechanisms of flaw initiating, spreading over the composite volume and leading to the ultimate failure are very complicated. So far, clearly description for the damage evolution and fracture behavior in composites remains a challenge work. Both the complex mechanical and damage characteristics can also make the optimization design for composites very difficult. Because of lacking enough data cumulation and available standards, the composite design efficiency usually depends on the designer's

Another important problem for composites is that they are susceptible to impact damages due to the lack of reinforcement in the out-of-plane direction. In a high energy impact, only small total penetration appears in composites. While in the low or medium energy impact, matrix crack will occur and interact, inducing delamination process. Fibre breakage would also happen on the opposite side to the impact [5]. Moreover, damages can be induced in composites by incorrect operations during manufacture and assembly, aging or service

The common damages in composites are fibre breakage, matrix cracking, fibre-matrix debonding and delamination between plies, most of which occur beneath the top surfaces and are barely visible. They can severely degrade the performance of composites and should

The conventional non-destructive testing (NDT) methods, such as ultrasonic, X-ray, thermography and eddy current methods can be adopted for detecting damages in composites. However, these NDT methods, merely allowing the off-line testing in a local manner with complicated and heavy equipments, are labor-extensive and time-consuming especially for large-scale structures. Meanwhile, disassembling the tested structures may be required to ensure the inspection area accessible, which can increase the maintenance costs

Structural health monitoring (SHM), an emerging technique developed from NDT, combines advanced sensor technology with intelligent algorithms to interrogate the

sports and recreation market is also one of the primary consumers of composites [3].

experiences and the final structures are easily prone to be over-designed [4].

**1.3. Requirements of SHM for composite materials** 

be identified in time to avoid catastrophic structural failures.

In SHM, various sensors are integrated with target structures to obtain different structural information, such as temperature, stress, strain, vibration and so on. The familiar SHM sensors are resistance strain gages, fibre optic sensors, piezoelectric sensors, eddy current sensors, and microelectromechanical systems (MEMS) sensors.

## **2.1. Resistance strain gages**

Resistance strain gage is a traditional strain sensor element. The gage mainly consists of a resistance grid of thin wire or foil, connector and encapsulation layer, as shown in figure 1(a). With the strain-resistance effect, the grid senses the structure's strain as its resistance value, which can be finally converted to the voltage signal with Wheatstone bridge circuit (seen in figure 1(b)). Resistance strain gage, of very small thickness but high sensitivity, can be easily bonded onto the structures and applicable in high temperature or pressure conditions.

**Figure 1.** (a) Configuration of a resistance strain gage (b) Wheatstone bridge circuit

## **2.2. Fibre optic sensors**

Fibre optic sensors (FOSs) are competitive candidates for SHM applications because of their unique advantages of light weight, high stability and reliability, long life cycle, low power utilization, EMI immunity, high bandwidth, compatibility with optical date transmission and processing, etc. According to the sensing range, FOSs can be categorized into local,

quasi-distributed and distributed sensors [9]. The most-commonly used local FOSs are interferometric sensors, such as Mach-Zehnder, Michelson and Fabry-Perot FOSs. These sensors can measure strains and deformations at local sites by detecting the phase shifts of relative optical waves.

Structural Health Monitoring for Composite Materials 41

Lead zirconium titanate ceramics (PZT) wafers and polyvinylidene fluoride (PVDF) films are the two common piezoelectric elements, as shown in figure 3. PZT wafers with high piezoelectric constant possess both excellent sensitivities as sensors and strong driving abilities as actuators, whereas the wafers are quite brittle due to ceramic inherent nature. In contrast, PVDF films have the advantages of high flexibility, low mass and cost and high internal damping [13]. However, because of the poor inverse piezoelectric properties and large compliance, PVDF films are usually preferred to be sensors [14]. To overcome the disadvantage of high brittleness of PZT wafers, piezoelectric composites, such as

The concept of using eddy currents for damage detection stems from the electromagnetic induction [15], with which the eddy current can be induced to the tested conductive structure and sensed by the identical or different windings of eddy current sensors. The main application of eddy current sensors is crack or corrosion detection for metallic parts even through coatings or layers which may be non-conducting. This makes the sensors useful for such the composite structures as parent metal materials with composite doubler

Due to the winding configurations, the conventional eddy current sensors with obtrusive size are hard to be integrated. Fortunately, with the development of micro-fabrication technique, the windings can be adhered or directly printed to a conformable substrate. As shown in figure 4, eddy current foil sensors, even in an array style with multiple sensing elements of various shapes, have been produced [16-17]. The new sensors are so thin and flexible that they can be easily surface-mounted or embeded between layers, offering the

**Figure 4.** (a) 4-element rosette eddy current array and (b) 9-element linear eddy current array [18]

(a) (b)

piezoelectric rubbers and piezoelectric paints, have been developed.

**Figure 3.** PZT and PVDF of various sizes and shapes [14]

**2.4. Eddy current sensors** 

repairs and metal-matrix composites.

potential for on-line and continuous monitoring.

Fiber Bragg grating (FBG) sensors, with multiplexing capacity, are a kind of typical quasidistributed FOSs. FBG is formed by inducing a periodic modulation of the refractive index in the core of a single mode optical fiber [10]. When light within a fiber passes through a FBG, constructive interference between the forward and contra-propagating light waves happens and leads to the narrowband back-reflection of light with Bragg wavelength *<sup>B</sup>* . Any local changes along with FBG can be manifested as that of *<sup>B</sup>* and therefore, from the measurement of the transmitted or reflected spectrum, as shown in figure 2, it is possible to monitor any strain-resulting parameters from temperatures to stress waves [9-11]. The major advantage of the sensor is that an array of wavelength-multiplexed FBGs can be deployed in a single fiber for quasi-distributed measurement. To further increase the number of FBGs, both spatial-division multiplexing and time-division multiplexing can be implemented.

**Figure 2.** Fiber Bragg grating principle [11]

With all segments of an optical fiber acting as sensors, distributed FOSs can fulfill the real distributed measurement, which is very attractive for SHM of large structures. The sensors are based on the modulation of light intensity in a fiber. The optical time domain reflectometry (OTDR) and Brillouin scattering are the two main distributed sensor methodologies, in which Rayleigh and Fresnel scatterings and Doppler shift in light frequency are used for measuring, respectively [9].

#### **2.3. Piezoelectric sensors**

Piezoelectric sensors are frequently used for measuring low or high frequency vibrations, such as Lamb waves or acoustic emission. Compared with conventional acoustic probes, e.g., wedge or comb Lamb wave transducers, piezoelectric sensors are more desired for SHM in view of their weights, sizes and costs. The sensors, made of piezoelectric materials, operate on piezoelectric principles. With direct piezoelectric effect, the sensors in a stress field can generate charge response and vice verse, an external electric field applied to the sensors can result in an induced strain field through inverse piezoelectric effect. Consequently, piezoelectric sensors can be employed both as actuators and sensors [12].

Lead zirconium titanate ceramics (PZT) wafers and polyvinylidene fluoride (PVDF) films are the two common piezoelectric elements, as shown in figure 3. PZT wafers with high piezoelectric constant possess both excellent sensitivities as sensors and strong driving abilities as actuators, whereas the wafers are quite brittle due to ceramic inherent nature. In contrast, PVDF films have the advantages of high flexibility, low mass and cost and high internal damping [13]. However, because of the poor inverse piezoelectric properties and large compliance, PVDF films are usually preferred to be sensors [14]. To overcome the disadvantage of high brittleness of PZT wafers, piezoelectric composites, such as piezoelectric rubbers and piezoelectric paints, have been developed.

**Figure 3.** PZT and PVDF of various sizes and shapes [14]

## **2.4. Eddy current sensors**

40 Composites and Their Applications

relative optical waves.

**Figure 2.** Fiber Bragg grating principle [11]

**2.3. Piezoelectric sensors** 

frequency are used for measuring, respectively [9].

quasi-distributed and distributed sensors [9]. The most-commonly used local FOSs are interferometric sensors, such as Mach-Zehnder, Michelson and Fabry-Perot FOSs. These sensors can measure strains and deformations at local sites by detecting the phase shifts of

Fiber Bragg grating (FBG) sensors, with multiplexing capacity, are a kind of typical quasidistributed FOSs. FBG is formed by inducing a periodic modulation of the refractive index in the core of a single mode optical fiber [10]. When light within a fiber passes through a FBG, constructive interference between the forward and contra-propagating light waves happens and leads to the narrowband back-reflection of light with Bragg wavelength *<sup>B</sup>*

measurement of the transmitted or reflected spectrum, as shown in figure 2, it is possible to monitor any strain-resulting parameters from temperatures to stress waves [9-11]. The major advantage of the sensor is that an array of wavelength-multiplexed FBGs can be deployed in a single fiber for quasi-distributed measurement. To further increase the number of FBGs, both spatial-division multiplexing and time-division multiplexing can be implemented.

With all segments of an optical fiber acting as sensors, distributed FOSs can fulfill the real distributed measurement, which is very attractive for SHM of large structures. The sensors are based on the modulation of light intensity in a fiber. The optical time domain reflectometry (OTDR) and Brillouin scattering are the two main distributed sensor methodologies, in which Rayleigh and Fresnel scatterings and Doppler shift in light

Piezoelectric sensors are frequently used for measuring low or high frequency vibrations, such as Lamb waves or acoustic emission. Compared with conventional acoustic probes, e.g., wedge or comb Lamb wave transducers, piezoelectric sensors are more desired for SHM in view of their weights, sizes and costs. The sensors, made of piezoelectric materials, operate on piezoelectric principles. With direct piezoelectric effect, the sensors in a stress field can generate charge response and vice verse, an external electric field applied to the sensors can result in an induced strain field through inverse piezoelectric effect. Consequently, piezoelectric sensors can be employed both as actuators and sensors [12].

Any local changes along with FBG can be manifested as that of *<sup>B</sup>*

.

and therefore, from the

The concept of using eddy currents for damage detection stems from the electromagnetic induction [15], with which the eddy current can be induced to the tested conductive structure and sensed by the identical or different windings of eddy current sensors. The main application of eddy current sensors is crack or corrosion detection for metallic parts even through coatings or layers which may be non-conducting. This makes the sensors useful for such the composite structures as parent metal materials with composite doubler repairs and metal-matrix composites.

Due to the winding configurations, the conventional eddy current sensors with obtrusive size are hard to be integrated. Fortunately, with the development of micro-fabrication technique, the windings can be adhered or directly printed to a conformable substrate. As shown in figure 4, eddy current foil sensors, even in an array style with multiple sensing elements of various shapes, have been produced [16-17]. The new sensors are so thin and flexible that they can be easily surface-mounted or embeded between layers, offering the potential for on-line and continuous monitoring.

**Figure 4.** (a) 4-element rosette eddy current array and (b) 9-element linear eddy current array [18]

## **2.5. MEMS sensors**

With the aid of advanced integrated circuit (IC) fabrication processes, MEMS is developed by co-fabricating microsensors, actuators and control functions in one silicon slice. MEMS is an intelligent system which can sense the circumstances and do some reactions by the microcircuit control [18]. At present, many MEMS sensors, such as MEMS accelerometers and pressure sensors, can be purchased commercially. Due to the extremely small size and large-scale integration degree, the sensors have the remarkable characteristics of light weight, flexibility in design, low power consumption and noise level, short response time, high reliability and economy, etc. To avoid the lengthy cables, the wireless communication capability can be added to the sensors with transmitter chips equipped.

Structural Health Monitoring for Composite Materials 43

structural states in conjunction with signal and information processing technologies, mechanical modeling analysis or priori-knowledge. Passive SHM only 'listens' to the structures but does not interact with them, as figure 6(a) illustrates. While in active SHM, structures are firstly excited with actuators in prescribed manners and interrogated by analyzing the received structural responses. Though both actuators and sensors are

required, as shown in figure 6(b), active SHM can be carried out whenever necessary.

sensor Signal

sensor

(a) Passive SHM

sensor

(b) Active SHM

Monitored structure actuator

actuator

sensor

collection &process

Signal collection &process

For composite materials, the common active SHM methods include Lamb wave, Electromechanical (E/M) impedance and active vibration-based methods. Acoustic emission, strain-

Lamb waves, first theoretically predicted by Horace Lamb in 1917, are a kind of guided ultrasonic waves existing in thin-wall structures. Because of the ability of long-distance transmission and high sensitivity to both the surface and the internal defects, Lamb waves

Lamb waves are usually excited and received by PZT wafers. FOS and PVDF can be also used as the wave sensors. Due to the multi-mode and dispersion characteristics, the propagation of Lamb waves is very complicated. In practical applications, a windowed toneburst is usually selected to generate the fundamental symmetric (S0) and anti-symmetric (A0) modes with the excitation frequency below the cut-off frequency of A1 mode. To

**Figure 6.** Schemes of passive SHM and active SHM

Exciting signal

**3.1. Lamb wave method** 

based method and CVM are the typical passive approaches.

Monitored structure

sensor sensor

are widely used as a promising tool for active SHM.

Besides Comparative Vacuum Monitoring (CVM) sensors which will be later introduced together with CVM method, there are other sensors, such as laser scanners and microwave sensors, could be applied in SHM.

With the advances in SHM requirements for both monitoring area and damage quantity, a great number of same or different sensors are arranged to form large sensor arrays to the monitored structure [19], leading to the appearance of various sensor-array layers. Similar to the above eddy current arrays shown in figure 4, the layers are generally made by encapsulating sensor elements with thin and flexible dielectric films in desired configurations. The benefits from the layers are [20]: a) rapidly and consistently arranging a large number of sensors is allowed; b) connecting wires are avoid to reduce EMI; c) the layers can be surface-mounted on existing structures or embeded as extra layers in composites during manufacturing. Besides the PZT-array layer, known as SMART Layer [21], the PZT-FOS hybrid array layer [6] and HELP (Hybrid Electromagnetic Performing) layer [22] have also emerged, as shown in figure 5.

**Figure 5.** (a) PZT-FOS hybrid array layer [22] (b) HELP layer [23]

## **3. Typical SHM methods for composite materials**

As illustrated in figure 6, SHM can be performed in either passive or active ways depending on whether actuators are used [23]. In passive SHM, various operational parameters, such as loads, stress, acoustic emission and circumstance condition, mainly concerned to infer the structural states in conjunction with signal and information processing technologies, mechanical modeling analysis or priori-knowledge. Passive SHM only 'listens' to the structures but does not interact with them, as figure 6(a) illustrates. While in active SHM, structures are firstly excited with actuators in prescribed manners and interrogated by analyzing the received structural responses. Though both actuators and sensors are required, as shown in figure 6(b), active SHM can be carried out whenever necessary.

**Figure 6.** Schemes of passive SHM and active SHM

For composite materials, the common active SHM methods include Lamb wave, Electromechanical (E/M) impedance and active vibration-based methods. Acoustic emission, strainbased method and CVM are the typical passive approaches.

## **3.1. Lamb wave method**

42 Composites and Their Applications

sensors, could be applied in SHM.

layer [22] have also emerged, as shown in figure 5.

**Figure 5.** (a) PZT-FOS hybrid array layer [22] (b) HELP layer [23]

**3. Typical SHM methods for composite materials** 

With the aid of advanced integrated circuit (IC) fabrication processes, MEMS is developed by co-fabricating microsensors, actuators and control functions in one silicon slice. MEMS is an intelligent system which can sense the circumstances and do some reactions by the microcircuit control [18]. At present, many MEMS sensors, such as MEMS accelerometers and pressure sensors, can be purchased commercially. Due to the extremely small size and large-scale integration degree, the sensors have the remarkable characteristics of light weight, flexibility in design, low power consumption and noise level, short response time, high reliability and economy, etc. To avoid the lengthy cables, the wireless communication

Besides Comparative Vacuum Monitoring (CVM) sensors which will be later introduced together with CVM method, there are other sensors, such as laser scanners and microwave

With the advances in SHM requirements for both monitoring area and damage quantity, a great number of same or different sensors are arranged to form large sensor arrays to the monitored structure [19], leading to the appearance of various sensor-array layers. Similar to the above eddy current arrays shown in figure 4, the layers are generally made by encapsulating sensor elements with thin and flexible dielectric films in desired configurations. The benefits from the layers are [20]: a) rapidly and consistently arranging a large number of sensors is allowed; b) connecting wires are avoid to reduce EMI; c) the layers can be surface-mounted on existing structures or embeded as extra layers in composites during manufacturing. Besides the PZT-array layer, known as SMART Layer [21], the PZT-FOS hybrid array layer [6] and HELP (Hybrid Electromagnetic Performing)

As illustrated in figure 6, SHM can be performed in either passive or active ways depending on whether actuators are used [23]. In passive SHM, various operational parameters, such as loads, stress, acoustic emission and circumstance condition, mainly concerned to infer the

(a) (b)

capability can be added to the sensors with transmitter chips equipped.

**2.5. MEMS sensors** 

Lamb waves, first theoretically predicted by Horace Lamb in 1917, are a kind of guided ultrasonic waves existing in thin-wall structures. Because of the ability of long-distance transmission and high sensitivity to both the surface and the internal defects, Lamb waves are widely used as a promising tool for active SHM.

Lamb waves are usually excited and received by PZT wafers. FOS and PVDF can be also used as the wave sensors. Due to the multi-mode and dispersion characteristics, the propagation of Lamb waves is very complicated. In practical applications, a windowed toneburst is usually selected to generate the fundamental symmetric (S0) and anti-symmetric (A0) modes with the excitation frequency below the cut-off frequency of A1 mode. To

achieve single S0 or A0 mode generation, frequency tuning or double-side generation methods can be utilized [24-25].

Structural Health Monitoring for Composite Materials 45

2

(2)

According to equation (1), the damage scattered signals ( ) *ij s t* measured by all transducer

<sup>2</sup> (,) ( ( , )) ( 1) *N N*

An image is gained with pixel values at all points calculated. Though delay-and-sum imaging method is very similar to the above ellipse location technique in theory, the special calibration for TOF of every damage scattered signal is not required. Furthermore, those methods [31-32] based on TR focusing are essentially identical to the imaging method.

Phased arrays are generally compact transducer arrays in linear, circular or other patterns. Every array element, commonly PZT wafer, is individually used as actuator and sensor in a round-robin fashion such that a group of sensor signals are collected. Based on the synthetic-beam principles [33], all the signals, supplied with different phase delays, can be combined into one synthetic beamforming signal at one given steering angle, which can be implemented in either time or wavenumber domain [34]. Through the similar processes for all angles, the virtual scanning can be achieved without any physical manipulation of the array. Since constructive interference for the damage scattered signals is actually realized during beamforming, the signal-to-noise (SNR) of diagnostic signals and inspection distance can be largely improved [35]. An image of the scanned area is finally generated by directly mapping all the synthetic signals with the known velocity. Note that phased arrays work in

In tomography imaging, an array of transducers should be arranged around the tested area and used for Lamb wave exciting and receiving in pitch-catch mode. A tomographic image can be reconstructed by using wave speed, waveform or amplitude as flaw-relevant features. The standard parallel projection, fan-beam or crosshole schemes are usually adopted in tomography technique [36]. The crosshole scheme with iterative nature and great flexibly is more suited for any geometry and incomplete data set. To increase the sensitivity of tomography with sparse arrays, reconstruction algorithm for probabilistic inspection of defects (RAPID) is introduced [37]. In RAPID, the probabilities of defect occurrence at a point can be estimated from the changing severity of the signal of each transducer pair and

The structural mechanical impendence, defined as the ratio of the applied force to the resulting velocity, can be easily affected by damages, such as cracks, disbonds and delaminations. However, direct measurement for the mechanical impendence is very hard. With PZT transducers, mechanical impendence is indirectly measured as E/M one for

1 1

*i ji Exy s t xy N N* 

*ij ij*

pairs can be time-shifted and summarized to get the pixel value at *O* as

where *N* is the transducer number.

pulse echo mode.

its relative position to the pair.

**3.2. E/M impedance method** 

damage detection in E/M impedance method [38].

Since defects in composites can bring about changes of geometric and mechanical boundary conditions, the phenomena of reflecting, scattering and energy attenuation could occur when propagating Lamb waves encounter the defects. Characteristic parameters can be then extracted from Lamb wave signals for damage monitoring.

Damage location is usually based on the time of flight (TOF) of Lamb wave signals. In ellipse location method [26], if the TOF of the damage scattered signal acquired by a transducer pair as well as the propagation velocity is known, an ellipse with the transducer pair at its foci can be determined to indicate the possible flaw locus. In order to identify the exact damage location, more ellipses are required to be constructed with other scattered signals from different transducer pairs and their intersection corresponds to the flaw site. Theoretically, a minimum of three ellipses can unambiguously locate the damage. When mode conversation severely takes place during damage scattering, the TOF of the damageinduced mode signals can be also used to estimate the defect point [27].

Time reversal (TR), based on spatial reciprocity and time invariance of linear wave equations, has been advocated as a baseline-free damage detection method. The presence of damage can induce nonlinearity and break down the reconstruction procedure of TR [28], resulting in divergence between the original and reconstructed waveforms. From the waveform difference, damage index can be then computed, in which original waveform rather than reference signal is involved in.

Damage imaging based on sensor arrays is often performed in Lamb wave monitoring to directly give a display of damage positions and intensities. The familiar imaging methods include delay-and-sum, phased array, and tomography methods.

In delay-and-sum imaging method [29-30], every point of the tested structure is considered as a potential flaw. As shown in figure 7, the traveling time (,) *ij t xy* of Lamb waves from actuator *i* at (,) *i j x y* to an imaging point *O* at (,) *x y* and then to sensor *j* at (,) *j j x y* is computed assuming that only one Lamb wave mode exists

$$\mathbf{t}\_{ij}(\mathbf{x}, y) = \mathbf{t}\_{off} + \sqrt{\left(\mathbf{x}\_i - \mathbf{x}\right)^2 + \left(y\_i - y\right)^2} \left/ \mathbf{c}\_i + \sqrt{\left(\mathbf{x}\_j - \mathbf{x}\right)^2 + \left(y\_j - y\right)^2} \right/\mathbf{c}\_j \tag{1}$$

where *off t* is the reference time, *<sup>i</sup> c* and *<sup>j</sup> c* are the group velocities for the wave mode propagating from *i* to *O* and from *O* and *j* , respectively.

**Figure 7.** Illustration of delay-and-sum imaging

According to equation (1), the damage scattered signals ( ) *ij s t* measured by all transducer pairs can be time-shifted and summarized to get the pixel value at *O* as

$$E(\mathbf{x}, y) = \left[ \frac{2}{N(N-1)} \sum\_{i=1}^{N} \sum\_{j=i+1}^{N} s\_{ij} \langle t\_{ij}(\mathbf{x}, y) \rangle \right]^2 \tag{2}$$

where *N* is the transducer number.

44 Composites and Their Applications

methods can be utilized [24-25].

rather than reference signal is involved in.

extracted from Lamb wave signals for damage monitoring.

induced mode signals can be also used to estimate the defect point [27].

include delay-and-sum, phased array, and tomography methods.

computed assuming that only one Lamb wave mode exists

propagating from *i* to *O* and from *O* and *j* , respectively.

**Figure 7.** Illustration of delay-and-sum imaging

achieve single S0 or A0 mode generation, frequency tuning or double-side generation

Since defects in composites can bring about changes of geometric and mechanical boundary conditions, the phenomena of reflecting, scattering and energy attenuation could occur when propagating Lamb waves encounter the defects. Characteristic parameters can be then

Damage location is usually based on the time of flight (TOF) of Lamb wave signals. In ellipse location method [26], if the TOF of the damage scattered signal acquired by a transducer pair as well as the propagation velocity is known, an ellipse with the transducer pair at its foci can be determined to indicate the possible flaw locus. In order to identify the exact damage location, more ellipses are required to be constructed with other scattered signals from different transducer pairs and their intersection corresponds to the flaw site. Theoretically, a minimum of three ellipses can unambiguously locate the damage. When mode conversation severely takes place during damage scattering, the TOF of the damage-

Time reversal (TR), based on spatial reciprocity and time invariance of linear wave equations, has been advocated as a baseline-free damage detection method. The presence of damage can induce nonlinearity and break down the reconstruction procedure of TR [28], resulting in divergence between the original and reconstructed waveforms. From the waveform difference, damage index can be then computed, in which original waveform

Damage imaging based on sensor arrays is often performed in Lamb wave monitoring to directly give a display of damage positions and intensities. The familiar imaging methods

In delay-and-sum imaging method [29-30], every point of the tested structure is considered as a potential flaw. As shown in figure 7, the traveling time (,) *ij t xy* of Lamb waves from actuator *i* at (,) *i j x y* to an imaging point *O* at (,) *x y* and then to sensor *j* at (,) *j j x y* is

where *off t* is the reference time, *<sup>i</sup> c* and *<sup>j</sup> c* are the group velocities for the wave mode

Actuator *i* (*xi , yi*) Sensor *j* (*xj , yj*)

*O* (*x , y*)

22 22 (,) ( ) ( ) ( ) ( ) *ij off i i ij <sup>j</sup> <sup>j</sup> t xy t x x y y c x x y y c* (1)

An image is gained with pixel values at all points calculated. Though delay-and-sum imaging method is very similar to the above ellipse location technique in theory, the special calibration for TOF of every damage scattered signal is not required. Furthermore, those methods [31-32] based on TR focusing are essentially identical to the imaging method.

Phased arrays are generally compact transducer arrays in linear, circular or other patterns. Every array element, commonly PZT wafer, is individually used as actuator and sensor in a round-robin fashion such that a group of sensor signals are collected. Based on the synthetic-beam principles [33], all the signals, supplied with different phase delays, can be combined into one synthetic beamforming signal at one given steering angle, which can be implemented in either time or wavenumber domain [34]. Through the similar processes for all angles, the virtual scanning can be achieved without any physical manipulation of the array. Since constructive interference for the damage scattered signals is actually realized during beamforming, the signal-to-noise (SNR) of diagnostic signals and inspection distance can be largely improved [35]. An image of the scanned area is finally generated by directly mapping all the synthetic signals with the known velocity. Note that phased arrays work in pulse echo mode.

In tomography imaging, an array of transducers should be arranged around the tested area and used for Lamb wave exciting and receiving in pitch-catch mode. A tomographic image can be reconstructed by using wave speed, waveform or amplitude as flaw-relevant features. The standard parallel projection, fan-beam or crosshole schemes are usually adopted in tomography technique [36]. The crosshole scheme with iterative nature and great flexibly is more suited for any geometry and incomplete data set. To increase the sensitivity of tomography with sparse arrays, reconstruction algorithm for probabilistic inspection of defects (RAPID) is introduced [37]. In RAPID, the probabilities of defect occurrence at a point can be estimated from the changing severity of the signal of each transducer pair and its relative position to the pair.

#### **3.2. E/M impedance method**

The structural mechanical impendence, defined as the ratio of the applied force to the resulting velocity, can be easily affected by damages, such as cracks, disbonds and delaminations. However, direct measurement for the mechanical impendence is very hard. With PZT transducers, mechanical impendence is indirectly measured as E/M one for damage detection in E/M impedance method [38].

The method utilizes PZT transducers as both actuators and sensors to acquire structural dynamic responses. The electro-mechanical coupling model between the transducer and the structure is shown in figure 8. In the model, the PZT wafer is axially connected to a single degree-of-freedom spring-mass-damper system represented for the structural impendence. Through the mechanical coupling between the transducer and the tested structure and electro-mechanical transduction inside the transducer, the structural impedance gets reflected in the electric one at the transducer terminals as [39-40]

$$Z(\alpha) = \left[ \operatorname{ion} \left( \overline{\boldsymbol{\varepsilon}}^{\boldsymbol{T}}\_{33} - \frac{Z\_s(\alpha)}{Z\_s(\alpha) + Z\_a(\alpha)} d\_{3x}^2 \widehat{Y}\_{xx}^E \right) \right]^{-1} \tag{3}$$

Structural Health Monitoring for Composite Materials 47

Note that the impedance measurement is often performed in ultrasonic frequency range. At such high frequencies, the dynamic response is dominated in local modes and the excitation wavelength is small enough to ensure the high sensitivity to incipient local flaws [38-40].

Active vibration-based method is a classical SHM technique. The basic idea behind the method is that structural dynamic characteristics are functions of the physical properties, such as mass, stiffness and damping [5, 41]. Therefore, damages, arising with physical property changes, can cause detectable differences in vibration responses. The dynamic characteristic parameters commonly used in the method include frequency, mode shape, power spectrum, mode curvature, frequency response function (FRF), mode flexibility

The method can be either model based or non-model based. The model-based methods undertake structural model analysis and use model characteristic parameters to identify defects, while non-model based ones permit damage detection relying on the vibration

Model-based methods are applied much more in practical SHM applications and can be roughly classified into three groups. One group of the methods, regarded as the forward problems, consists in calibrating model parameters in various known damage cases and defects can be then determined by comparing the measured parameters to the predicted ones. The main challenge of the method is how to obtain the sufficient and accurate characteristic parameters related to all structural circumstances. Particularly for large structures, experimental

In the second group of the methods, criteria or indicators are defined to examine model parameter variations for damage identifying. The ordinary natural frequency criteria are Cawley–Adams criterion and damage location assurance criterion (DLAC). Multiple damage location assurance criterion (MDLAC) is an extension of DLAC to detection multiple flaw sites [42]. Frequency response assurance criterion (FRAC), frequency domain assurance criterion

measurement is unpractical and finite element method (FEM) may a better choice.

A typical SHM system based on active vibrations is shown in figure 9.

**3.3. Active vibration-based method** 

matrix, energy transfer rate (ETR), etc.

**Figure 9.** A typical active vibration-based SHM system

characteristics independent of structural model.

where *Z*( ) is the electric impedance computed as the ratio between the input voltage and the output current of the PZT wafer. ( ) *Zs* and ( ) *Za* are the structure and PZT wafer mechanical impedances, respectively. *a* , 33 *T* , 3*<sup>x</sup> <sup>d</sup>* and <sup>ˆ</sup> *<sup>E</sup> Yxx* are the geometry constant, the complex dielectric constant of the PZT wafers at zero stress, the piezoelectric coupling constant and Young's modulus, respectively.

**Figure 8.** Electro-mechanical coupling model between the PZT transducer and the structure [39]

Equation (3) shows that, as long as the mechanical properties of PZT wafers keep invariable, *Z*( ) is uniquely determined by ( ) *Zs* . Thus, the variations of ( ) *Z* can be mainly attributed to those of structural integrity. In E/M impedance monitoring, ( ) *Z* over a specified bandwidth, i.e., the complex impedance spectrum, is obtained by driving the transducer with sinusoid voltage sweeping and compared with its baseline. Usually, the existence of flaw exhibits as the resonance frequency or amplitude modification in the spectrum. Since the imaginary part of ( ) *Z* is temperature-sensitive due to 33 *T* in equation (3), the real part is more reactive to defects and can be considered for damage assessment. For instance, a damage index is computed as the Euclidean norm of the real portion of the spectrum [39], i.e.,

$$DI = \sqrt{\frac{\sum\_{N} \left[ R\_{\epsilon} (Z\_i^1) - R\_{\epsilon} (Z\_i^0) \right]^2}{\sum\_{N} \left[ R\_{\epsilon} (Z\_i^0) \right]^2}} \tag{4}$$

where *N* is the number of sampling points in the spectrum. <sup>1</sup> *Zi* and <sup>0</sup> *Zi* are the electric impedances measured in current and health states at frequency sampling point *i* , respectively.

Note that the impedance measurement is often performed in ultrasonic frequency range. At such high frequencies, the dynamic response is dominated in local modes and the excitation wavelength is small enough to ensure the high sensitivity to incipient local flaws [38-40].

## **3.3. Active vibration-based method**

46 Composites and Their Applications

where *Z*( ) 

*Z*( ) 

spectrum [39], i.e.,

respectively.

The method utilizes PZT transducers as both actuators and sensors to acquire structural dynamic responses. The electro-mechanical coupling model between the transducer and the structure is shown in figure 8. In the model, the PZT wafer is axially connected to a single degree-of-freedom spring-mass-damper system represented for the structural impendence. Through the mechanical coupling between the transducer and the tested structure and electro-mechanical transduction inside the transducer, the structural impedance gets

33 3

*s a*

*Z Z* 

 

*T E s*

 

is the electric impedance computed as the ratio between the input voltage and

 and ( ) *Za* 

( ) <sup>ˆ</sup> ( ) () ()

**Figure 8.** Electro-mechanical coupling model between the PZT transducer and the structure [39]

attributed to those of structural integrity. In E/M impedance monitoring, ( ) *Z*

*N*

*DI*

*N*

Equation (3) shows that, as long as the mechanical properties of PZT wafers keep invariable,

specified bandwidth, i.e., the complex impedance spectrum, is obtained by driving the transducer with sinusoid voltage sweeping and compared with its baseline. Usually, the existence of flaw exhibits as the resonance frequency or amplitude modification in the

(3), the real part is more reactive to defects and can be considered for damage assessment. For instance, a damage index is computed as the Euclidean norm of the real portion of the

<sup>2</sup> 1 0

() ()

*ei ei*

*RZ RZ*

where *N* is the number of sampling points in the spectrum. <sup>1</sup> *Zi* and <sup>0</sup> *Zi* are the electric impedances measured in current and health states at frequency sampling point *i* ,

*e i*

 

*R Z*

( )

<sup>2</sup> <sup>0</sup>

*T* 

complex dielectric constant of the PZT wafers at zero stress, the piezoelectric coupling

*<sup>Z</sup> Z ia d Y*

1

, 3*<sup>x</sup> <sup>d</sup>* and <sup>ˆ</sup> *<sup>E</sup> Yxx* are the geometry constant, the

(3)

are the structure and PZT wafer

can be mainly

over a

in equation

*T* 

(4)

2

. Thus, the variations of ( ) *Z*

is temperature-sensitive due to 33

*x xx*

reflected in the electric one at the transducer terminals as [39-40]

the output current of the PZT wafer. ( ) *Zs*

mechanical impedances, respectively. *a* , 33

constant and Young's modulus, respectively.

is uniquely determined by ( ) *Zs*

spectrum. Since the imaginary part of ( ) *Z*

  Active vibration-based method is a classical SHM technique. The basic idea behind the method is that structural dynamic characteristics are functions of the physical properties, such as mass, stiffness and damping [5, 41]. Therefore, damages, arising with physical property changes, can cause detectable differences in vibration responses. The dynamic characteristic parameters commonly used in the method include frequency, mode shape, power spectrum, mode curvature, frequency response function (FRF), mode flexibility matrix, energy transfer rate (ETR), etc.

A typical SHM system based on active vibrations is shown in figure 9.

**Figure 9.** A typical active vibration-based SHM system

The method can be either model based or non-model based. The model-based methods undertake structural model analysis and use model characteristic parameters to identify defects, while non-model based ones permit damage detection relying on the vibration characteristics independent of structural model.

Model-based methods are applied much more in practical SHM applications and can be roughly classified into three groups. One group of the methods, regarded as the forward problems, consists in calibrating model parameters in various known damage cases and defects can be then determined by comparing the measured parameters to the predicted ones. The main challenge of the method is how to obtain the sufficient and accurate characteristic parameters related to all structural circumstances. Particularly for large structures, experimental measurement is unpractical and finite element method (FEM) may a better choice.

In the second group of the methods, criteria or indicators are defined to examine model parameter variations for damage identifying. The ordinary natural frequency criteria are Cawley–Adams criterion and damage location assurance criterion (DLAC). Multiple damage location assurance criterion (MDLAC) is an extension of DLAC to detection multiple flaw sites [42]. Frequency response assurance criterion (FRAC), frequency domain assurance criterion (FDAC) [43], global shape correlation (GSC) function and global amplitude correlation (GAC) function are the criteria of FRFs [44]. Modal assurance criterion (MAC) can quantify the correlation between measured and analytical mode shapes in a scalar number from zero to unity. The co-ordinate MAC (COMAC) and partial MAC (PMAC) are the developed forms of MAC [5]. The discrepancies of the other model properties, such as mode shape curvature and dynamic flexibility, can be also computed as damage indicators.

Structural Health Monitoring for Composite Materials 49

AE signal

Signal envelop

Event impulse

Ring impulse

AE signal

Using the time information of AE signals, AE source location can be realized. Taking the triangulation method for example, the location principle is illustrated in figure 12. At least three sensors, <sup>1</sup>*P* , <sup>2</sup>*P* and <sup>3</sup>*P* , should be used to decide a triangle 123 *PPP* . As figure 12 shows, supposing AE source *S* is inside 123 *PPP* and the angles between *S* and the three sensors

1 2 <sup>2</sup> <sup>1</sup>

and <sup>3</sup>*P* , respectively. 12 *t* and 23 *t* are the arrival time differences between 1*P* and 2*P* , and

Note that equation (3) can be also applicable when *S* is outside 123 *PPP* . By solving the

sin( ) ( sin sin ) 0

23 3 2 2 23 3 2 2

 

ˆ ˆ sin( ) sin sin( ) 0

 

 

*Cg* are the velocities for the AE waves propagating from *S* to 1*P* , <sup>2</sup>*P*

12 1 2 12 2 1

2 3 3 2

can be gained to locate *S* .

*g g g g*

ˆ sin sin( ) sin sin( ) 0

*CC t S L C C S*

12 1 3 2 2 23 3 1 2

*g g g g*

 

*CC t LC C*

where <sup>12</sup> *L* , <sup>13</sup> *L* and <sup>23</sup> *L* are the lengths of three sides of 123 *PPP* . 2

*L SL*

 

*Cg* and <sup>3</sup>

between 2*P* and 3*P* , respectively.

 

, respectively. According to the geometric relationship, a nonlinear

 

> 

ˆ

(3)

*S* is the internal angle

**Figure 10.** Extracting procedure for AE event

**Figure 11.** Extracting procedure for ring-down count

 and 3 

*Cg* , <sup>2</sup>

 , 2 and 3 

equation can be finally derived as

are 1 , 2 

<sup>123</sup> *PPP* . 1

equation (3), 1

The last group of the methods is based on the structural model modification. The discrepancy between the original and modified models can provide the damage information. Mathematically, model modification is a constrained optimization problem based on the structural equations of motion, the nominal model and the measured data [45].

Note that compared with the aforementioned active Lamb wave or E/M impedance methods, the vibration frequencies in the active vibration-based method are generally much lower.

## **3.4. Acoustic emission method**

Acoustic emission (AE) can be defined as the sudden release of localized strain energy in the form of transient elastic wave, due to a distortion or change in the structural integrity of material [46]. Many AEs arise during damage processes within structures. These AEs are referred to as primary ones while the secondary AEs are the others induced from external sources, such as impacts [47].

AE phenomena could appear evidently even when a structure is in microscopic-level damage status, which provides the possibility for defect forecasting and real-time monitoring. Generally, the procedure of AE testing can be summarized as: a) AE waves originate from AE source and propagate to the sensors; b) AE waves are captured by the sensors and converted to electrical signals; c) The AE signals are processed and interpreted to evaluate structural condition. Since only sensors are used to passively detect AE signals, AE method is a passive SHM technique.

From an AE signal, the parameters of AE event, ring-down count, count rate and total count are traditionally extracted to describe the damage mechanisms. The extracting procedures for AE event and ring-down count are illustrated in figures 10 and 11, respectively. As figure 10 shows, providing the envelope picked up from an AE signal with a proper voltage threshold *V*<sup>1</sup> , a square impulse is obtained and related to an AE event. The impulse number over unit time and the accumulative impulse number are respectively defined as event count rate and total event count. If a threshold is directly set to the AE waveform, the ringdown count is gotten by quantitatively recording the resultant ring impulses, as shown in figure 11. Ring-down count per event is the so-called AE rate. The other signal features including amplitude, duration, rise time and energy can be also correlated with the defect characteristics [48]. Additionally, because different damages could result in different frequency contents, the spectrum of the AE signal can be calculated for damage discrimination [49].

**Figure 10.** Extracting procedure for AE event

48 Composites and Their Applications

**3.4. Acoustic emission method** 

sources, such as impacts [47].

discrimination [49].

AE method is a passive SHM technique.

lower.

(FDAC) [43], global shape correlation (GSC) function and global amplitude correlation (GAC) function are the criteria of FRFs [44]. Modal assurance criterion (MAC) can quantify the correlation between measured and analytical mode shapes in a scalar number from zero to unity. The co-ordinate MAC (COMAC) and partial MAC (PMAC) are the developed forms of MAC [5]. The discrepancies of the other model properties, such as mode shape curvature and

The last group of the methods is based on the structural model modification. The discrepancy between the original and modified models can provide the damage information. Mathematically, model modification is a constrained optimization problem based on the structural equations of motion, the nominal model and the measured data [45]. Note that compared with the aforementioned active Lamb wave or E/M impedance methods, the vibration frequencies in the active vibration-based method are generally much

Acoustic emission (AE) can be defined as the sudden release of localized strain energy in the form of transient elastic wave, due to a distortion or change in the structural integrity of material [46]. Many AEs arise during damage processes within structures. These AEs are referred to as primary ones while the secondary AEs are the others induced from external

AE phenomena could appear evidently even when a structure is in microscopic-level damage status, which provides the possibility for defect forecasting and real-time monitoring. Generally, the procedure of AE testing can be summarized as: a) AE waves originate from AE source and propagate to the sensors; b) AE waves are captured by the sensors and converted to electrical signals; c) The AE signals are processed and interpreted to evaluate structural condition. Since only sensors are used to passively detect AE signals,

From an AE signal, the parameters of AE event, ring-down count, count rate and total count are traditionally extracted to describe the damage mechanisms. The extracting procedures for AE event and ring-down count are illustrated in figures 10 and 11, respectively. As figure 10 shows, providing the envelope picked up from an AE signal with a proper voltage threshold *V*<sup>1</sup> , a square impulse is obtained and related to an AE event. The impulse number over unit time and the accumulative impulse number are respectively defined as event count rate and total event count. If a threshold is directly set to the AE waveform, the ringdown count is gotten by quantitatively recording the resultant ring impulses, as shown in figure 11. Ring-down count per event is the so-called AE rate. The other signal features including amplitude, duration, rise time and energy can be also correlated with the defect characteristics [48]. Additionally, because different damages could result in different frequency contents, the spectrum of the AE signal can be calculated for damage

dynamic flexibility, can be also computed as damage indicators.

**Figure 11.** Extracting procedure for ring-down count

Using the time information of AE signals, AE source location can be realized. Taking the triangulation method for example, the location principle is illustrated in figure 12. At least three sensors, <sup>1</sup>*P* , <sup>2</sup>*P* and <sup>3</sup>*P* , should be used to decide a triangle 123 *PPP* . As figure 12 shows, supposing AE source *S* is inside 123 *PPP* and the angles between *S* and the three sensors are 1 , 2 and 3 , respectively. According to the geometric relationship, a nonlinear equation can be finally derived as

$$\begin{cases} \mathcal{C}\_{\mathcal{g}\_{1}} \mathcal{C}\_{\mathcal{g}\_{2}} \Delta t\_{12} \sin(\theta\_{1} + \theta\_{2}) - L\_{12} (\mathcal{C}\_{\mathcal{g}\_{2}} \sin \theta\_{2} - \mathcal{C}\_{\mathcal{g}\_{1}} \sin \theta\_{1}) = 0 \\ \mathcal{C}\_{\mathcal{g}\_{2}} \mathcal{C}\_{\mathcal{g}\_{3}} \Delta t\_{23} \sin(\theta\_{3} + \hat{\mathcal{S}}\_{2} - \theta\_{2}) - L\_{23} \left( \mathcal{C}\_{\mathcal{g}\_{3}} \sin \theta\_{3} - \mathcal{C}\_{\mathcal{g}\_{2}} \sin(\hat{\mathcal{S}}\_{2} - \theta\_{2}) \right) = 0 \\ L\_{12} \sin \theta\_{1} \sin(\theta\_{3} + \hat{\mathcal{S}}\_{2} - \theta\_{2}) - L\_{23} \sin \theta\_{3} \sin(\theta\_{1} + \theta\_{2}) = 0 \end{cases} \tag{3}$$

where <sup>12</sup> *L* , <sup>13</sup> *L* and <sup>23</sup> *L* are the lengths of three sides of 123 *PPP* . 2 ˆ *S* is the internal angle <sup>123</sup> *PPP* . 1 *Cg* , <sup>2</sup> *Cg* and <sup>3</sup> *Cg* are the velocities for the AE waves propagating from *S* to 1*P* , <sup>2</sup>*P* and <sup>3</sup>*P* , respectively. 12 *t* and 23 *t* are the arrival time differences between 1*P* and 2*P* , and between 2*P* and 3*P* , respectively.

Note that equation (3) can be also applicable when *S* is outside 123 *PPP* . By solving the equation (3), 1 , 2 and 3 can be gained to locate *S* .

Structural Health Monitoring for Composite Materials 51

from the baseline. In the other way, a theoretical model for the structure is established and analyzed to acquire the strain data corresponding to various structural states. Comparing the data to the actually acquired ones directly or with criterions, the structural integrity is evaluated. The key issue lies in this methodology is how to make the model exact enough

CVM method is a very mature technique and is ready for deployment onto operational platforms [54]. CVM has been developed by Structural Monitoring System Ltd. (SMS), with the original patents being granted in 1995. The CVM system has three primary components: a CVM sensor, fluid flow meter and stable low vacuum source [55]. The CVM sensor is directly adhered to the surface of the monitored structure to form a series of long and narrow galleries, which are alternately placed in the low vacuum or atmosphere states, as figure 14 illustrates. With the stable vacuum reference provided by the vacuum source, the air pressure of vacuum galleries is measured by the flow meter. If no flaw presents, the galleries remain sealed and there should be no leaks and pressure changes happening. However, if a flaw develops and breaks the galleries, air will flow along the breakage passage from the atmosphere to the vacuum galleries, increasing the pressure [54, 56].

Obviously, the sensitivity of the CVM sensor is determined by the gallery wall thickness. Now, the commercially available sensor can have a sensitivity down to 250*μ*m with an accuracy better than 4% [55]. Since the exposed structural surface becomes one part of the galleries, CVM method is very suitable for surface crack or corrosion detection in metals. Using embedded CVM sensor, the applicability of the method for monitoring crack growth,

Furthermore, the rate of pressure rising can be the indication of damage size.

debonding or delamination in composite structures has been also demonstrated.

especially for the complex real-life structure.

**Figure 14.** Measuring principle of CVM [61]

**3.6. CVM method** 

**Figure 12.** Illustration of triangulation method

For plate-like structures, modal AE (MAE) [50-52] is often presented based on elastic wave theory. In MAE, AE signal is analyzed in terms of different propagating wave modes, among which the basic A0 and S0 modes are mostly concerned.

## **3.5. Strain-based method**

Strain-based method is an effective passive SHM method, because the presence of damage in the structure under normal operational loads can alter the local strain distribution due to the changing load path [53]. Besides the resistance strain gages, FOSs are usually applied in the method to measure the distributed strains. Figure 13 gives a typical strain distribution measurement system based on an array of multiplexed FBG sensors.

**Figure 13.** A strain distribution measurement system based on FBG sensors

In practical applications, the strain-based method can be performed in two ways. In one way, the strain distribution of the intact structure is measured as the baseline in advance. Damage can be then detected when the current strain measurement significantly diverges from the baseline. In the other way, a theoretical model for the structure is established and analyzed to acquire the strain data corresponding to various structural states. Comparing the data to the actually acquired ones directly or with criterions, the structural integrity is evaluated. The key issue lies in this methodology is how to make the model exact enough especially for the complex real-life structure.

## **3.6. CVM method**

50 Composites and Their Applications

**Figure 12.** Illustration of triangulation method

**3.5. Strain-based method** 

among which the basic A0 and S0 modes are mostly concerned.

measurement system based on an array of multiplexed FBG sensors.

**Figure 13.** A strain distribution measurement system based on FBG sensors

In practical applications, the strain-based method can be performed in two ways. In one way, the strain distribution of the intact structure is measured as the baseline in advance. Damage can be then detected when the current strain measurement significantly diverges

For plate-like structures, modal AE (MAE) [50-52] is often presented based on elastic wave theory. In MAE, AE signal is analyzed in terms of different propagating wave modes,

Strain-based method is an effective passive SHM method, because the presence of damage in the structure under normal operational loads can alter the local strain distribution due to the changing load path [53]. Besides the resistance strain gages, FOSs are usually applied in the method to measure the distributed strains. Figure 13 gives a typical strain distribution CVM method is a very mature technique and is ready for deployment onto operational platforms [54]. CVM has been developed by Structural Monitoring System Ltd. (SMS), with the original patents being granted in 1995. The CVM system has three primary components: a CVM sensor, fluid flow meter and stable low vacuum source [55]. The CVM sensor is directly adhered to the surface of the monitored structure to form a series of long and narrow galleries, which are alternately placed in the low vacuum or atmosphere states, as figure 14 illustrates. With the stable vacuum reference provided by the vacuum source, the air pressure of vacuum galleries is measured by the flow meter. If no flaw presents, the galleries remain sealed and there should be no leaks and pressure changes happening. However, if a flaw develops and breaks the galleries, air will flow along the breakage passage from the atmosphere to the vacuum galleries, increasing the pressure [54, 56]. Furthermore, the rate of pressure rising can be the indication of damage size.

Obviously, the sensitivity of the CVM sensor is determined by the gallery wall thickness. Now, the commercially available sensor can have a sensitivity down to 250*μ*m with an accuracy better than 4% [55]. Since the exposed structural surface becomes one part of the galleries, CVM method is very suitable for surface crack or corrosion detection in metals. Using embedded CVM sensor, the applicability of the method for monitoring crack growth, debonding or delamination in composite structures has been also demonstrated.

**Figure 14.** Measuring principle of CVM [61]

All the above methods can be summarized in table 1.


Structural Health Monitoring for Composite Materials 53

amplifier and the specimen, is shown in figure 16. Lamb wave detection system is built based on an industrial computer, in which a LAI200-ISA arbitrary wave generator (up to 50MHz DAC clock, ±5V output scale, 12 bit resolution), a charge amplifier and a PCI-9812 analog input card (10MHz sampling rate, ±5V sampling scale and 12 bit acquisition resolution) are integrated to generate Lamb wave signals, amplify and collect sensor signals. Matrix switch controls the working sequence of all PZT pairs and power amplifier is used to amplify the

**D2**

**P5**

**D1**

*y*

**P8**

**P4**

*x*

Lamb wave

**P1**

excitation signal to enlarge the monitoring area in the plate.

**P2**

**P6**

**Figure 15.** Configuration of the specimen of Lamb wave imaging

composite plate

**Figure 16.** Figure 16 Experiment setup of Lamb wave imaging

(x, y)/(mm) (x, y)/(mm)

Power amplifier detection system

P1 (200 , 200) P6 (-200 , 0) P2 (-200 , 200) P7 (0 , -200) P3 (-200 , -200) P8 (200 , 0) P4 (200 , -200) D1 (60 , 7 0) P5 (0 , 200) D2 (10 , -30) **Table 2.** The coordinates (x, y) of PZT wafers and damages in the epoxy glass composite plate

**P3 P7**

Matrix switch The epoxy glass

*O*

**Table 1.** Typical SHM methods for composites

## **4. SHM examples on composite materials**

To verify the SHM methods, two examples of Lamb wave imaging and impact location for composite structures are arranged as the representations of active and passive methods, respectively.

## **4.1. Lamb wave imaging**

The tested specimen is a quasi-isotropic epoxy glass-fiber composite plate with the dimension of 600mm×600mm×2mm. Eight PZT wafers P1~P8 are mounted on the plate to form a sparse PZT array, as shown in figure 15. The diameter of each PZT is 8mm and its thickness is 0.48 mm. Two identical hexagonal hollow screws, denoted as D1 and D2, are bonded on the plate to simulate damages. The positions of PZT wafers and damages are listed in table 2. The overall experimental setup, including Lamb wave detection system, matrix switch, power amplifier and the specimen, is shown in figure 16. Lamb wave detection system is built based on an industrial computer, in which a LAI200-ISA arbitrary wave generator (up to 50MHz DAC clock, ±5V output scale, 12 bit resolution), a charge amplifier and a PCI-9812 analog input card (10MHz sampling rate, ±5V sampling scale and 12 bit acquisition resolution) are integrated to generate Lamb wave signals, amplify and collect sensor signals. Matrix switch controls the working sequence of all PZT pairs and power amplifier is used to amplify the excitation signal to enlarge the monitoring area in the plate.

**Figure 15.** Configuration of the specimen of Lamb wave imaging

52 Composites and Their Applications

Methods Used sensors

Piezoelectric sensor

Piezoelectric

Piezoelectric sensor &

accelerometer

Piezoelectric sensor & AE sensor

**Table 1.** Typical SHM methods for composites

**4. SHM examples on composite materials** 

Resistance strain

Lamb Wave method

E/M impedance method

Active

vibration-based method

Strain-based method

Acoustic emission

respectively.

**4.1. Lamb wave imaging** 

All the above methods can be summarized in table 1.

Monitoring objects

Debonding

Delamination

sensor D D E E E Local monitoring, off-line

gauge E D E E E On-line, relying on loads

D E D E E On-line

To verify the SHM methods, two examples of Lamb wave imaging and impact location for composite structures are arranged as the representations of active and passive methods,

The tested specimen is a quasi-isotropic epoxy glass-fiber composite plate with the dimension of 600mm×600mm×2mm. Eight PZT wafers P1~P8 are mounted on the plate to form a sparse PZT array, as shown in figure 15. The diameter of each PZT is 8mm and its thickness is 0.48 mm. Two identical hexagonal hollow screws, denoted as D1 and D2, are bonded on the plate to simulate damages. The positions of PZT wafers and damages are listed in table 2. The overall experimental setup, including Lamb wave detection system, matrix switch, power

CVM CVM sensor D D E E E Local monitoring, mature method

E D E E E

Crack

Strain

Impact

Characteristics

off-line FOS D D E E E Global monitoring, requiring PZT actuators,

monitoring

vibration monitoring

FOS E D E E E On-line & off-line, low frequency (<1kHZ)

FOS E D E E E Distribution measurement, on-line, rely on loads

D D E E E Global monitoring, high sensitivity, on-line &

limited by high-frequency modulation

On-line & off-line, medium and high frequency vibration and acceleration

**Figure 16.** Figure 16 Experiment setup of Lamb wave imaging


**Table 2.** The coordinates (x, y) of PZT wafers and damages in the epoxy glass composite plate

Normalized voltage (V)

As shown in figure 17, a symmetrical modulated 5-cycle sine burst with the central frequency of 50 kHz is adopted to excite diagnostic waves of single A0 mode into the composite plate. The damage scattered signals can be obtained by subtracting the baseline response of the undamaged plate from the response of the damaged plate under the sin burst excitation. Figure 18 shows the damage scattered signals measured by P1-P5 pair. The wavepacket scattered from D1 can be observed from figure 18(a). When D1 and D2 exist, the two damage scattered wavepackts appear in figure 18(b).

**Figure 18.** Damage scattered signals measured by P1-P5 pair

0 100 200 300 400 500

Time ( s)

After the group velocity of the A0 mode at 50 kHz is measured as 1331.4m/s in the composite plate, the damage images can be constructed by using the envelopes of the twenty-eight scattered signals acquired by all the PZT pairs in the sparse array based on equation (2). The imaging results are shown in figure 19 where the symbol 'X' denotes the actual damage location. As displayed in figure19 (a) and (b), each flaw point is clearly and accurately represented by a bright focalized spot.

(a) D1 (b) D1 and D2

0 100 200 300 400 500

Structural Health Monitoring for Composite Materials 55

The experiment system for impact location, mainly composed of an aircraft wing box and an integrated SHM system, is shown in figure 20. The size of the wing box is 1000mm×1800mm× 200mm. The top panel is made of carbon fiber composite material and the bottom panel is made of aluminum. The panels are fastened to steel box frame. There are in total six T-shaped stiffeners with a distance of 130mm between each other on the panels. Vertical to the stiffeners there are five rows of bolt holes with a distance of 280mm. The experiment system is built on the top panel of the carbon fiber composite material. An array of smart layers [52] with three PZT wafers is attached on the inner surface of the top panel. Four impacts are produced to the plate using a hammer (seen in figure 20). The detailed positions of the impacts and the used

PZT wafers (P1, P3, P4, P7, P16, P19, P20 and P22) are shown in figure 21 and table 3.

**Figure 20.** Experiment setup of impact location [52]

**Figure 21.** Configuration of the specimen of impact location [52]

**4.2. Impact location** 

Time ( s)

**Figure 19.** Imaging results

#### **4.2. Impact location**

54 Composites and Their Applications

**Figure 17.** Excitation signal


0

Normalized voltage (V)

1


Voltage (V)

two damage scattered wavepackts appear in figure 18(b).

0 20 40 60 80 100

Scattered wavepacket from D1

Time ( s)

**Figure 18.** Damage scattered signals measured by P1-P5 pair

X (mm)


0 100 200 300 400 500

Time ( s)

represented by a bright focalized spot.

**Figure 19.** Imaging results

Y (mm)


As shown in figure 17, a symmetrical modulated 5-cycle sine burst with the central frequency of 50 kHz is adopted to excite diagnostic waves of single A0 mode into the composite plate. The damage scattered signals can be obtained by subtracting the baseline response of the undamaged plate from the response of the damaged plate under the sin burst excitation. Figure 18 shows the damage scattered signals measured by P1-P5 pair. The wavepacket scattered from D1 can be observed from figure 18(a). When D1 and D2 exist, the


0

Normalized voltage (V)

1

Amplitude (V)

(a) Signal waveform (b) Signal spectrum

0 50 100 150

0 100 200 300 400 500

from D2

Scattered wavepacket

Time ( s)

X (mm)


Frequency (kHz)

Scattered wavepacket from D1

After the group velocity of the A0 mode at 50 kHz is measured as 1331.4m/s in the composite plate, the damage images can be constructed by using the envelopes of the twenty-eight scattered signals acquired by all the PZT pairs in the sparse array based on equation (2). The imaging results are shown in figure 19 where the symbol 'X' denotes the actual damage location. As displayed in figure19 (a) and (b), each flaw point is clearly and accurately

(a) D1 (b) D1 and D2

Y (mm)


(a) D1 (b) D1 and D2

The experiment system for impact location, mainly composed of an aircraft wing box and an integrated SHM system, is shown in figure 20. The size of the wing box is 1000mm×1800mm× 200mm. The top panel is made of carbon fiber composite material and the bottom panel is made of aluminum. The panels are fastened to steel box frame. There are in total six T-shaped stiffeners with a distance of 130mm between each other on the panels. Vertical to the stiffeners there are five rows of bolt holes with a distance of 280mm. The experiment system is built on the top panel of the carbon fiber composite material. An array of smart layers [52] with three PZT wafers is attached on the inner surface of the top panel. Four impacts are produced to the plate using a hammer (seen in figure 20). The detailed positions of the impacts and the used PZT wafers (P1, P3, P4, P7, P16, P19, P20 and P22) are shown in figure 21 and table 3.

**Figure 21.** Configuration of the specimen of impact location [52]


Structural Health Monitoring for Composite Materials 57

common sensors used in SHM as well as typical SHM methods for composite materials are then introduced. Two examples, Lamb wave imaging for a glass-fiber composite plate and

Though much development of SHM has been achieved, a great deal of work is still required

*Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu Province, P.R. China* 

*National Key Lab of Electromagnetic Environmental Effect and Electro-optical Engineering, Nanjing,* 

This work is supported by EU-FP7 SICA programme (Grant No.FP7-PEOPLE-2010-IRSES-

[2] Jones RM. Mechanics of composite materials (2nd edition). Philadelphia PA: Taylor &

[3] Miracle DB, Donaldson SL. Introduction to Composites. In: Miracle DB, Donaldson SL (ed.) ASM Handbook Volume 21: Composites. ASM International; 2001. p3-17 [4] Awad ZK, Aravinthan T, Zhuge Y, et al. A review of optimization techniques used in the design of fibre composite structures for civil engineering applications. Materials and

[5] Montalvao D, Maia NMM and Ribeiro AMR. A Review of Vibration-based Structural Health Monitoring with Special Emphasis on Composite Materials. The Shock and

[6] Qing XL, Kumar A, Zhang C. A hybrid piezoelectric/fiber optic diagnostic system for structural health monitoring. Smart Materials and Structures 2005; 14(3) S98-S103. [7] Qing XL, Chan HL, Beard SJ, et al. An Active Diagnostic System for Structural Health Monitoring of Rocket Engines. Journal of intelligent material systems and structures

[8] Ihn JB, Chang FK. Pitch-catch Active Sensing Methods in Structural Health Monitoring

[9] Lia HN, Lia DS and Song GB. Recent applications of fiber optic sensors to health monitoring in civil engineering. Engineering Structures 2004; 26(11): 1647-1657.

for Aircraft Structures. Structural Health Monitoring 2008; 7(1): 5-19.

[1] Campbell FC. Structural Composite Materials. Ohio: ASM International; 2010.

impact location in an aircraft composite wing box, are also arranged.

for the further practical SHM applications in composite materials.

Jian Cai, Lei Qiu, Shenfang Yuan, PeiPei Liu and Dong Liang *The Aeronautic Key Lab for Smart Materials and Structures,* 

269202), Natural Science Foundation of China (Grant No.50830201).

**Author details** 

*Jiangsu Province, P.R. China*

**Acknowledgement** 

**6. References** 

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Lihua Shi

**Table 3.** The coordinates (x, y) of PZT wafers and impacts in the top panel

The impact responses are fed into the integrated SHM system. Figure 22 shows a waterfall plot of normalized responses produced by the impact at IST1. Here, the narrow-band components with central frequency of 100kHz are extracted from all the responses and their envelopes are then computed for arrival time determination, which can be performed with the complex wavelet transform [57]. After the arrival times are decided by the first peaks in the obtained envelopes, the impact can be located based on equation (3). Note that the anisotropic properties in the composite panel should be considered during impact location. The location result is given in table 4, in which the location error is defined as the spatial interval between the actual and the estimated impact sites.

**Figure 22.** Acquired impact responses [52]


**Table 4.** Impact location result in the carbon-fiber composite panel

## **5. Summary and conclusions**

SHM for composite materials is briefly described in the chapter. Firstly, an introduction involving advantages, problems and SHM requirements of composites is made. The common sensors used in SHM as well as typical SHM methods for composite materials are then introduced. Two examples, Lamb wave imaging for a glass-fiber composite plate and impact location in an aircraft composite wing box, are also arranged.

Though much development of SHM has been achieved, a great deal of work is still required for the further practical SHM applications in composite materials.

## **Author details**

Jian Cai, Lei Qiu, Shenfang Yuan, PeiPei Liu and Dong Liang *The Aeronautic Key Lab for Smart Materials and Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, Jiangsu Province, P.R. China* 

Lihua Shi

56 Composites and Their Applications

(x, y)/(mm) (x, y)/(mm)

P1 (-210 , 225) P20 (-200 , 75) P3 (210 , -75) P22 (-200 , -225) P4 (210 , -225) IST1 (-120 , 150) P7 (-100 , 225) IST2 (0 , 150) P16 (-50 , -225) IST3 (0 , 0) P19 (-200 , 225) IST4 (0 , -150)

The impact responses are fed into the integrated SHM system. Figure 22 shows a waterfall plot of normalized responses produced by the impact at IST1. Here, the narrow-band components with central frequency of 100kHz are extracted from all the responses and their envelopes are then computed for arrival time determination, which can be performed with the complex wavelet transform [57]. After the arrival times are decided by the first peaks in the obtained envelopes, the impact can be located based on equation (3). Note that the anisotropic properties in the composite panel should be considered during impact location. The location result is given in table 4, in which the location error is defined as the spatial

**Table 3.** The coordinates (x, y) of PZT wafers and impacts in the top panel

interval between the actual and the estimated impact sites.

Impact Number Impact location result

**Table 4.** Impact location result in the carbon-fiber composite panel

Actual site Estimated site Location error

IST1 (-120mm, 150mm) (-130mm, 125mm) 27mm IST2 (0mm, 150mm) (15mm, 120mm) 16mm IST3 (0mm, 0mm) (0mm, -20mm) 20mm IST4 (0mm, -150mm) (-20mm, -120mm) 36mm

SHM for composite materials is briefly described in the chapter. Firstly, an introduction involving advantages, problems and SHM requirements of composites is made. The

**Figure 22.** Acquired impact responses [52]

**5. Summary and conclusions** 

*National Key Lab of Electromagnetic Environmental Effect and Electro-optical Engineering, Nanjing, Jiangsu Province, P.R. China*

## **Acknowledgement**

This work is supported by EU-FP7 SICA programme (Grant No.FP7-PEOPLE-2010-IRSES-269202), Natural Science Foundation of China (Grant No.50830201).

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[18] Varadan VK, Varadan VV. Microsensors, microelectromechanical systems (MEMS), and electronics for smart structures and systems. Smart Materials and Structures 2000; 9(6):

[19] Yuan SF, Liang DK, Shi LH, et al. Recent Progress on Distributed Structural Health Monitoring Research at NUAA. Journal of Intelligent Material Systems and Structures

[20] Qing XL, Beard SJ, Kumar A, et al. Advances in the development of built-in diagnostic system for filament wound composite structures. Composites Science and Technology

[21] Lin M, Chang FG. 1998. Design and Fabrication of Built-in Diagnostic for Composite Structures. In: 12th American Society of Composites Technical Conference, Baltimore,

[22] Lemistre MB, Balageas DL. A Hybrid Electromagnetic Acousto-ultrasonic Method for SHM of Carbon/epoxy Structures. Structural Health Monitoring 2003; 2(2): 153-160. [23] Giurgiutiu V. Structural Health Monitoring with Piezoelectric Wafer Active Sensors.

[24] Bottai GS, Chrysochoidis NA, Giurgiutiu V, et al. Analytical and experimental evaluation of piezoelectric wafer active sensors performances for Lamb waves based

structural health monitoring in composite laminates, 2007, Proceedings of SPIE. [25] Su Z, Ye L. Selective generation of Lamb wave modes and their propagation characteristics in defective composite laminates. Journal of Materials: Design and


[43] Pascual R, Golinval JC, Razeto M. A frequency domain correlation technique for model correlation and updating. International Modal Analysis Conference (IMAC), 15th, Orlando, FL; UNITED STATES; 3-6 Feb. 1997. pp. 587-592. 1997.

**Chapter 4** 

© 2012 Jee and Lee, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Jee and Lee, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Acoustic Emission of Composite Vessel** 

There are about 10 million vehicles run on natural gas in the world. There are about 1.7 million low-pressure LPG vehicles [1], and 17,000 high-pressure CNG vehicles running in

Generally for CNG vehicles, type II vessel, where the increase in used pressure and lighter weight are achieved through fiber-reinforced composite material, which is wrapped in hoopdirection on the steel liner is used. Since 1984, the U.S. experienced more than 80 cases of vehicle fuel tank-related accidents [3] and Korea also experienced 8 cases in which the CNG tank exploded; thus, there is a need for the development of inspection technology for highpressure fuel tanks. In the case of the U.S., the inspection technology of high-pressure fuel tanks were developed by DOD and NASA as an inspection technology for missile fuel tanks[4] but as the use of high-pressure fuel tanks for transport increased DOT executed a research on inspection technology for vehicles based on the research results of NASA and reported that among several NDT technology, Acoustic Emission(AE) has a possibility of being used as an inspection technology for vehicles[5,6]. The gas vessel, which is made of fiber-reinforced composite material, is unlike vessel made of only steel materials in that when the damage increases the acoustic generation activity increases but when the degree of damage increases even more, the acoustic generation activity rather decreases [7]. A study of defect detection and failure analysis for composite Materials using acoustic emission is progressing steadily [9-11,14].

Experiment vessel used in this research is a 64 Liter CNG fuel tank used in vehicles. The thickness of the liner in the shell is about 6 mm and was made using the DDI (Deep Drawing Ironing) method[12,13] using 34CrMo4 steel plate, and is a type-II vessel in which

Hyun-Sup Jee and Jong-O Lee

Korea and the number is increasing.[2]

http://dx.doi.org/10.5772/47877

**1. Introduction** 

**2. Experiment** 

**2.1. Experimental vessel** 

glass fiber is hoop-wrapped on the shell of the liner.

Additional information is available at the end of the chapter


## **Chapter 4**

## **Acoustic Emission of Composite Vessel**

Hyun-Sup Jee and Jong-O Lee

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/47877

## **1. Introduction**

60 Composites and Their Applications

10(6): 713-721.

[43] Pascual R, Golinval JC, Razeto M. A frequency domain correlation technique for model correlation and updating. International Modal Analysis Conference (IMAC), 15th,

[44] Zang C, Friswell MI, Imregun M. Structural Health Monitoring and Damage Assessment Using Measured FRFs from Multiple Sensors, Part I: The Indicator of

[45] Doebling SW, Farrar CR, Prime MB. Review of Vibration-Based Damage Identification

[46] Gostautas RS, Ramirez G, Peterman RJ. Acoustic Emission Monitoring and Analysis of Glass Fiber-Reinforced Composites Bridge Decks. Journal of Bridge Engineering 2005;

[47] Nair A, Cai CS. Acoustic emission monitoring of bridges: Review and case studies.

[48] Huang M, Jiang L, Liaw PK. Using Acoustic Emission in Fatigue and Fracture Materials

[49] Groot PJ, Wijnen PA, Janssen RB. Real-time frequency determination of acoustic emission for different fracture mechanisms in carbon epoxy composites. Composites

[50] Morscher GN, Modal acoustic emission of damage accumulation in a woven SiC/SiC

[51] Surgeon M, Wevers M. Modal analysis of acoustic emission signals from CFRP

[52] Qiu L, Yuan SF, Zhang XY, et al. A time reversal focusing based impact imaging method and its evaluation on complex composite structures. Smart Materials and

[53] Silva-Munoz RA, Lopez-Anido RA. Structural health monitoring of marine composite structural joints using embedded fiber Bragg grating strain sensors. Composite

[54] Mrad N. State of Development of Advanced Sensory Systems for Structural Health Monitoring Applications. Proceedings of the NATO RTO AVT-144 Workshop on Enhanced Aircraft Platform Availability Through Advanced Maintenance Concepts and

Technologies, Vilnius, Lithuania, 3-5 October 2006 (DRDC Atlantic SL-2008-260). [55] Wishaw M, Barton DP. Comparative Vacuum Monitoring: a New Method of In-Situ Real-Time Crack Detection and Monitoring. In: Proceding of 10th Asia-Pacific

[56] Stehmeier H, Speckmann H. Comparative Vacuum Monitoring (CVM) Monitoring of fatigue cracking in aircraft structures. 2nd European Workshop on Structural Health Monitoring July 7-9, 2004 Amazeum Conference Centre at Deutsches Museum, Munich,

[57] Jeong H. Analysis of plate wave propagation in anisotropic laminates using a wavelet

Correlation Criteria. Key Engineering Materials 2003; 245-246(131): 131-140.

Orlando, FL; UNITED STATES; 3-6 Feb. 1997. pp. 587-592. 1997.

http://www.tms.org/pubs/journals/JOM/9811/Huang/Huang-9811.html

composite. Composites Science and Technology 1999; 59(5): 687-697.

Methods. Shock and Vibration Digest 1998; 30(2): 91-104.

Engineering Structures 2010; 32(6): 1704-1714.

Science and Technology 1995; 55(4): 405-412.

laminates. NDT&E International 1999; 32(6): 311-322.

Conference On Nondestructive Testing, 2001. Brisbane.

transform. NDT&E International 2001; 34(3): 185-190.

Research. JOM-e 1998; 50(11): 1-14.

Structures 2011; 20(10): 105014.

Structures 2009; 89(2): 224-234.

Germany.

There are about 10 million vehicles run on natural gas in the world. There are about 1.7 million low-pressure LPG vehicles [1], and 17,000 high-pressure CNG vehicles running in Korea and the number is increasing.[2]

Generally for CNG vehicles, type II vessel, where the increase in used pressure and lighter weight are achieved through fiber-reinforced composite material, which is wrapped in hoopdirection on the steel liner is used. Since 1984, the U.S. experienced more than 80 cases of vehicle fuel tank-related accidents [3] and Korea also experienced 8 cases in which the CNG tank exploded; thus, there is a need for the development of inspection technology for highpressure fuel tanks. In the case of the U.S., the inspection technology of high-pressure fuel tanks were developed by DOD and NASA as an inspection technology for missile fuel tanks[4] but as the use of high-pressure fuel tanks for transport increased DOT executed a research on inspection technology for vehicles based on the research results of NASA and reported that among several NDT technology, Acoustic Emission(AE) has a possibility of being used as an inspection technology for vehicles[5,6]. The gas vessel, which is made of fiber-reinforced composite material, is unlike vessel made of only steel materials in that when the damage increases the acoustic generation activity increases but when the degree of damage increases even more, the acoustic generation activity rather decreases [7]. A study of defect detection and failure analysis for composite Materials using acoustic emission is progressing steadily [9-11,14].

## **2. Experiment**

## **2.1. Experimental vessel**

Experiment vessel used in this research is a 64 Liter CNG fuel tank used in vehicles. The thickness of the liner in the shell is about 6 mm and was made using the DDI (Deep Drawing Ironing) method[12,13] using 34CrMo4 steel plate, and is a type-II vessel in which glass fiber is hoop-wrapped on the shell of the liner.

© 2012 Jee and Lee, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Jee and Lee, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Acoustic Emission of Composite Vessel 63

**Figure 2.** Diagram of Experimentation : (a) Burst and (b) fatigue test of the composite vessel

burst test was carried out. Figure 3(b) shows the conditions for pressurization.

**3. Result and research** 

*3.1.1. Damage mechanism of composite vessel* 

liner of that part and finally destroying the vessel.

**3.1. Burst test** 

The burst pressure was estimated to be 600 bar under the pressured conditions. By raising the pressure to 30%, 50%, 60%, 70%, 80% and 90% of estimated burst pressure and keeping each pressure stage for 10 minutes as in Figure 3(a), we acquired the AE signal from each stage. As can be seen in the figure, pressure was put on the vessel with a pump to control the pressure and acoustic emission signals were detected using acoustic emission sensors attached to the vessel. And the signal were processed and analyzed after fed into AE equipment. The fatigue test repeated 20000 cycles between 0 and 207 bar, afterwards which has a used pressure of 0 and afterwards, the pressure was continuously increased and the

(b)

(a)

First, the composite materials wrapped in the metal liner were separated due to the matrix crack and then each layer was separated from each other. Then, some section of the fuel tank was weakened due to the cutting of some reinforced fiber, causing the destruction of metal

**Figure 1.** Schematic diagram of experimental vessel


**Table 1.** Chemical composition of vessel liner (unit : wt.%)

## **2.2. Method of experiment**

For the test, the acoustic emission sensor is R15I (PAC) with the resonance frequency of 150 kHz and cable of RG58A/U (10m) is put on the middle of shell using the vacuum grease.

The detected AE signal is put into the DiSP-52 Acoustic emission workstation (PAC) for processing. In addition, the water was used as medium for burst test. The threshold value of test was set at 45dB. The source of simulated sound was the destruction of the 2H Pentel pencil lead. The average sensitivity of sensor was 98dB within 1 inch from sensor.

**Figure 2.** Diagram of Experimentation : (a) Burst and (b) fatigue test of the composite vessel

The burst pressure was estimated to be 600 bar under the pressured conditions. By raising the pressure to 30%, 50%, 60%, 70%, 80% and 90% of estimated burst pressure and keeping each pressure stage for 10 minutes as in Figure 3(a), we acquired the AE signal from each stage. As can be seen in the figure, pressure was put on the vessel with a pump to control the pressure and acoustic emission signals were detected using acoustic emission sensors attached to the vessel. And the signal were processed and analyzed after fed into AE equipment. The fatigue test repeated 20000 cycles between 0 and 207 bar, afterwards which has a used pressure of 0 and afterwards, the pressure was continuously increased and the burst test was carried out. Figure 3(b) shows the conditions for pressurization.

## **3. Result and research**

## **3.1. Burst test**

62 Composites and Their Applications

**Figure 1.** Schematic diagram of experimental vessel

1.00 0.40

P+S ≤0.020

**Table 1.** Chemical composition of vessel liner (unit : wt.%)

0.40 0.10

For the test, the acoustic emission sensor is R15I (PAC) with the resonance frequency of 150 kHz and cable of RG58A/U (10m) is put on the middle of shell using the vacuum grease.

The detected AE signal is put into the DiSP-52 Acoustic emission workstation (PAC) for processing. In addition, the water was used as medium for burst test. The threshold value of test was set at 45dB. The source of simulated sound was the destruction of the 2H Pentel

pencil lead. The average sensitivity of sensor was 98dB within 1 inch from sensor.

0.38 0.25

**2.2. Method of experiment** 

Max Min C Mn Si P S Cr Mo

0.015 -

0.010 -

1.20 0.80 0.40 0.15

#### *3.1.1. Damage mechanism of composite vessel*

First, the composite materials wrapped in the metal liner were separated due to the matrix crack and then each layer was separated from each other. Then, some section of the fuel tank was weakened due to the cutting of some reinforced fiber, causing the destruction of metal liner of that part and finally destroying the vessel.

Acoustic Emission of Composite Vessel 65

pressure on the vessel and keeps the pressure for some time until some sound emits from it. This is called the analysis of creep effect. The values in Figure 5 show that the pressure up to 360 bar, or 60% of the expected burst press is weak, showing that the vessel is not likely to receive significant damages. But after 70% of the expected pressure, the signals of 60dB or more are often shown, which may mean that there is a lot of creep effect and that the

(a) (b)

(c) (d)

(e) (f)

significant damages have been done to the vessel.

**Figure 3.** Loading sequence : (a) Burst and (b) fatigue test

**Figure 4.** Composite vessel after burst test

#### *3.1.2. AE signal generated during burst test*

In this test, considering the flow noise during the initial pressure, I excluded the signal during the first two minutes from the data but included the remaining 8 minutes data for evaluation. Generally, for the evaluation of the soundness of vessel, a tester imposes pressure on the vessel and keeps the pressure for some time until some sound emits from it. This is called the analysis of creep effect. The values in Figure 5 show that the pressure up to 360 bar, or 60% of the expected burst press is weak, showing that the vessel is not likely to receive significant damages. But after 70% of the expected pressure, the signals of 60dB or more are often shown, which may mean that there is a lot of creep effect and that the significant damages have been done to the vessel.

64 Composites and Their Applications

**Figure 3.** Loading sequence : (a) Burst and (b) fatigue test

Pressure

Operating Pressure (207 bar)

10 min

**Figure 4.** Composite vessel after burst test

*3.1.2. AE signal generated during burst test* 

In this test, considering the flow noise during the initial pressure, I excluded the signal during the first two minutes from the data but included the remaining 8 minutes data for evaluation. Generally, for the evaluation of the soundness of vessel, a tester imposes

(a)

(b)

<sup>0</sup> 4k 8k 12k 16k 20k Cycle

Bursting

Acoustic Emission of Composite Vessel 67

**Figure 7.** Mean rise time during load holding

**Figure 8.** Mean duration during load holding

related to the growth of the cracks.

Unlike the metal vessel, the pressure vessel that is made with composite materials has various damage mechanisms including the matrix crack, crack growth, separation between layers, cutting of fiber, and destruction of metal liner. Figure 9 shows that at the load of 30 % stage, the amplitude of signal rises due to the increased sound emission from matrix crack of composite materials. It also shows that at the load of 50-60 % of the estimated burst pressure, the sound emission is weakened and thus the amplitude of the signal is also reduced. At the load of 70 % pressure, the interlayer separation and the cut of reinforced fiber start and the amplitude of the signal increases again. Then, more reinforcement fibers become severed and the metal liner begins to burst, showing a little increase in amplitude.

As shown below, the composite material pressure vessel has many damage mechanisms in it. The initial damage mechanism contains the matrix crack due to the stress as well as the crack grown. As shown in Figure 10 (a), the rise time appears at the range of 10 μs and 100 μs. This shows the creation of matrix crack and the growth of this crack. The signal from around 10 μs is related to the creation of cracks while the signals from 100 μs look to be

**Figure 5.** Amplitude vs Time plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80% (f)90% and (g)fractured

The pressure rise to the 80 % and 90 % of the expected burst pressure shows a lot of signals with high amplitudes. At the pressure stage of 540 bar or higher, where the burst is likely to occur, the signals with the amplitude of 80-100 dB continue to occur while giving the continuous leak signal from 1352 seconds to 1361 seconds. The burst occured when the pressure rapidly went down to 400 bar. Figure 6 shows the total hits from 2 minutes after the start of each load. There is a rapid increase in the number of hits when the load pressure goes over 70 % of the expected burst pressure where the significant damage is likely to happen. In the final destruction, the number of total hits is low because the time to destruction was short, considering the felicity ratio and the pressure rising up to the fracture.

**Figure 6.** Total hits during load holding

Figure 7 and 8 show that the average rise time and duration for the signals occurred during the holding time of each load. The rise time and duration become shorter up to loads up to 70 % while those from time of 80 % to the time of burst get longer, during which the damage is likely to be greater.

**Figure 7.** Mean rise time during load holding

and (g)fractured

fracture.

**Figure 6.** Total hits during load holding

is likely to be greater.

**Figure 5.** Amplitude vs Time plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80% (f)90%

(g)

The pressure rise to the 80 % and 90 % of the expected burst pressure shows a lot of signals with high amplitudes. At the pressure stage of 540 bar or higher, where the burst is likely to occur, the signals with the amplitude of 80-100 dB continue to occur while giving the continuous leak signal from 1352 seconds to 1361 seconds. The burst occured when the pressure rapidly went down to 400 bar. Figure 6 shows the total hits from 2 minutes after the start of each load. There is a rapid increase in the number of hits when the load pressure goes over 70 % of the expected burst pressure where the significant damage is likely to happen. In the final destruction, the number of total hits is low because the time to destruction was short, considering the felicity ratio and the pressure rising up to the

Figure 7 and 8 show that the average rise time and duration for the signals occurred during the holding time of each load. The rise time and duration become shorter up to loads up to 70 % while those from time of 80 % to the time of burst get longer, during which the damage

**Figure 8.** Mean duration during load holding

Unlike the metal vessel, the pressure vessel that is made with composite materials has various damage mechanisms including the matrix crack, crack growth, separation between layers, cutting of fiber, and destruction of metal liner. Figure 9 shows that at the load of 30 % stage, the amplitude of signal rises due to the increased sound emission from matrix crack of composite materials. It also shows that at the load of 50-60 % of the estimated burst pressure, the sound emission is weakened and thus the amplitude of the signal is also reduced. At the load of 70 % pressure, the interlayer separation and the cut of reinforced fiber start and the amplitude of the signal increases again. Then, more reinforcement fibers become severed and the metal liner begins to burst, showing a little increase in amplitude.

As shown below, the composite material pressure vessel has many damage mechanisms in it. The initial damage mechanism contains the matrix crack due to the stress as well as the crack grown. As shown in Figure 10 (a), the rise time appears at the range of 10 μs and 100 μs. This shows the creation of matrix crack and the growth of this crack. The signal from around 10 μs is related to the creation of cracks while the signals from 100 μs look to be related to the growth of the cracks.

Acoustic Emission of Composite Vessel 69

(f)

**Figure 10.** Rise time vs amplitude plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80%

(g)

(c) (d)

(f)90% and (g)fractured

(e)

**Figure 9.** Mean amplitude during load holding

AE signals at the load of 50% and 60% show that the matrix crack and growth occur in this range instead of the additional causes for damages. At the load of 70%, a lot of the rise time of AE signals come from 500 μs appear. It is likely to indicate that the new damage elements such as cut of reinforcement fiber appear. The rapid increase in AE signals around 10 μs shows that the growth of matrix cracks such as growth of existing cracks and interlayer separation occur at a fast pace. Then, at the load of 80~90%, it is estimated that rapid damages such as growth of the previous matrix cracks and the cutting of reinforcement fiber occur, making much sound from cutting of reinforcement fiber. The 800 μs of rise time at this stage is likely to be caused by the cutting of several lines of reinforcement fibers. Then, at 100% load, a composite of damage mechanisms occur, increasing the damage of vessel and then destroying the last metal liner.

Figure 11 shows the total count of sound emission signals which occur during the 2 minutes of holding time. These variables are better indications than mean rise time in Figure 7, mean duration of Figure 8 or mean amplitude of Figure 9. It was discovered that it was difficult to assess the damage against the vessel with variables such as mean amplitude, total hit, mean rise time, or mean duration but the total count and the total signal strength of the sound emitted when the load is pressed on, activation and vessel's damage were better indications for damages on the vessel.

**Figure 9.** Mean amplitude during load holding

for damages on the vessel.

AE signals at the load of 50% and 60% show that the matrix crack and growth occur in this range instead of the additional causes for damages. At the load of 70%, a lot of the rise time of AE signals come from 500 μs appear. It is likely to indicate that the new damage elements such as cut of reinforcement fiber appear. The rapid increase in AE signals around 10 μs shows that the growth of matrix cracks such as growth of existing cracks and interlayer separation occur at a fast pace. Then, at the load of 80~90%, it is estimated that rapid damages such as growth of the previous matrix cracks and the cutting of reinforcement fiber occur, making much sound from cutting of reinforcement fiber. The 800 μs of rise time at this stage is likely to be caused by the cutting of several lines of reinforcement fibers. Then, at 100% load, a composite of damage mechanisms occur, increasing the damage of vessel and then destroying the last metal liner.

Figure 11 shows the total count of sound emission signals which occur during the 2 minutes of holding time. These variables are better indications than mean rise time in Figure 7, mean duration of Figure 8 or mean amplitude of Figure 9. It was discovered that it was difficult to assess the damage against the vessel with variables such as mean amplitude, total hit, mean rise time, or mean duration but the total count and the total signal strength of the sound emitted when the load is pressed on, activation and vessel's damage were better indications

(a) (b)

**Figure 10.** Rise time vs amplitude plot during load holding : (a)30% (b)50% (c)60% (d)70% (e)80% (f)90% and (g)fractured

Acoustic Emission of Composite Vessel 71

 30% 50% 60% 70% 80% 90% 100%

> 30% 50% 60% 70% 80% 90% 100%

load of 30 % to 60 %, even though there was increase in the number of signals, there was not a big increase in amplitude. Accordingly, we now that up to 60 %, there is not much of a creep affect but just the low amplitude of signals. I think that this is mainly caused by the creation and growth of cracks of substrate. After pressing, there were the creep effects, which show the significant increase in AE signal and the signals with the amplitude of 85 dB. In addition, as the distribution of amplitude appears different from the previous one, I think that the damages on the vessel are significant. It seems that there are more creations and growths of cracks in the composite materials and the cutting of reinforcement fiber and peeling of substrate cause the damage mechanism. In addition, the slope of the distribution of amplitude is known to be related to the mechanism of the signals[8], In the load of 70 % or higher, the slop looks similar, meaning that the damage is caused by the similar damage

**45 50 55 60 65 70 75 80 85 90 95 100**

Amplitude (*dB*)

**0 1000 2000 3000 4000 5000**

Count

mechanism.

**Figure 13.** Amplitude distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**Figure 14.** Count distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**100000**

**Figure 11.** Total count during load holding

**Figure 12.** Total signal strength during load holding

#### *3.1.3. Distribution of AE signal during the holding time*

I did not use the signals during the initial 2 minutes among the 10 minutes to minimize the flow noise which often includes the initial stages of pressing in the pressure test. After that, I observed the number of hits and creep effect of the sound emission signals which occur during the holding time. Even though there were creation and growth of cracks on substrate under the load of 60 % of the estimated burst pressure, they did not cause much damage to the vessel. But it was found that at the load of 70 % pressure, the vessel was quickly destroyed.[4] Figure 13 shows the distribution of amplitude of signals occurring after 2 minutes of the pressure holding at each load. At the load of 60 % or less, there were not a lot of signals of 60 dB or higher but less than 7 0dB. At this time, I could estimate that at the initial stage of the vessel damage, the composite material substrate wrapping around the vessel were showing some cracks and their growth. I could observe the cracks with my own eyes at the 2000th cycle of fatigue test when the pressure was at 207 bar. The used pressure was more than 30 % of the estimated burst pressure (180 bar). During the burst test, at the load of 30 % to 60 %, even though there was increase in the number of signals, there was not a big increase in amplitude. Accordingly, we now that up to 60 %, there is not much of a creep affect but just the low amplitude of signals. I think that this is mainly caused by the creation and growth of cracks of substrate. After pressing, there were the creep effects, which show the significant increase in AE signal and the signals with the amplitude of 85 dB. In addition, as the distribution of amplitude appears different from the previous one, I think that the damages on the vessel are significant. It seems that there are more creations and growths of cracks in the composite materials and the cutting of reinforcement fiber and peeling of substrate cause the damage mechanism. In addition, the slope of the distribution of amplitude is known to be related to the mechanism of the signals[8], In the load of 70 % or higher, the slop looks similar, meaning that the damage is caused by the similar damage mechanism.

**Figure 13.** Amplitude distribution during load holding

70 Composites and Their Applications

**Figure 11.** Total count during load holding

**Figure 12.** Total signal strength during load holding

*3.1.3. Distribution of AE signal during the holding time* 

I did not use the signals during the initial 2 minutes among the 10 minutes to minimize the flow noise which often includes the initial stages of pressing in the pressure test. After that, I observed the number of hits and creep effect of the sound emission signals which occur during the holding time. Even though there were creation and growth of cracks on substrate under the load of 60 % of the estimated burst pressure, they did not cause much damage to the vessel. But it was found that at the load of 70 % pressure, the vessel was quickly destroyed.[4] Figure 13 shows the distribution of amplitude of signals occurring after 2 minutes of the pressure holding at each load. At the load of 60 % or less, there were not a lot of signals of 60 dB or higher but less than 7 0dB. At this time, I could estimate that at the initial stage of the vessel damage, the composite material substrate wrapping around the vessel were showing some cracks and their growth. I could observe the cracks with my own eyes at the 2000th cycle of fatigue test when the pressure was at 207 bar. The used pressure was more than 30 % of the estimated burst pressure (180 bar). During the burst test, at the

**Figure 14.** Count distribution during load holding

Figure 14 shows the distribution of counts of AE signals coming from each load of pressure. When the signal with counts of 500 or less occurred under the load of 60 %, the damage was caused by the creation of growth of substrate crack under the load of 60 % pressure. At the pressure of 80-90 % where the significant damage is done to vessel, the number of counts was 1000 or more. At the burst stage the number of counts was 2,500 or more.

Acoustic Emission of Composite Vessel 73

 30% 50% 60% 70% 80% 90% 100%

**Figure 16.** Rise time distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**100000**

*3.1.4. Mean frequency of AE signal during the holding time* 

hoop did not affect the burst pressure of the vessel [8].

To estimate the damage with the sound emission signal, the signal of 60 dB or higher is used. Figure 17 shows the distribution of amplitude for hits of 60 dB or higher for each load of pressure. AE time becomes lower until the pressure of 70 % but at 80 %, it becomes faster. As the pressure rises at constant pace during the test, at 70 % pressure, there is no AE signal until 60 % pressures, showing the Kaiser effect. Accordingly, up to 60 % pressure, it looks like that there was no significant damage to the vessel. As the pressure rises up to 70 % that may have damaged the vessel, it looks like that AE happened due to the Felicity effect when the pressure goes up to 80 % as the vessel was damaged during the rise up to 80 %. At the 90 and 100 % pressure, there is the Felicity effect. Particularly, in the pressure range of up to 100 %, which experienced the load of 90 % or higher, there is AE signals of 60 dB or more from the beginning, showing that the damage is significant. In the actual burst test, the damage mechanism of Type II vessel, or the matrix crack in the direction of initial hoop occurs and then the creation of cracks and its growth and the interlayer separation (which belongs to matrix crack but has different damage mode) occur [4]. At the load of 60 %, the matrix crack in the direction of hoop was shown. But, the matrix crack in the direction of

**0 100 200 300 400 500 600 700 800 900 1000 1100**

*s*)

Rise time (

Figure 18 shows the count of hits where sound exceeds 60 dB at each stage of burst test. The rate decreases from 7 % to 1 % during the pressure of 30 % to 60 %. Then, the signals of 60 dB or higher rapidly go up to 2 0% at the load of 70 %. The rate goes up to 30 % at the load of 100 %. The signals of 60 dB or higher at an early stage may be caused by the creation of matrix crack on the composite materials. The reduction thereafter may be caused by the low amplitude (60 dB or less) rather than the creation of crack. At the load of 70 % or higher, there are not only the matrix cracks but also the damages by the new damage mechanism such as the cut of fibers, increasing the signals of 60 dB or higher. The measurement of the

count of hits which are 60 dB or higher is a good tool to assess the damage.

Figure 15 shows that the duration of signal occurs with the range of 500 μs at the load of up to 70 % and that the signal of over 10000 μs started to appear at the load of 80 % or higher. At the load of 90 %, the 20000 μs appears during the final burst stage where the signals of 25000 ~ 50000 μs appeared.

**Figure 15.** Duration distribution during load holding

Figure 16 shows the rise time of 100 μs or less and the signal of 100-200 μs at the load of up to the initial 30 %. As we understand the distribution of the amplitude from the fatigue test of 20,000 cycles, there may be no mechanism other than the creation and growth of cracks at the load of 30 % pressure. It means that the signal of 200 μs or less is caused by the creation and growth of cracks of the vessel materials. Even though there were 1 and 4 signals of 300 μs or higher at the load of 50-60 %, they are negligible considering the total number of signals. It seems that the damage was mainly caused by creation and growth of cracks on composite materials rather than by new damage mechanisms. At the load of 70 % pressure, the signal of 250-450 μs, which is longer than at previous load stage appeared, meaning that there may be new damage mechanism. It seems that it was caused by the additional damage mechanism such as the cutting of reinforcement fibers and interlayer peeling due to the increase in internal pressures. At the load of 80-90 %, the rise time of 500 μs or more is observed. It may be affected by the composite damage mechanism such as the growth of existing cracks, cutting of reinforcement fiber and inter-layer separation happening at the neighboring location. This trend is true of the distribution of counts and durations as specified above. The observation of the sound emission signals coming out of the neighboring location through the composite damage mechanism would be a good tool to assess the vessel.

**Figure 16.** Rise time distribution during load holding

25000 ~ 50000 μs appeared.

**Figure 15.** Duration distribution during load holding

**1**

**10**

**100**

Number of hits

**1000**

**10000**

**100000**

Figure 14 shows the distribution of counts of AE signals coming from each load of pressure. When the signal with counts of 500 or less occurred under the load of 60 %, the damage was caused by the creation of growth of substrate crack under the load of 60 % pressure. At the pressure of 80-90 % where the significant damage is done to vessel, the number of counts

Figure 15 shows that the duration of signal occurs with the range of 500 μs at the load of up to 70 % and that the signal of over 10000 μs started to appear at the load of 80 % or higher. At the load of 90 %, the 20000 μs appears during the final burst stage where the signals of

Figure 16 shows the rise time of 100 μs or less and the signal of 100-200 μs at the load of up to the initial 30 %. As we understand the distribution of the amplitude from the fatigue test of 20,000 cycles, there may be no mechanism other than the creation and growth of cracks at the load of 30 % pressure. It means that the signal of 200 μs or less is caused by the creation and growth of cracks of the vessel materials. Even though there were 1 and 4 signals of 300 μs or higher at the load of 50-60 %, they are negligible considering the total number of signals. It seems that the damage was mainly caused by creation and growth of cracks on composite materials rather than by new damage mechanisms. At the load of 70 % pressure, the signal of 250-450 μs, which is longer than at previous load stage appeared, meaning that there may be new damage mechanism. It seems that it was caused by the additional damage mechanism such as the cutting of reinforcement fibers and interlayer peeling due to the increase in internal pressures. At the load of 80-90 %, the rise time of 500 μs or more is observed. It may be affected by the composite damage mechanism such as the growth of existing cracks, cutting of reinforcement fiber and inter-layer separation happening at the neighboring location. This trend is true of the distribution of counts and durations as specified above. The observation of the sound emission signals coming out of the neighboring location through the

**0 10000 20000 30000 40000 50000**

*s***)**  30% 50% 60% 70% 80% 90% 100%

Duration **(**

composite damage mechanism would be a good tool to assess the vessel.

was 1000 or more. At the burst stage the number of counts was 2,500 or more.

#### *3.1.4. Mean frequency of AE signal during the holding time*

To estimate the damage with the sound emission signal, the signal of 60 dB or higher is used. Figure 17 shows the distribution of amplitude for hits of 60 dB or higher for each load of pressure. AE time becomes lower until the pressure of 70 % but at 80 %, it becomes faster. As the pressure rises at constant pace during the test, at 70 % pressure, there is no AE signal until 60 % pressures, showing the Kaiser effect. Accordingly, up to 60 % pressure, it looks like that there was no significant damage to the vessel. As the pressure rises up to 70 % that may have damaged the vessel, it looks like that AE happened due to the Felicity effect when the pressure goes up to 80 % as the vessel was damaged during the rise up to 80 %. At the 90 and 100 % pressure, there is the Felicity effect. Particularly, in the pressure range of up to 100 %, which experienced the load of 90 % or higher, there is AE signals of 60 dB or more from the beginning, showing that the damage is significant. In the actual burst test, the damage mechanism of Type II vessel, or the matrix crack in the direction of initial hoop occurs and then the creation of cracks and its growth and the interlayer separation (which belongs to matrix crack but has different damage mode) occur [4]. At the load of 60 %, the matrix crack in the direction of hoop was shown. But, the matrix crack in the direction of hoop did not affect the burst pressure of the vessel [8].

Figure 18 shows the count of hits where sound exceeds 60 dB at each stage of burst test. The rate decreases from 7 % to 1 % during the pressure of 30 % to 60 %. Then, the signals of 60 dB or higher rapidly go up to 2 0% at the load of 70 %. The rate goes up to 30 % at the load of 100 %. The signals of 60 dB or higher at an early stage may be caused by the creation of matrix crack on the composite materials. The reduction thereafter may be caused by the low amplitude (60 dB or less) rather than the creation of crack. At the load of 70 % or higher, there are not only the matrix cracks but also the damages by the new damage mechanism such as the cut of fibers, increasing the signals of 60 dB or higher. The measurement of the count of hits which are 60 dB or higher is a good tool to assess the damage.

Acoustic Emission of Composite Vessel 75

related to the source mechanism. It shows the creation and growth of matrix crack at the load of up to 60 % and the additional damage mechanism such as the cut of fiber at the load of 70 % or higher. The reverberation frequency shows a little difference between below and above 70% but is constant in most stages while the average frequency was also constant

**20 30 40 50 60 70 80 90 100 110**

**load stage (%)**

**20 30 40 50 60 70 80 90 100 110**

**load stage (%)**

**20 30 40 50 60 70 80 90 100 110**

**load stage (%)**

with a little rise at the load of initial 30 % pressure.

**200**

**Figure 19.** Mean initial frequency during each loading stage

**0**

**50**

**100**

**initial frequency(kHz)**

**150**

**Figure 20.** Mean reverberation frequency during each loading stage

**0**

**50**

**100**

**reverbration frequency(kHz)**

**150**

**200**

**Figure 21.** Mean average frequency during each loading stage

**0**

**50**

**100**

**average frquency(kHz)**

**150**

**200**

**Figure 17.** Amplitude distribution during loading at each loading stage (over 60 dB)

**Figure 18.** Ratio of hits over 60 dB / total hits during each loading stage

Figure 19-21 show the mean initial, reverberation and average frequency of AE signal at each load stage. The frequency is obtained not from the analysis of wave form but from the duration and counts. The frequency shows the difference between above and below 70 % load. As for the initial frequency, it did not show a big change at the load of up to 60 % or 100 kHz but it goes up at the load of 70 % until going down a little. This is likely to be related to the source mechanism. It shows the creation and growth of matrix crack at the load of up to 60 % and the additional damage mechanism such as the cut of fiber at the load of 70 % or higher. The reverberation frequency shows a little difference between below and above 70% but is constant in most stages while the average frequency was also constant with a little rise at the load of initial 30 % pressure.

**Figure 19.** Mean initial frequency during each loading stage

74 Composites and Their Applications

**Figure 17.** Amplitude distribution during loading at each loading stage (over 60 dB)

30

50

60

70

80

90

100

**amplitude(dB)**

0 300 600 900 1200 1500

**time(sec)**

**Figure 18.** Ratio of hits over 60 dB / total hits during each loading stage

**0**

**10**

**20**

**hits over 60 dB/total hits (%)**

**30**

**40**

Figure 19-21 show the mean initial, reverberation and average frequency of AE signal at each load stage. The frequency is obtained not from the analysis of wave form but from the duration and counts. The frequency shows the difference between above and below 70 % load. As for the initial frequency, it did not show a big change at the load of up to 60 % or 100 kHz but it goes up at the load of 70 % until going down a little. This is likely to be

**0 20 40 60 80 100**

**load stage(%)**

**Figure 20.** Mean reverberation frequency during each loading stage

**Figure 21.** Mean average frequency during each loading stage

## **3.2. Fatigue test**

## *3.2.1. Identification and Verification of Acoustic Emission Location*

Figure 22 shows the results of measuring the elastic wave speed of the artificial acoustic emission source for the acoustic emission location on the fiber-wrapped composite material. As shown on the picture, the speed of longitudinal elastic wave was 4512 m/sec and the elastic wave speed of the wrapped direction was 5689 m/sec showing the characteristic of anisotropy. Thus, the location was identified using anisotropic vessel source location that uses the difference of time for the acoustic emission signal is to be reached.

Acoustic Emission of Composite Vessel 77

thickness of the wrapping composite material. The direction of the realized defect was

Figure 25 shows the average number of hit per sensor during the fatigue test of vessel. The vessel with the artificial defect has more number of hits compared to the sound vessel and when you see the tendency in increase and decrease, the number of hits for transverse defect vessel and sound vessel decreases as the number of cycle increases; however, the number of hits for longitudinal defect vessel decreases in 4000 cycle and then increases and then

The reason the number of hit in the early stages is big is related to the initiation and growth of the matrix rupture in the comparatively weak areas within the vessel. The number of hits decrease and the initiation and growth of ruptures are comparatively slowed until sufficient

In the case of longitudinal defects vessel, the amount of resilience needed to bring about progress in the defect (growth of rupture) is comparatively smaller than other vessel, so in the 8000th and 12000th fatigue test, the number of hits is bigger than in other vessel and because there is a need for another accumulation of resilience, the number of hits decrease afterwards. Such phenomena are clearly distinct in the composite material as mentioned in the introduction and in the early stages of damage, resilience increases like the number of hits and when there are some increases in damage, the resilience afterwards is comparatively low.

**Figure 25.** The number of hits per channel during fatigue test for artificial defect and sound vessel

Figure 26 shows the number of events extracted during the fatigue test on longitudinal defect and transverse defect vessel. An event shows the number of sources calculating the

amount of resilience is stored to bring about new initiation and growth.

longitudinal and transverse, two types of artificial defects.

**Figure 24.** Schematic diagram of artificial defect

decreases in 8000 and 12000 cycles.

**Figure 22.** The elastic wave velocity with degree between propagation and wrapping direction

Figure 23 is the location of the sensor and the result of the identification test of the location using the artificial acoustic emission source. The four sensors were attached in staggered locations in channel 5, 6, 7, 8 and channel 7 refers to the backside of 8. The acoustic emission source is located diagonally in equal intervals between channel 5 and 8 and between channel 6 and 8.

**Figure 23.** Confirm of Source location

## *3.2.2. Fatigue Test and Location of Artificial Defect*

Figure 24 shows the size of the artificial defect realized on the composite material which is wrapped and the length is 50 mm, width 3 mm, and the depth is 3 mm which is 50 % of the thickness of the wrapping composite material. The direction of the realized defect was longitudinal and transverse, two types of artificial defects.

**Figure 24.** Schematic diagram of artificial defect

76 Composites and Their Applications

*3.2.1. Identification and Verification of Acoustic Emission Location* 

uses the difference of time for the acoustic emission signal is to be reached.

**Figure 22.** The elastic wave velocity with degree between propagation and wrapping direction

Figure 23 is the location of the sensor and the result of the identification test of the location using the artificial acoustic emission source. The four sensors were attached in staggered locations in channel 5, 6, 7, 8 and channel 7 refers to the backside of 8. The acoustic emission source is located diagonally in equal intervals between channel 5 and 8 and between

Figure 24 shows the size of the artificial defect realized on the composite material which is wrapped and the length is 50 mm, width 3 mm, and the depth is 3 mm which is 50 % of the

Figure 22 shows the results of measuring the elastic wave speed of the artificial acoustic emission source for the acoustic emission location on the fiber-wrapped composite material. As shown on the picture, the speed of longitudinal elastic wave was 4512 m/sec and the elastic wave speed of the wrapped direction was 5689 m/sec showing the characteristic of anisotropy. Thus, the location was identified using anisotropic vessel source location that

**3.2. Fatigue test** 

channel 6 and 8.

**Figure 23.** Confirm of Source location

*3.2.2. Fatigue Test and Location of Artificial Defect* 

Figure 25 shows the average number of hit per sensor during the fatigue test of vessel. The vessel with the artificial defect has more number of hits compared to the sound vessel and when you see the tendency in increase and decrease, the number of hits for transverse defect vessel and sound vessel decreases as the number of cycle increases; however, the number of hits for longitudinal defect vessel decreases in 4000 cycle and then increases and then decreases in 8000 and 12000 cycles.

The reason the number of hit in the early stages is big is related to the initiation and growth of the matrix rupture in the comparatively weak areas within the vessel. The number of hits decrease and the initiation and growth of ruptures are comparatively slowed until sufficient amount of resilience is stored to bring about new initiation and growth.

In the case of longitudinal defects vessel, the amount of resilience needed to bring about progress in the defect (growth of rupture) is comparatively smaller than other vessel, so in the 8000th and 12000th fatigue test, the number of hits is bigger than in other vessel and because there is a need for another accumulation of resilience, the number of hits decrease afterwards. Such phenomena are clearly distinct in the composite material as mentioned in the introduction and in the early stages of damage, resilience increases like the number of hits and when there are some increases in damage, the resilience afterwards is comparatively low.

**Figure 25.** The number of hits per channel during fatigue test for artificial defect and sound vessel

Figure 26 shows the number of events extracted during the fatigue test on longitudinal defect and transverse defect vessel. An event shows the number of sources calculating the

location of the acoustic emission source within vessel using the acoustic emission signal that hit the sensor and in the case transverse defect vessel which includes a transverse defect, the number of events is noticeably decreased as the number of cycle increases, but in the case of longitudinal defect vessel which has longitudinal defect, the number of events increases a lot in 8000th cycle and does not change much as the number of cycle increases.

Acoustic Emission of Composite Vessel 79

**Figure 27.** The ratio of events/ hits per sensor during fatigue test for artificial defect vessel

shown clearly.

steel liner.

*3.2.3. AE parameters during fatigue test* 

In the case of c), in the 8000th fatigue cycle, 108 events were clustered around the artificial defect and as mentioned in the explanation in Figure 27 more than 50% of the occurred hits were signals related to the artificial defect. Afterwards, events that occurred in d) ~f) were those that mostly occurred near the artificial defect so we can acknowledge that the damage on the composite material is progressed around the artificial defect. Figure 29 shows the surroundings of the longitudinal artificial defect after the 20000th cycle and shows that at the end of the defect, there is a matrix rupture progressing in a hoop-direction and although not clear in the picture, at the end of the depth in the artificial defect, delaminating was observed on the overall defect. In the case of vessel with transverse defects, after the 8,000th fatigue test, less than 1 event was created and the location of the artificial defect could not be

On the other hand, after the 20000th fatigue test, in the burst test, the location of the burst is marked in c) of Figure 28 and events are also observed in a), d), e), and f). The source of the acoustic emission signal is assumed to be the fatigue rupture in the weak areas of the steel liner rather than in the composite material. The final burst in the case of the longitudinal defect accompanies matrix rupture and delaminating as mentioned above during the fatigue test and burst test so the whole vessel area, which is the length of the defect, is thinner in terms of thickness like the depth of the defect and thus is weaker than other areas and it is thought that it burst at the final burst location in which the fatigue rupture occurred in the

Figure 30 shows the relationship between the amplitude of the signal occurring during the early three cycles and the rise time, and can be clearly distinguished as around 10 μs and over 100 μs and the grey mark shows the rise time while holding the load during the three cycles and also at 90 %, which is the highest, has a rise time of about 10 μs. Generally while load holding, it can be inferred that there is likely to be growth of an existent crack rather

**Figure 26.** The number of events during fatigue test for artificial defect vessel

Figure 27 compares the number of hit per sensor and the number of events using Figure 25 and 26 and shows the event/hit ratio according to the increase in the number of cycle. This is a figure that shows the % of the number of hits that can precisely show the source among the total number of hits. In the case of longitudinal defect vessel, it was 41.8 % in the 4000th cycle, 55.7 % in the 8000th cycle. An event can be calculated using the difference in time that the resilience from the source (acoustic emission signal) sent through the walls of the vessel reaches the sensor and has to be extracted from at least 3 sensors. Hits that do not go by such standards cannot be used to calculate events and if the location was identified by simulation or if the signal is weak or is static, it cannot be recorded as an event. In the case of longitudinal defect vessel, the number of hits is small in the 4000th and 8000th cycle but the growth of the rupture is relatively easy and it sends hits to at least 3 sensors with signals with sufficient amplitude so the number of event/hit rate increases.

Figure 28 is the result of the location due to the acoustic emission test performed during the fatigue test on longitudinal defect vessel. Figure 28 (a) shows the location of 25 events during the first 3 fatigue test cycles. It does not show the location of artificial defects but shows the overall looks on the whole vessel. Such results are not shown in pictures but also in transverse defects vessel, 49 events in Figure 26 shows looks like a) on the whole vessel and it seems that signals were created on the weakest areas of the whole vessel when the first pressure was put. In the case of b), after the 4,000th cycle, in the 3rd fatigue test, 23 events were shown to be clustered around the artificial defect. As for the vessel with transverse defects, after the 4,000th fatigue test cycle, in the 3rd fatigue test, 12 events were created but they were all over the whole vessel so we could not show the location of the artificial defect.

**Figure 27.** The ratio of events/ hits per sensor during fatigue test for artificial defect vessel

In the case of c), in the 8000th fatigue cycle, 108 events were clustered around the artificial defect and as mentioned in the explanation in Figure 27 more than 50% of the occurred hits were signals related to the artificial defect. Afterwards, events that occurred in d) ~f) were those that mostly occurred near the artificial defect so we can acknowledge that the damage on the composite material is progressed around the artificial defect. Figure 29 shows the surroundings of the longitudinal artificial defect after the 20000th cycle and shows that at the end of the defect, there is a matrix rupture progressing in a hoop-direction and although not clear in the picture, at the end of the depth in the artificial defect, delaminating was observed on the overall defect. In the case of vessel with transverse defects, after the 8,000th fatigue test, less than 1 event was created and the location of the artificial defect could not be shown clearly.

On the other hand, after the 20000th fatigue test, in the burst test, the location of the burst is marked in c) of Figure 28 and events are also observed in a), d), e), and f). The source of the acoustic emission signal is assumed to be the fatigue rupture in the weak areas of the steel liner rather than in the composite material. The final burst in the case of the longitudinal defect accompanies matrix rupture and delaminating as mentioned above during the fatigue test and burst test so the whole vessel area, which is the length of the defect, is thinner in terms of thickness like the depth of the defect and thus is weaker than other areas and it is thought that it burst at the final burst location in which the fatigue rupture occurred in the steel liner.

#### *3.2.3. AE parameters during fatigue test*

78 Composites and Their Applications

location of the acoustic emission source within vessel using the acoustic emission signal that hit the sensor and in the case transverse defect vessel which includes a transverse defect, the number of events is noticeably decreased as the number of cycle increases, but in the case of longitudinal defect vessel which has longitudinal defect, the number of events increases a

Figure 27 compares the number of hit per sensor and the number of events using Figure 25 and 26 and shows the event/hit ratio according to the increase in the number of cycle. This is a figure that shows the % of the number of hits that can precisely show the source among the total number of hits. In the case of longitudinal defect vessel, it was 41.8 % in the 4000th cycle, 55.7 % in the 8000th cycle. An event can be calculated using the difference in time that the resilience from the source (acoustic emission signal) sent through the walls of the vessel reaches the sensor and has to be extracted from at least 3 sensors. Hits that do not go by such standards cannot be used to calculate events and if the location was identified by simulation or if the signal is weak or is static, it cannot be recorded as an event. In the case of longitudinal defect vessel, the number of hits is small in the 4000th and 8000th cycle but the growth of the rupture is relatively easy and it sends hits to at least 3 sensors with signals

Figure 28 is the result of the location due to the acoustic emission test performed during the fatigue test on longitudinal defect vessel. Figure 28 (a) shows the location of 25 events during the first 3 fatigue test cycles. It does not show the location of artificial defects but shows the overall looks on the whole vessel. Such results are not shown in pictures but also in transverse defects vessel, 49 events in Figure 26 shows looks like a) on the whole vessel and it seems that signals were created on the weakest areas of the whole vessel when the first pressure was put. In the case of b), after the 4,000th cycle, in the 3rd fatigue test, 23 events were shown to be clustered around the artificial defect. As for the vessel with transverse defects, after the 4,000th fatigue test cycle, in the 3rd fatigue test, 12 events were created but they were all over the whole vessel so we could not show the location of the artificial defect.

lot in 8000th cycle and does not change much as the number of cycle increases.

**Figure 26.** The number of events during fatigue test for artificial defect vessel

with sufficient amplitude so the number of event/hit rate increases.

Figure 30 shows the relationship between the amplitude of the signal occurring during the early three cycles and the rise time, and can be clearly distinguished as around 10 μs and over 100 μs and the grey mark shows the rise time while holding the load during the three cycles and also at 90 %, which is the highest, has a rise time of about 10 μs. Generally while load holding, it can be inferred that there is likely to be growth of an existent crack rather

On ris fat of av μs aft

n the other hand, e time noticeably tigue test, there a the cracks created erage rise time of and afterwards ter the 12000th cy

 in the case of a s y decreased and are not many initi d in early stages. f related hits for e decreased to 34 ycle and then decr

sound cylinder, a increased little u iations of new ma On the other han events occurred d μs after the 800 reased again.

Acoustic Em

mission of Composit

e Vessel 81

verage 4000th growth cts, the was 56 about

atigue test, the av h test. After the 4 you can see the g vessel with defec hree fatigue test w creased to 82 μs

nals of events occ and is clustered a d that the initiatio Especially, a high ect as shown in b) ke a) or d) so the g

curring around on and her rise ), c), e), growth

curring ring in

n other around vessel h other

ls, and vity of sensors

e of the signal occ of signals occurr

curs far away in e cracks occurs a trix crack in the nals overlap with

rs to detect signal cause the sensitiv d using resonant s

racks.

after the 4000th fa up to the 20000th atrix cracks and y nd, in the case of during the first th 0th cycle and inc

for sound vessel

plitude of the sign hits and rise time 00 μs. It is judged he overall vessel. r around the defe ll on the vessel lik the initiation of cr

average rise time verage rise time

vidual cracks occ rowth of multiple he growth of ma nd generated sign

.wideband sensor

als. However, bec s are also detected

ring initial 3 cycles

between the amp ssel and related h red around 100-8 n weak areas of th ce forms a cluster is scattered overal wer rise time than t

atigue cycle, the a igher than the av

growth of indiv ith defects, the gr rgy needed for th sound vessel. An ignal is extended. equency, we use of detected signa nt sensors, signals

me distribution dur

the relationship b ee cycles of the ve and is also scatter ack will happen in ed when the sourc when the source i e defect has a low

after the 4000th fa e shown to be hi

ound vessel, the case of vessel wi e the elastic ener ller than that of s the rise time of si to analysis of fre urier transform o all behind resonan

**Fig**

**gure 30.** The rise tim

gure 31 a) shows uring the first thre μs in the figure a owth of matrix cra me can be observe of Figure 31 than w cracks around the

n the other hand, defect vessel are und vessel.

the case of a so acks. But, in the c e defects because ith defect is smal gnals. Therefore, t enerally, in order alyze by the Fou deband sensors fa

Fig du 10 gro tim f) o of

On in sou

In cra the wi sig Ge an wi

**Figure 28.** The result of source location with cycle for longitudinal defect : a)0, b)4000, c)8000, d)12000, e)16000, f)20000

**Figure 29.** Longitudinal defect and matrix crack after 20000 cycle fatigue test

than initiation of a new matrix crack and thus can be said that it is a growth of crack around 10 μs. And the rise time of AE signal which occurs during the initiation of a matrix crack can be said to be more than 100μs. This accord to the result that the rise time of the AE signal occurring during the initiation of matrix crack during the burst test is around 100 μs and that the rise time of the AE signal occurring during the growth of the crack is around 10 μs. [4]

On ris fat of av μs aft n the other hand, e time noticeably tigue test, there a the cracks created erage rise time of and afterwards ter the 12000th cy in the case of a s y decreased and are not many initi d in early stages. f related hits for e decreased to 34 ycle and then decr sound cylinder, a increased little u iations of new ma On the other han events occurred d μs after the 800 reased again. after the 4000th fa up to the 20000th atrix cracks and y nd, in the case of during the first th 0th cycle and inc atigue test, the av h test. After the 4 you can see the g vessel with defec hree fatigue test w creased to 82 μs verage 4000th growth cts, the was 56 about

**Fig gure 30.** The rise tim me distribution dur ring initial 3 cycles for sound vessel

80 Composites and Their Applications

e)16000, f)20000

[4]

**Figure 28.** The result of source location with cycle for longitudinal defect : a)0, b)4000, c)8000, d)12000,

Matrix crack

Artificial defect (Longitudinal direction)

than initiation of a new matrix crack and thus can be said that it is a growth of crack around 10 μs. And the rise time of AE signal which occurs during the initiation of a matrix crack can be said to be more than 100μs. This accord to the result that the rise time of the AE signal occurring during the initiation of matrix crack during the burst test is around 100 μs and that the rise time of the AE signal occurring during the growth of the crack is around 10 μs.

**Figure 29.** Longitudinal defect and matrix crack after 20000 cycle fatigue test

Fig du 10 gro tim f) o of gure 31 a) shows uring the first thre μs in the figure a owth of matrix cra me can be observe of Figure 31 than w cracks around the the relationship b ee cycles of the ve and is also scatter ack will happen in ed when the sourc when the source i e defect has a low between the amp ssel and related h red around 100-8 n weak areas of th ce forms a cluster is scattered overal wer rise time than t plitude of the sign hits and rise time 00 μs. It is judged he overall vessel. r around the defe ll on the vessel lik the initiation of cr nals of events occ and is clustered a d that the initiatio Especially, a high ect as shown in b) ke a) or d) so the g racks. curring around on and her rise ), c), e), growth

On in sou n the other hand, defect vessel are und vessel. after the 4000th fa e shown to be hi atigue cycle, the a igher than the av average rise time verage rise time e of the signal occ of signals occurr curring ring in

In cra the wi sig the case of a so acks. But, in the c e defects because ith defect is smal gnals. Therefore, t ound vessel, the case of vessel wi e the elastic ener ller than that of s the rise time of si growth of indiv ith defects, the gr rgy needed for th sound vessel. An ignal is extended. vidual cracks occ rowth of multiple he growth of ma nd generated sign curs far away in e cracks occurs a trix crack in the nals overlap with n other around vessel h other

Ge an wi enerally, in order alyze by the Fou deband sensors fa to analysis of fre urier transform o all behind resonan equency, we use of detected signa nt sensors, signals .wideband sensor als. However, bec s are also detected rs to detect signal cause the sensitiv d using resonant s ls, and vity of sensors and the frequency is calculated by rise time, duration, and count. Frequency by calculation was used and analyzed in this research because resonant sensors were used for the source location.

Acoustic Emission of Composite Vessel 83

Figure 32 shows the distribution of initial frequency versus the amplitude with fatigue cycle but it is dispersed around 100-200 kHz, which is unrelated to the fatigue cycle. It is in accord with the result that the average frequency of signal which is generated due to initiation and growth of matrix crack is around 100 kHz in the burst test using resonant type sensors [7].

(a) (b)

(c) (d)

**Figure 32.** The initial frequency distribution of hits connected with event during fatigue cycle for

(e) (f)

longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

**Figure 31.** The rise time distribution of hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Figure 32 shows the distribution of initial frequency versus the amplitude with fatigue cycle but it is dispersed around 100-200 kHz, which is unrelated to the fatigue cycle. It is in accord with the result that the average frequency of signal which is generated due to initiation and growth of matrix crack is around 100 kHz in the burst test using resonant type sensors [7].

82 Composites and Their Applications

and the frequency is calculated by rise time, duration, and count. Frequency by calculation was used and analyzed in this research because resonant sensors were used for the source location.

(a) (b)

(c) (d)

**Figure 31.** The rise time distribution of hits connected with event during fatigue cycle for longitudinal

(e) (f)

defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

**Figure 32.** The initial frequency distribution of hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Acoustic Emission of Composite Vessel 85

on signals relating to all events of Figure 33 c) was 73 kHz and these accords to the fact that the average reverberation frequency is between 50 - 75 kHz in all stage of burst which includes the damage mechanism of composite materials in the cast of burst test.[7] On the other hand, the reverberation frequency during the burst test of the same vessel was known to occur in all stage of the burst test in the range of 150 - 350 kHz[8], and in such case, because it includes the mechanism of all damage, it is hard to differentiate damage mechanism as a frequency. There were 23 event signals that had a reverberation frequency in the 150 - 350 kHz area in Figure 33 c) and 16 of them, which are 70 %, occurred in the matrix crack area of Figure 29, which is an observation of artificial defects after the 20000th fatigue test. More than 90 % of the related 44 hits(150 - 350 kHz) were signals with rise time lower than 100 μs and average rise time of 31 μs. As mentioned in 3.2.1, it is assumed that it

Figure 34 shows the amplitude distribution of accumulated hits according to the number of fatigue test and its slope has two types of shapes. Generally, the slope in the amplitude distribution of accumulated hit is known to be related to the mechanism of the source [8] and in vessel with defects as used in the experiment, as mentioned in the previous chapter, it includes mechanisms such as the initiation of matrix crack, growth of the created cracks (including delaminating), and the initiation and growth of liner fatigue

Initiation and growth of liner fatigue crack will be mentioned in the following chapter and in the view of the estimated result of the damage mechanism according to the number of fatigue explained in the previous chapter, the case of the initiation of matrix cracks is estimated to have a slope of ①(0.04) and the growth of cracks, a slope of

In order to analyze the characteristics of acoustic emission signals that occurred in the final burst position, event signals observed around the final burst location within 200 mm hoop direction in terms of length as marked in Figure 29 were analyzed. 18 events were occurred during the 20000th fatigue cycle and the number of related hits was 59. Figure 35 shows accumulated amplitude distribution and there are only 3 that are over 60 dB and 49 of them are below 50 dB. You can see the slope as ③ but if you observe closely, it is possible to observe that the same slope exists as ① in Figure 34 and also that it include the initiation of matrix crack. Seeing it as showing the slope of ③, as a signal according to a sole damage mechanism, it is estimated to be related to liner damage and

Figure 36 shows the distribution of rise time on the amplitude of the signals occurred in the final burst location. There is almost no rise time above 100 μs and is dispersed around 10 μs. It can be thought that the rise time which occurs during the growth of steel liner fatigue

is due to the growth of matrix crack.

cracks.

②(0.12).

the size is 0.06

*3.2.4. Amplitude distribution slop during fatigue test* 

crack is similar to that during the growth of matrix crack.

**Figure 33.** The reverberation frequency distribution of hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Figure 33 shows the distribution of reverberation frequency on amplitude according to the number of fatigue tests but besides the oval area in Figure 33 c) which is the result of the 8000th fatigue test, most are dispersed below 150 kHz. The average reverberation frequency on signals relating to all events of Figure 33 c) was 73 kHz and these accords to the fact that the average reverberation frequency is between 50 - 75 kHz in all stage of burst which includes the damage mechanism of composite materials in the cast of burst test.[7] On the other hand, the reverberation frequency during the burst test of the same vessel was known to occur in all stage of the burst test in the range of 150 - 350 kHz[8], and in such case, because it includes the mechanism of all damage, it is hard to differentiate damage mechanism as a frequency. There were 23 event signals that had a reverberation frequency in the 150 - 350 kHz area in Figure 33 c) and 16 of them, which are 70 %, occurred in the matrix crack area of Figure 29, which is an observation of artificial defects after the 20000th fatigue test. More than 90 % of the related 44 hits(150 - 350 kHz) were signals with rise time lower than 100 μs and average rise time of 31 μs. As mentioned in 3.2.1, it is assumed that it is due to the growth of matrix crack.

## *3.2.4. Amplitude distribution slop during fatigue test*

84 Composites and Their Applications

**Figure 33.** The reverberation frequency distribution of hits connected with event during fatigue cycle

Figure 33 shows the distribution of reverberation frequency on amplitude according to the number of fatigue tests but besides the oval area in Figure 33 c) which is the result of the 8000th fatigue test, most are dispersed below 150 kHz. The average reverberation frequency

for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

Figure 34 shows the amplitude distribution of accumulated hits according to the number of fatigue test and its slope has two types of shapes. Generally, the slope in the amplitude distribution of accumulated hit is known to be related to the mechanism of the source [8] and in vessel with defects as used in the experiment, as mentioned in the previous chapter, it includes mechanisms such as the initiation of matrix crack, growth of the created cracks (including delaminating), and the initiation and growth of liner fatigue cracks.

Initiation and growth of liner fatigue crack will be mentioned in the following chapter and in the view of the estimated result of the damage mechanism according to the number of fatigue explained in the previous chapter, the case of the initiation of matrix cracks is estimated to have a slope of ①(0.04) and the growth of cracks, a slope of ②(0.12).

In order to analyze the characteristics of acoustic emission signals that occurred in the final burst position, event signals observed around the final burst location within 200 mm hoop direction in terms of length as marked in Figure 29 were analyzed. 18 events were occurred during the 20000th fatigue cycle and the number of related hits was 59. Figure 35 shows accumulated amplitude distribution and there are only 3 that are over 60 dB and 49 of them are below 50 dB. You can see the slope as ③ but if you observe closely, it is possible to observe that the same slope exists as ① in Figure 34 and also that it include the initiation of matrix crack. Seeing it as showing the slope of ③, as a signal according to a sole damage mechanism, it is estimated to be related to liner damage and the size is 0.06

Figure 36 shows the distribution of rise time on the amplitude of the signals occurred in the final burst location. There is almost no rise time above 100 μs and is dispersed around 10 μs. It can be thought that the rise time which occurs during the growth of steel liner fatigue crack is similar to that during the growth of matrix crack.

Acoustic Emission of Composite Vessel 87

**Figure 35.** The amplitude distribution of accumulated hits connected with event during fatigue cycle

�

�

We burst the vessel with two types of artificial defect after carrying out 20000 cycles fatigue test on a sound vessel and continuously increasing the pressure where the burst pressure is 590~615 bar with the difference in the pressure of the vessel were within 5 % and thus was

Generally, 20000 cycles of fatigue is equivalent to a vessel used for more than 50 years if you are to put pressure on the vessel once a day although, of course, in the case of a real gas vessel, gas is used as a pressure medium so it may be different from the case in which machine oil is used as a pressure medium, but if the vessel with artificial defects and sound vessel were tested in the same conditions, the burst pressure is shown to be almost the same. Thus, in the case of the size of artificial defect used in this research, the direction of defect is

**Figure 36.** Distribution of rise time on the amplitude of the signals

shown to have almost no effect on the life of the vessel.

*3.2.5. Burst test after 20000 cycles fatigue test* 

irrelevant to the existence of defects.

for longitudinal defect vessel

**Figure 34.** The amplitude distribution of accumulated hits connected with event during fatigue cycle for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

**Figure 35.** The amplitude distribution of accumulated hits connected with event during fatigue cycle for longitudinal defect vessel

**Figure 36.** Distribution of rise time on the amplitude of the signals

#### *3.2.5. Burst test after 20000 cycles fatigue test*

86 Composites and Their Applications

**Figure 34.** The amplitude distribution of accumulated hits connected with event during fatigue cycle

for longitudinal defect vessel: a) 0, b) 4000, c) 8000, d) 12000, e) 16000, f) 20000 cycles

We burst the vessel with two types of artificial defect after carrying out 20000 cycles fatigue test on a sound vessel and continuously increasing the pressure where the burst pressure is 590~615 bar with the difference in the pressure of the vessel were within 5 % and thus was irrelevant to the existence of defects.

Generally, 20000 cycles of fatigue is equivalent to a vessel used for more than 50 years if you are to put pressure on the vessel once a day although, of course, in the case of a real gas vessel, gas is used as a pressure medium so it may be different from the case in which machine oil is used as a pressure medium, but if the vessel with artificial defects and sound vessel were tested in the same conditions, the burst pressure is shown to be almost the same. Thus, in the case of the size of artificial defect used in this research, the direction of defect is shown to have almost no effect on the life of the vessel.

Figure 37 is a picture that shows the burst location in the vessel with the artificial defect. As shown in the picture, we can see that in the case of a vessel with a transverse defect, the final burst location is in the general burst location (cylinder and head area) for a well-constructed type II vessel. However, in the case of a vessel with a longitudinal defect, in both vessels, the final burst location was within the transverse vessel in which the defect was located. We think that this is because in the case of the longitudinal defect, the wrapped fiber is cut in 3 mm depth and in 50 mm length so the effect in which the thickness of the composite material is big, but in the case of the transverse defect, the fiber is cut only in 3 mm depth and in 3 mm length so the effect is small.

Acoustic Emission of Composite Vessel 89

the vessel. If the pressure is over 420 bar, or 70 % of the estimated burst pressure, the damage to the vessel becomes greater, meaning that the creep effect becomes larger. 2. The sound emission signal variables such as mean amplitude, mean rise time, means duration, and rise time amplitude correlation can be obtained when the vessel is damaged at each stage of pressure load. Though the variables were not enough to

3. It was discovered that the total count, and total signal strength at the pressure holding stage were the sound emission variables, which represent the degree of damages on the

4. The rate of number of hits with 60dB or higher in amplitudes in the number of total hits

5. We can estimate the damage mechanism through mean rise time, mean amplitude and

After manufacturing a sound vessel and a vessel with artificial defect for the composite vessel, we executed acoustic emission test during fatigue test and came up with the

1. Vessel with two types of artificial defects (longitudinal and transverse) and a sound vessel was put in 20000 fatigue test and the pressure was continuously increased and then was burst, the burst pressure was 590~615 bar and the differences in pressure on

3. Acoustic Emission Signal, which occurs during the fatigue test, occurred more in vessel with defects rather than in sound vessel, and as the number of fatigue test accumulated, the number of hits increased more in vessel with longitudinal defects than in those with

4. In the case of vessel with longitudinal defects, events were clustered around the artificial defect and more than 50 % of the occurred hits were signals that were related to artificial defects and the source location was precisely found on the defect location but in the case of vessel with transverse defect, events rarely occurred and even if they

5. Longitudinal defect of the vessel created matrix rupture and delaminating of the composite material during the fatigue test and the burst test and the thickness of the whole vessel area became thinner like the length of the defect and thus was weaker than other areas of the vessel. And the final burst was at the location in which the

6. The position of the longitudinal defect was shown well using the identification of acoustic emission location during the fatigue test and the average rise time of acoustic emission signal related to events occurring here was about 30-90 μs and signals with a shorter rise time can be observed more in the growth rather than in the initiation of matrix cracks.

occurred, the source location relevant to a defect did not match.

fatigue rupture of the steel liner occurred.

the vessel were less than 5 % and was not relevant to the existence of defects. 2. There is a correlation between the direction of defect and the final burst location and the longitudinal defect had a greater effect on the final burst location of the vessel rather

evaluate but were effective to estimate the damage mechanism.

is likely to indicate a damage of vessel.

vessel.

**4.2. Fatigue test** 

following conclusion.

than the transverse defect.

transverse defects.

frequency analysis.

In this research, we cannot precisely know how much the depth of the defect has to be in order for the final burst pressure to change; however, the direction of the defect and the final burst location do have a correlation and we can infer that the longitudinal defect has a bigger effect on the final burst location.

**Figure 37.** Position of artificial defect and burst location : a) longitudinal, b) transverse

## **4. Conclusion**

## **4.1. Burst test**

By increasing the loading of pressure up to the expected burst pressure, I obtained the sound emission signal during 10 minutes holding time after loading. Considering that there would be flow noises during the initial 2 minutes, I obtained data for the remaining 8 minutes except the initial 2 minutes to analyze the AE variables.

1. Up to 360 bar, or 60 % of the estimated burst pressure, which is equivalent to 1.8 times of the usage pressure, it seems that there is little creep effect as there is little damage to the vessel. If the pressure is over 420 bar, or 70 % of the estimated burst pressure, the damage to the vessel becomes greater, meaning that the creep effect becomes larger.


## **4.2. Fatigue test**

88 Composites and Their Applications

and in 3 mm length so the effect is small.

bigger effect on the final burst location.

**4. Conclusion** 

**4.1. Burst test** 

Figure 37 is a picture that shows the burst location in the vessel with the artificial defect. As shown in the picture, we can see that in the case of a vessel with a transverse defect, the final burst location is in the general burst location (cylinder and head area) for a well-constructed type II vessel. However, in the case of a vessel with a longitudinal defect, in both vessels, the final burst location was within the transverse vessel in which the defect was located. We think that this is because in the case of the longitudinal defect, the wrapped fiber is cut in 3 mm depth and in 50 mm length so the effect in which the thickness of the composite material is big, but in the case of the transverse defect, the fiber is cut only in 3 mm depth

In this research, we cannot precisely know how much the depth of the defect has to be in order for the final burst pressure to change; however, the direction of the defect and the final burst location do have a correlation and we can infer that the longitudinal defect has a

**Figure 37.** Position of artificial defect and burst location : a) longitudinal, b) transverse

Defect position

a b

minutes except the initial 2 minutes to analyze the AE variables.

By increasing the loading of pressure up to the expected burst pressure, I obtained the sound emission signal during 10 minutes holding time after loading. Considering that there would be flow noises during the initial 2 minutes, I obtained data for the remaining 8

Defect position (back side)

1. Up to 360 bar, or 60 % of the estimated burst pressure, which is equivalent to 1.8 times of the usage pressure, it seems that there is little creep effect as there is little damage to After manufacturing a sound vessel and a vessel with artificial defect for the composite vessel, we executed acoustic emission test during fatigue test and came up with the following conclusion.


7. The initial frequency is distributed around 100-200 kHz and signals with reverberation frequency higher than 150 kHz are related to the growth of matrix cracks.

**Chapter 5** 

© 2012 Liu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Liu et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In this chapter, a technique for delamination identification in laminated composites using the zero-order mode of Lamb waves, i.e., S0 mode and A0 mode, without referring to the baseline data, is described. Through measuring the propagation speed of a wave and the traveling time of a reflected wave from the delamination, the delamination position can be

**Locating Delamination in Composite** 

Yaolu Liu, Alamusi, Jinhua Li, Huiming Ning, Liangke Wu, Weifeng Yuan, Bin Gu and Ning Hu

Additional information is available at the end of the chapter

**of Lamb Waves** 

http://dx.doi.org/10.5772/49991

**1. Introduction** 

studies.

**Laminated Beams Using the Zero-Order Mode** 

To improve the safety and reliability of various engineering structure, it is essential to develop efficient techniques for non-destructive damage detection or structural health monitoring. Lamb wave can travel a long distance in plate-like and shell-like structures made of materials even with high attenuation ratio (e.g. Carbon Fibre/Epoxy Polymer composites). To take this advantage, many researchers have recently explored the possibility of using Lame waves for damage identification [1]. To date, many developed Lamb wavebased techniques are generally based on so called two-stage prediction models by which the difference in the signals between a defective structure and a benchmark (intact structure) can be evaluated. Then, the residual error is easy to be defined no matter what information extracted from the signals is used, such as the information in time domain [2, 3] or frequency domain [4, 5]. Therefore, a benchmark or baseline signal is essential for the detection, which is very reliable and suitable for monitoring the propagation of damage. Also, tremendous efforts have been put to the delamination identification, which could be treated as a problem of inverse pattern recognition using calibrated numerical methods such as artificial neural network [6]. The interaction between Lamb waves and delamination has also been investigated numerically and theoretically [7-9]. However, the complex wave scattering phenomenon in a delamination area has not been clearly understood in these

8. The slope of accumulated amplitude distribution is about 0.04 during the initiation of matrix cracks and is about 0.12 during the growth. However, signals estimated to be liner fatigue crack growth have a slope of 0.06 and a rise time similar to that of during the growth of matrix cracks.

## **Author details**

Hyun-Sup Jee and Jong-O Lee *Korea Institute of Materials Science, South Korea* 

## **5. References**


## **Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves**

Yaolu Liu, Alamusi, Jinhua Li, Huiming Ning, Liangke Wu, Weifeng Yuan, Bin Gu and Ning Hu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/49991

## **1. Introduction**

90 Composites and Their Applications

**Author details** 

**5. References** 

the growth of matrix cracks.

*Korea Institute of Materials Science, South Korea* 

Fuel Tanks, FaAA-SF-R-97-05-04

Japan (ISIJ), 64(14),2019-2028

Composite Structures, 40(2), 149-158

[1] Statics of korean Association for Natural Gas Vehicles (2009)

quality by Acoustic emission, Journal of KSNT, 16(2), 79

emission of composite plate, Journal of KSCM, 18(5), 15-20

pressure gas cylinder, Journal of KSPE, 19(2), 177-186

Composite Science and Technology, 68, 1144-1155

of laminated composite structures, Journal of KSCM, 16(6), 16.

Hyun-Sup Jee and Jong-O Lee

7. The initial frequency is distributed around 100-200 kHz and signals with reverberation

8. The slope of accumulated amplitude distribution is about 0.04 during the initiation of matrix cracks and is about 0.12 during the growth. However, signals estimated to be liner fatigue crack growth have a slope of 0.06 and a rise time similar to that of during

[2] Mark Toughiry (2002) Examination Of The Nondestructive Evaluation Of Composite Gas Cylinders, United States Department of Transportation, NTIAC/A7621-18:CRC-CD8.1, 10 [3] General Motors Corporation (1997) Development of Inspection Technology for NGV

[4] H. S. Jee, J. O. Lee, N. H. Ju and J. K. Lee (2011) Study of acoustic emission parameters

[5] J. O. Lee, J. S. Lee, U. H. Yoon and S. H. Lee (1996) Evaluation of adhesive bonding

[6] H. S. Jee, J. O. Lee, N. H. Ju, J. K. Lee and C. H. So (2011) Development of in-service inspection for type-ll gas cylinder, Proceeding for Spring Conference of KIGAS [7] H. S. Jee, J. O. Lee, N. H. Ju, J. K. Lee and C. H. So (2011) Damage Evaluation for High Pressure Fuel Tank by Analysis of AE Parameters Journal of KSCM, 24(4), 25 [8] S. Yuyama, T. Kishi and Y. Hisamatsu (1982) Detection and Analysis of Crevice Corrosion-SCC Process by the Use of AE Technique, The Iron and Steel Institute of

[9] S. H. Paik, S. H. Park and S. J. Kim (2001) Three dimensional FE analysis of acoustic

[10] J. S. Park, K. S. Kim and H. S. Lee (2003) A study on the acoustic emission characteristics

[11] L. Dong and J. Mistry (1998) Acoustic emission monitoring of composite cylinder,

[12] J. C. Choi, J. S. Jung, C. Kim, Y. Choi and J. H. Yoon (2002) A study on the Development of computer-aided Process planning system for the deep drawing & Ironing of high

[13] Y. Choi, J. H. Yooh, Y. S. Park and J. C. Choi (2004) A study on the Die Design for manufacturing of High Pressure Gas cylinder, Journal of KSPE, 21(7), 153-162 [14] A. Bussiba, M. Kupiec, S. Ifergane, R. Piat and T. Bohlke (2008) Damage evaluation and fracture events sequence in various composites by acoustic emission technique,

during a burst test for CNG vehicle fuel tank, Journal of KSME, 35(9), 1131

frequency higher than 150 kHz are related to the growth of matrix cracks.

To improve the safety and reliability of various engineering structure, it is essential to develop efficient techniques for non-destructive damage detection or structural health monitoring. Lamb wave can travel a long distance in plate-like and shell-like structures made of materials even with high attenuation ratio (e.g. Carbon Fibre/Epoxy Polymer composites). To take this advantage, many researchers have recently explored the possibility of using Lame waves for damage identification [1]. To date, many developed Lamb wavebased techniques are generally based on so called two-stage prediction models by which the difference in the signals between a defective structure and a benchmark (intact structure) can be evaluated. Then, the residual error is easy to be defined no matter what information extracted from the signals is used, such as the information in time domain [2, 3] or frequency domain [4, 5]. Therefore, a benchmark or baseline signal is essential for the detection, which is very reliable and suitable for monitoring the propagation of damage. Also, tremendous efforts have been put to the delamination identification, which could be treated as a problem of inverse pattern recognition using calibrated numerical methods such as artificial neural network [6]. The interaction between Lamb waves and delamination has also been investigated numerically and theoretically [7-9]. However, the complex wave scattering phenomenon in a delamination area has not been clearly understood in these studies.

In this chapter, a technique for delamination identification in laminated composites using the zero-order mode of Lamb waves, i.e., S0 mode and A0 mode, without referring to the baseline data, is described. Through measuring the propagation speed of a wave and the traveling time of a reflected wave from the delamination, the delamination position can be

© 2012 Liu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Liu et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

accurately identified. Moreover, to understand the complex interaction of Lamb waves with a long delamination damage, the numerical simulations have been carried out.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 93

*tNf*

 

(1)

two signals can yield the pure A0 mode. In experiments, an excitation signal in the following

0.5[1 cos(2 / )sin(2 )], / ( ) 0, / *ft N ft t N f P t*

 

**Figure 1.** Schematic view of models for experiments using S0 mode (unit: mm)

アクチュエータ(PZT) センサ(PZT)

1005mm

**Figure 2.** Schematic view of models for experiments using A0 mode (unit: mm)

**Table 1.** Material properties of PZT and CFRP lamina

Material properties

PZT E11=62 GPa, E33=49 GPa, d33=472 pC/N, d31=-210 pC/N, *ρ*=7500 kg/m3

*ν*12=*ν*13=0.3, *ν*23=0.45, *ρ*=1600 kg/m3

4.8

擬似はく離(10/11層目)

10th and 11th plies

10

CFRP lamina E11=115 GPa, E22=E33=9 GPa, G12=G23=5.5 GPa, G13=3 GPa,

 

where *f* is the central frequency in Hz and *N* is the number of sinusoidal cycles within a pulse. In this experiment, the signal with *f*=100 kHz (Fig. 1 for S0 mode) or *f*=50 kHz (Fig. 2 for A0 mode) and *N*=5 was used and the electrical voltage on the actuator was 10 V. The

Without the baseline data, it is still easy to estimate the arrival of the reflections from the beam boundaries, provided that the wave propagation speed and the dimensions of the beam are known. Then, if the reflected signal from the delamination is not overlapped with the reflections from the boundaries, it can be simply collected by the sensor. Therefore, it is still a technical challenge when the delamination is located close to the beam boundaries and the reflected signal of the delamination is completely overlapped with the reflections

<sup>x</sup> <sup>z</sup> <sup>y</sup> <sup>30</sup>

Sensor (PZT) Delamination between

455 320 200

Eq. (1), was adopted to generate S0 or A0 mode.

material properties of PZT are listed in Table 1.

Actuator (PZT)

from the boundaries.

z

## **2. Detective technique without the baseline data**

## **2.1. Experiments and numerical analyses**

In general, compared with S0mode, the attenuation of A0 mode in structures is more severe. Here, S0 mode corresponds to an axial deformation mode while A0 mode corresponds to a flexural deformation mode. Therefore, it should be paid more attention to the detective capability of both modes for a delamination damage in the different interfaces along the thickness direction. Moreover, the experimental setup of S0 mode is slightly different with that of A0 mode.

## *2.1.1. Materials and experimental procedure*

For S0 mode, as shown in Fig. 1, a CFRP laminated composite beam of stack sequence of [010/9012/010] was used. An artificial delamination damage with different lengths for several cases, i.e., 30 mm, 20 mm and 10 mm, respectively, was intentionally created at the interface between the 10th and 11th plies by inserting a Teflon film with a thickness of 25 μm. A PZT actuator was attached on the top surface of the left end of the beam. The PZT actuator had a diameter of 10 mm and a thickness of 0.5 mm. In this case, the generated waves from the actuator merged into the reflected waves from the left end of beam, which leads to a simpler signal. And the same PZT unit was used as a sensor to pick up the reflected wave from the delamination damage. Because the propagation speed of S0 mode is much higher than that of A0 mode, confirmed as 4 times higher in the used composites from our testing, it is expected that in a specified time domain, no A0 mode will be collected due to its slow speed, making the analysis of S0 modes much easier.

For the A0 mode, a similar composite beam was used, as shown in Fig. 2. Two kinds of stack sequence of the laminated beam were used, i.e., [010/9012/010] and [012/04/04/012]. Note that [012/04/04/012] is a unidirectional laminate, and it can be simply expressed as [032]. To simplify the description of delamination later, this expression is used. Like the case of S0 mode, a delamination damage with different lengths, i.e., 30 mm, 20 mm and 10 mm, was intentionally created at the interface between two plies by inserting a Teflon film with a thickness of 25 μm. The distance between the center of the delamination and the left end of beam is 790 mm. To generate the A0 mode, two actuators were attached on the top and bottom surfaces of the beam with applied out-of-phase voltages because the difference of signals in the two actuators can produce the pure *A*0 mode, as shown in Fig. 2. Although this arrangement can generate a relatively pure A0 mode, there are still reflections in S0 mode due to the mode change caused by the scattering between the Lamb waves and the delamination. To pick up the pure *A0* mode, two PZT sensors were attached. We know that two components of A0 mode in the two sensor signals are of the electrical charges of opposite signs, and two components of S0 mode in the two sensor signals are of the electrical charges of same signs. Naturally, the difference of two signals can yield the pure A0 mode. In experiments, an excitation signal in the following Eq. (1), was adopted to generate S0 or A0 mode.

92 Composites and Their Applications

that of A0 mode.

accurately identified. Moreover, to understand the complex interaction of Lamb waves with

In general, compared with S0mode, the attenuation of A0 mode in structures is more severe. Here, S0 mode corresponds to an axial deformation mode while A0 mode corresponds to a flexural deformation mode. Therefore, it should be paid more attention to the detective capability of both modes for a delamination damage in the different interfaces along the thickness direction. Moreover, the experimental setup of S0 mode is slightly different with

For S0 mode, as shown in Fig. 1, a CFRP laminated composite beam of stack sequence of [010/9012/010] was used. An artificial delamination damage with different lengths for several cases, i.e., 30 mm, 20 mm and 10 mm, respectively, was intentionally created at the interface between the 10th and 11th plies by inserting a Teflon film with a thickness of 25 μm. A PZT actuator was attached on the top surface of the left end of the beam. The PZT actuator had a diameter of 10 mm and a thickness of 0.5 mm. In this case, the generated waves from the actuator merged into the reflected waves from the left end of beam, which leads to a simpler signal. And the same PZT unit was used as a sensor to pick up the reflected wave from the delamination damage. Because the propagation speed of S0 mode is much higher than that of A0 mode, confirmed as 4 times higher in the used composites from our testing, it is expected that in a specified time domain, no A0 mode will be collected due to its slow

For the A0 mode, a similar composite beam was used, as shown in Fig. 2. Two kinds of stack sequence of the laminated beam were used, i.e., [010/9012/010] and [012/04/04/012]. Note that [012/04/04/012] is a unidirectional laminate, and it can be simply expressed as [032]. To simplify the description of delamination later, this expression is used. Like the case of S0 mode, a delamination damage with different lengths, i.e., 30 mm, 20 mm and 10 mm, was intentionally created at the interface between two plies by inserting a Teflon film with a thickness of 25 μm. The distance between the center of the delamination and the left end of beam is 790 mm. To generate the A0 mode, two actuators were attached on the top and bottom surfaces of the beam with applied out-of-phase voltages because the difference of signals in the two actuators can produce the pure *A*0 mode, as shown in Fig. 2. Although this arrangement can generate a relatively pure A0 mode, there are still reflections in S0 mode due to the mode change caused by the scattering between the Lamb waves and the delamination. To pick up the pure *A0* mode, two PZT sensors were attached. We know that two components of A0 mode in the two sensor signals are of the electrical charges of opposite signs, and two components of S0 mode in the two sensor signals are of the electrical charges of same signs. Naturally, the difference of

a long delamination damage, the numerical simulations have been carried out.

**2. Detective technique without the baseline data** 

**2.1. Experiments and numerical analyses** 

*2.1.1. Materials and experimental procedure* 

speed, making the analysis of S0 modes much easier.

$$P(t) = \begin{cases} 0.5[1 - \cos(2\pi ft / N)\sin(2\pi ft)]\nu & t \le N / f \\ 0, \quad t > N / f \end{cases} \tag{1}$$

where *f* is the central frequency in Hz and *N* is the number of sinusoidal cycles within a pulse. In this experiment, the signal with *f*=100 kHz (Fig. 1 for S0 mode) or *f*=50 kHz (Fig. 2 for A0 mode) and *N*=5 was used and the electrical voltage on the actuator was 10 V. The material properties of PZT are listed in Table 1.

Without the baseline data, it is still easy to estimate the arrival of the reflections from the beam boundaries, provided that the wave propagation speed and the dimensions of the beam are known. Then, if the reflected signal from the delamination is not overlapped with the reflections from the boundaries, it can be simply collected by the sensor. Therefore, it is still a technical challenge when the delamination is located close to the beam boundaries and the reflected signal of the delamination is completely overlapped with the reflections from the boundaries.

**Figure 1.** Schematic view of models for experiments using S0 mode (unit: mm)

**Figure 2.** Schematic view of models for experiments using A0 mode (unit: mm)


**Table 1.** Material properties of PZT and CFRP lamina

## *2.1.2. Finite element analysis procedure*

To explore the wave propagation in laminates with a long delamination case, a threedimensional 8-noded brick hybrid element proposed by the authors [10] and the explicit time integration algorithm were used in the finite element simulations without considering the dynamic contact effects like those in some previous studies [11-13]. For the delamination area, one node used on the intact interface along the through-thickness direction, and double nodes were used on the delamination interface, i.e., one belongs to the elements of upper delaminated portion, and the other belongs to the elements of lower delaminated portion. The contact effects in the delamination area were neglected. The same signal stated in Eq. (1) was also used here. Furthermore, the actuator and the sensor were discretized using 3D brick elements. If an electrical field is applied on the actuator, the tensional or compressive strains will be generated in it from a proper relation, which connects the applied voltage and the generated internal strains. The induced stresses from the generated strains can be used to calculate the elemental nodal forces, which form axial force and bending moment in the beam simultaneously. Then, both S0 and A0 modes can be automatically generated. The material properties of CFRP are shown in Table 1.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 95

detect the delamination position, as shown in Fig. 4. For the 20 mm long delamination, a slightly higher error in the delamination position is observed. The reason is the overlapping between the reflected signals from the delamination and those from the right end of the beam, causing an increased error in determination of the arrival time of a reflected wave

Incident wave

**Figure 3.** Comparison between signals of delaminated and intact beams (a 30 mm delamination

Identified Identified

0 100 200 300

*f*=100kHz, *N*=5

Time (s)

30mm

Reflected wave from boundary

25mm

<sup>13</sup> mm 20mm

Reflected wave from delamination

 Delaminated beam Intact beam

from the delamination.

damage between the 10th and the 11th plies)



0.0

Voltage (V)

0.5

1.0

**Figure 4.** Delamination positions identified experimentally

## **2.2. Results of S0 mode**

## *2.2.1. Experimental analysis*

Firstly, to evaluate the wave propagation speed, an intact beam with two sensors attached, was employed. The distance between the two sensors was 420 mm. By using the wavelet transformation technique [14], the arrival times of the incident waves to the two sensors were determined and the wave propagation speed of S0 mode at *f*=100 kHz was estimated as 6210 m/s. To verify the experimental results, the theoretical wave speed of the S0 mode was estimated based on the transfer matrix method [15]. The calculated speed was 6467 m/s if the material properties of CFRP were taken as the values of Table 1. On the other hand, the corresponding propagation speed of A0 mode was estimated as 1506 m/s, much lower that the S0 mode. The higher theoretical wave speed of S0 mode may be due to the transfer matrix method used here, which is actually for an infinite plate. For a beam with a finite width, the influences of Poisson's ratio and boundary conditions are expected.

A comparison of the signals from an intact beam and a delaminated one (a 30 mm delamination damage) for the case in Fig. 1 is shown in Fig. 3. It is clear that there is a reflected wave in the signals from the delaminated beam, which is located between the incident and the reflected waves. However, no clear reflected wave could be detected when the delamination length was reduced to 10 mm. No A0 mode can be observed in Fig. 3, since its incident wave is not fast enough to reach the sensor within 300 μs. For a signal with *f*=100 kHz and *N*=5, the minimum detectable length of the delamination was 20 mm, which is about 1/3 of the wavelength of the S0 mode at 100 kHz. It is reasonable to assume that a higher excitation frequency may have the benefit of detecting smaller delamination cases. Once the arrival time of a reflected wave from the delamination is determined, the difference between the arrival times of the incident and reflected waves can be used to detect the delamination position, as shown in Fig. 4. For the 20 mm long delamination, a slightly higher error in the delamination position is observed. The reason is the overlapping between the reflected signals from the delamination and those from the right end of the beam, causing an increased error in determination of the arrival time of a reflected wave from the delamination.

94 Composites and Their Applications

**2.2. Results of S0 mode** 

*2.2.1. Experimental analysis* 

*2.1.2. Finite element analysis procedure* 

To explore the wave propagation in laminates with a long delamination case, a threedimensional 8-noded brick hybrid element proposed by the authors [10] and the explicit time integration algorithm were used in the finite element simulations without considering the dynamic contact effects like those in some previous studies [11-13]. For the delamination area, one node used on the intact interface along the through-thickness direction, and double nodes were used on the delamination interface, i.e., one belongs to the elements of upper delaminated portion, and the other belongs to the elements of lower delaminated portion. The contact effects in the delamination area were neglected. The same signal stated in Eq. (1) was also used here. Furthermore, the actuator and the sensor were discretized using 3D brick elements. If an electrical field is applied on the actuator, the tensional or compressive strains will be generated in it from a proper relation, which connects the applied voltage and the generated internal strains. The induced stresses from the generated strains can be used to calculate the elemental nodal forces, which form axial force and bending moment in the beam simultaneously. Then, both S0 and A0 modes can be

automatically generated. The material properties of CFRP are shown in Table 1.

width, the influences of Poisson's ratio and boundary conditions are expected.

Firstly, to evaluate the wave propagation speed, an intact beam with two sensors attached, was employed. The distance between the two sensors was 420 mm. By using the wavelet transformation technique [14], the arrival times of the incident waves to the two sensors were determined and the wave propagation speed of S0 mode at *f*=100 kHz was estimated as 6210 m/s. To verify the experimental results, the theoretical wave speed of the S0 mode was estimated based on the transfer matrix method [15]. The calculated speed was 6467 m/s if the material properties of CFRP were taken as the values of Table 1. On the other hand, the corresponding propagation speed of A0 mode was estimated as 1506 m/s, much lower that the S0 mode. The higher theoretical wave speed of S0 mode may be due to the transfer matrix method used here, which is actually for an infinite plate. For a beam with a finite

A comparison of the signals from an intact beam and a delaminated one (a 30 mm delamination damage) for the case in Fig. 1 is shown in Fig. 3. It is clear that there is a reflected wave in the signals from the delaminated beam, which is located between the incident and the reflected waves. However, no clear reflected wave could be detected when the delamination length was reduced to 10 mm. No A0 mode can be observed in Fig. 3, since its incident wave is not fast enough to reach the sensor within 300 μs. For a signal with *f*=100 kHz and *N*=5, the minimum detectable length of the delamination was 20 mm, which is about 1/3 of the wavelength of the S0 mode at 100 kHz. It is reasonable to assume that a higher excitation frequency may have the benefit of detecting smaller delamination cases. Once the arrival time of a reflected wave from the delamination is determined, the difference between the arrival times of the incident and reflected waves can be used to

**Figure 3.** Comparison between signals of delaminated and intact beams (a 30 mm delamination damage between the 10th and the 11th plies)

**Figure 4.** Delamination positions identified experimentally

## *2.2.2. Numerical simulation*

A comparison between the experimental result and numerical simulation for the case of Fig. 1 is shown in Fig.5. A good agreement between two results is observed. For convenience, the amplitude of the first arrival S0 mode in the simulated waves was calibrated using the experimental data. The attenuation coefficients of the CFRP in the numerical model were determined by matching the amplitudes of the reflected waves from the right end of the beam to the experimental data. When the actual sensor thickness, i.e., 0.5 mm, was used in numerical simulations, a small reflected wave from the sensor could be observed immediately next to the incident wave. This was also confirmed by the experimental result. Corresponding to a 0.05 mm sensor thickness, however, no obvious reflection from the sensor could be observed in the numerical simulation. Therefore, the sensor thickness of 0.05 mm was eventually chosen in the simulations. Note that the selection of sensor thickness does not affect the simulation results as the calculated amplitudes will be calibrated using the experimental data. Similar to the experimental results, the numerical simulation showed that the reflected wave from the 10 mm delamination was very weak.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 97

**Figure 5.** Comparison of experimental and numerical results for a 30 mm delamination damage

0 100 200 300

*f*=100kHz, *N*=5

Experimental

Time (s)

Reflected wave from delamination

 Numerical (sensor thickness=0.05 mm) Numerical (sensor thickness=0.5 mm)

**Figure 6.** Comparison of experimental and numerical results for a 30 mm delamination damage at the

0 100 200 300

*f*=80kHz, *N*=5

 Experimental Numerical

Time (s)

between the 10th and the 11th plies

mid-plane between the 16th and the 17th plies




0.0

Voltage (V)

0.5

1.0

1.5



0.0

Voltage (V)

0.4

0.8

To understand the effect of delamination position along the through-thickness direction on the propagation of a Lamb wave, a 30 mm delamination damage was created in the midplane of the laminated beam, i.e., between the 16th and the 17th plies. With the signal of *f*=80 kHz and *N*=5, the comparison between the numerical and experimental results is shown in Fig.6. In Fig.6, no obvious reflected waves from the delamination can be observed. This finding is consistent with the work of Guo et al. [7]. They showed that the delamination at the positions of zero shear stress along the through-thickness direction, such as the midplane of a beam, had no effect on Lamb wave propagation in a S0 mode.

To examine the capability of the numerical simulation to identify the location of delamination, the length of delamination L was increased from 30 to 90 mm. The length of the beam was also increased up to 1500 mm. Under a condition of *f*=100 kHz and *N*=5, the propagation speed of S0 mode was firstly calculated in an intact beam with two sensors. The estimated propagation speed was 6260m/s, which is very close to the experimental result. Based on this speed, the numerically identified positions for various delamination lengths are shown in Fig. 7(a). It is surprising to note that all predicted positions are beyond the right end of the delamination. It has been demonstrated that a higher propagation speed of S0 mode in the delaminated 0°layer can make the predicted delamination positions behind the actual delamination [16, 17]. From the FEM simulations, the estimated speed of S0 mode in the 0°delaminated layer was 8483 m/s. If we use this speed in the delaminated region only (in the intact region, we still use 6260 m/s), with the known left end of the delamination, the predicted delamination positions or the reflected positions of S0 mode are shown in Fig. 7(b). Compare it with Fig. 7(a), we can find that there is no obvious difference in the delamination positions for short delamination cases, e.g. 30 and 50 mm. However, for a longer delamination case (e.g., 70 and 90 mm), an increased deviation from the actual delamination is observed when 8483 m/s was used in the prediction. It implies that the use of the actual wave speed of the delaminated layer cannot improve the prediction and a reflected wave with observable intensity is normally from the right end of the delamination.

*2.2.2. Numerical simulation* 

A comparison between the experimental result and numerical simulation for the case of Fig. 1 is shown in Fig.5. A good agreement between two results is observed. For convenience, the amplitude of the first arrival S0 mode in the simulated waves was calibrated using the experimental data. The attenuation coefficients of the CFRP in the numerical model were determined by matching the amplitudes of the reflected waves from the right end of the beam to the experimental data. When the actual sensor thickness, i.e., 0.5 mm, was used in numerical simulations, a small reflected wave from the sensor could be observed immediately next to the incident wave. This was also confirmed by the experimental result. Corresponding to a 0.05 mm sensor thickness, however, no obvious reflection from the sensor could be observed in the numerical simulation. Therefore, the sensor thickness of 0.05 mm was eventually chosen in the simulations. Note that the selection of sensor thickness does not affect the simulation results as the calculated amplitudes will be calibrated using the experimental data. Similar to the experimental results, the numerical simulation showed

To understand the effect of delamination position along the through-thickness direction on the propagation of a Lamb wave, a 30 mm delamination damage was created in the midplane of the laminated beam, i.e., between the 16th and the 17th plies. With the signal of *f*=80 kHz and *N*=5, the comparison between the numerical and experimental results is shown in Fig.6. In Fig.6, no obvious reflected waves from the delamination can be observed. This finding is consistent with the work of Guo et al. [7]. They showed that the delamination at the positions of zero shear stress along the through-thickness direction, such as the mid-

To examine the capability of the numerical simulation to identify the location of delamination, the length of delamination L was increased from 30 to 90 mm. The length of the beam was also increased up to 1500 mm. Under a condition of *f*=100 kHz and *N*=5, the propagation speed of S0 mode was firstly calculated in an intact beam with two sensors. The estimated propagation speed was 6260m/s, which is very close to the experimental result. Based on this speed, the numerically identified positions for various delamination lengths are shown in Fig. 7(a). It is surprising to note that all predicted positions are beyond the right end of the delamination. It has been demonstrated that a higher propagation speed of S0 mode in the delaminated 0°layer can make the predicted delamination positions behind the actual delamination [16, 17]. From the FEM simulations, the estimated speed of S0 mode in the 0°delaminated layer was 8483 m/s. If we use this speed in the delaminated region only (in the intact region, we still use 6260 m/s), with the known left end of the delamination, the predicted delamination positions or the reflected positions of S0 mode are shown in Fig. 7(b). Compare it with Fig. 7(a), we can find that there is no obvious difference in the delamination positions for short delamination cases, e.g. 30 and 50 mm. However, for a longer delamination case (e.g., 70 and 90 mm), an increased deviation from the actual delamination is observed when 8483 m/s was used in the prediction. It implies that the use of the actual wave speed of the delaminated layer cannot improve the prediction and a reflected wave with observable intensity is normally from the right end of the delamination.

that the reflected wave from the 10 mm delamination was very weak.

plane of a beam, had no effect on Lamb wave propagation in a S0 mode.

**Figure 5.** Comparison of experimental and numerical results for a 30 mm delamination damage between the 10th and the 11th plies

**Figure 6.** Comparison of experimental and numerical results for a 30 mm delamination damage at the mid-plane between the 16th and the 17th plies

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 99

more easily overlapped in a shorter delamination case, e.g., the 10 mm one, resulting in a higher total reflection. Therefore, the intensity of the reflection from the delamination does not certainly depend on the length of the delamination. When the delamination is located on the mid-plane of the laminates, i.e., [010/906//906/010], the results are shown in Fig. 10. Similar to Fig. 9, the reflection can be identified clearly. Also, the reflection from 10 mm delamination is still stronger than that of 30 mm. For the short delamination, e.g., smaller than 30 mm, reflections from the two ends of the delamination should be overlapped if we consider the duration time of a 5 cycle signal (*N*/*f* with the number of cycles *N* and wave central frequency *f*), the length of delamination and the travelling speed of A0 mode. By comparing Figs. 10(a) and 10(b), it can found that the duration of the reflected signal from the 30 mm delamination is obviously longer than that of the 10 mm delamination although its amplitude is smaller. It implies that the overlapping degree of two reflections from the two ends of the 30 mm delamination is lower than that of the 10 mm delamination, which leads to the lower intensity reflection from the 30 mm one. For the laminates with [012/04//04/012] and [012//04/04/012], the anti-symmetric A0 mode still works very well and the reflection from the delamination can be clearly identified. Only the difference of two sensor

signals for the case of [012/04//04/012] with 10 mm delamination is illustrated in Fig. 11.

the numerical results to the experimental ones.

From Fig. 9 to Fig. 11, a good agreement between experimental and numerical results is observed. For convenience, the amplitude of the first arrival A0 mode in the simulated waves was calibrated using the experimental data. The attenuation coefficients of the CFRP in the numerical model were determined by matching the amplitude of the reflected waves from the right end of the beam to that of experimental data. Naturally, the material properties of CFRP in Table 1 were also adjusted slightly within a reasonable range to match

To determine the wave propagation speed of A0 mode, two sets of distant sensor pairs were attached on an intact laminated beam. The wave speed was determined from the distance and the difference of arrival times between the two sets of sensor pairs. For the laminate with [010/906/906/010], the experimental and numerical wave speeds were 1555 m/s and 1470 m/s, respectively. For the case of [012/04/04/012], the experimental and numerical wave speeds were 1723 m/s and 1616 m/s, respectively. From these results, it can be found that the wave speed of [012/04/04/012] is only slightly higher than that of [010/906/906/010]. The reason is that the wave speed of A0 mode is determined by the bending stiffness of laminates, which is dominated by the fibre orientation of outer layers near top and bottom surfaces of laminates. In both cases, 0° degree layers are located near the two surfaces of laminates, which results in the comparatively small difference of wave speeds in both cases. For the case of [010/906/906/010], if 90° degree layers

were located near the two surfaces of laminates, it would be a completely different story.

With the knowledge of the wave speeds, the delamination positions for various situations were evaluated, as shown in Table 2. The distances between identified positions and the actual centers of delamination are listed. In this table, the negative values denote that the identified positions are located on the left side of the delamination centers, and positive values represent that the identified positions are on the right side of the delamination centers. In this table, it is clear that the delamination positions in almost all laminates have been

**Figure 7.** Numerical identification of positions of various delamination cases

## **2.3. Results of A0 mode**

To confirm the effectiveness of present excitation and sensing techniques for A0 mode (Fig. 2), numerical simulation was firstly conducted for the laminate with [010//906/906/010] and 30 mm delamination under a condition of *f*=50 kHz and *N*=5. Note that "//" denotes the position of the delamination. The signals from upper and lower sensors are plotted in Fig. 8(a). From the first wave packet, it can be observed that the pure A0 mode can be excited by two actuators with the out-of-phase applied voltages and two signals with a mutual phase difference of 180° are generated. Also, after the incident wave, the direct reflected signal from the delamination can be clearly observed. As mentioned before, the reflected signals from the delamination may include two modes, i.e., S0 mode and A0 mode as marked in Fig. 8(a). It can be explained by the wave mode change when the incident A0 mode interacts with the delamination. In Fig. 8(a), the reflections of the transmitted waves from the boundary can be identified, which also contain the S0 mode and A0 mode. The difference of two signals between the two sensors in Fig. 8(a) is shown in Fig. 8(b). It is clear that the S0 mode has been completely removed, and only pure A0 mode exists.

The results of the laminate [010//906/906/010] with 30 mm and 10 mm delamination damages are shown in Fig 9. Sampling time for both numerical and experimental results was 1.0×10-4 ms. For the delamination of the lengths of 30 mm and 10mm, the reflections from the delamination can be clearly identified. In Figs. 9(a) and 9(b), it is interesting to note that the reflection signal from the 10 mm delamination is stronger than that from the 30 mm delamination. The reason may be that the reflections from the two ends of delamination are more easily overlapped in a shorter delamination case, e.g., the 10 mm one, resulting in a higher total reflection. Therefore, the intensity of the reflection from the delamination does not certainly depend on the length of the delamination. When the delamination is located on the mid-plane of the laminates, i.e., [010/906//906/010], the results are shown in Fig. 10. Similar to Fig. 9, the reflection can be identified clearly. Also, the reflection from 10 mm delamination is still stronger than that of 30 mm. For the short delamination, e.g., smaller than 30 mm, reflections from the two ends of the delamination should be overlapped if we consider the duration time of a 5 cycle signal (*N*/*f* with the number of cycles *N* and wave central frequency *f*), the length of delamination and the travelling speed of A0 mode. By comparing Figs. 10(a) and 10(b), it can found that the duration of the reflected signal from the 30 mm delamination is obviously longer than that of the 10 mm delamination although its amplitude is smaller. It implies that the overlapping degree of two reflections from the two ends of the 30 mm delamination is lower than that of the 10 mm delamination, which leads to the lower intensity reflection from the 30 mm one. For the laminates with [012/04//04/012] and [012//04/04/012], the anti-symmetric A0 mode still works very well and the reflection from the delamination can be clearly identified. Only the difference of two sensor signals for the case of [012/04//04/012] with 10 mm delamination is illustrated in Fig. 11.

98 Composites and Their Applications

**2.3. Results of A0 mode** 

**Figure 7.** Numerical identification of positions of various delamination cases

mode has been completely removed, and only pure A0 mode exists.

To confirm the effectiveness of present excitation and sensing techniques for A0 mode (Fig. 2), numerical simulation was firstly conducted for the laminate with [010//906/906/010] and 30 mm delamination under a condition of *f*=50 kHz and *N*=5. Note that "//" denotes the position of the delamination. The signals from upper and lower sensors are plotted in Fig. 8(a). From the first wave packet, it can be observed that the pure A0 mode can be excited by two actuators with the out-of-phase applied voltages and two signals with a mutual phase difference of 180° are generated. Also, after the incident wave, the direct reflected signal from the delamination can be clearly observed. As mentioned before, the reflected signals from the delamination may include two modes, i.e., S0 mode and A0 mode as marked in Fig. 8(a). It can be explained by the wave mode change when the incident A0 mode interacts with the delamination. In Fig. 8(a), the reflections of the transmitted waves from the boundary can be identified, which also contain the S0 mode and A0 mode. The difference of two signals between the two sensors in Fig. 8(a) is shown in Fig. 8(b). It is clear that the S0

(a) Results using 6260 m/s (b) Results using 8483 m/s in 0o delaminated layer

The results of the laminate [010//906/906/010] with 30 mm and 10 mm delamination damages are shown in Fig 9. Sampling time for both numerical and experimental results was 1.0×10-4 ms. For the delamination of the lengths of 30 mm and 10mm, the reflections from the delamination can be clearly identified. In Figs. 9(a) and 9(b), it is interesting to note that the reflection signal from the 10 mm delamination is stronger than that from the 30 mm delamination. The reason may be that the reflections from the two ends of delamination are From Fig. 9 to Fig. 11, a good agreement between experimental and numerical results is observed. For convenience, the amplitude of the first arrival A0 mode in the simulated waves was calibrated using the experimental data. The attenuation coefficients of the CFRP in the numerical model were determined by matching the amplitude of the reflected waves from the right end of the beam to that of experimental data. Naturally, the material properties of CFRP in Table 1 were also adjusted slightly within a reasonable range to match the numerical results to the experimental ones.

To determine the wave propagation speed of A0 mode, two sets of distant sensor pairs were attached on an intact laminated beam. The wave speed was determined from the distance and the difference of arrival times between the two sets of sensor pairs. For the laminate with [010/906/906/010], the experimental and numerical wave speeds were 1555 m/s and 1470 m/s, respectively. For the case of [012/04/04/012], the experimental and numerical wave speeds were 1723 m/s and 1616 m/s, respectively. From these results, it can be found that the wave speed of [012/04/04/012] is only slightly higher than that of [010/906/906/010]. The reason is that the wave speed of A0 mode is determined by the bending stiffness of laminates, which is dominated by the fibre orientation of outer layers near top and bottom surfaces of laminates. In both cases, 0° degree layers are located near the two surfaces of laminates, which results in the comparatively small difference of wave speeds in both cases. For the case of [010/906/906/010], if 90° degree layers were located near the two surfaces of laminates, it would be a completely different story.

With the knowledge of the wave speeds, the delamination positions for various situations were evaluated, as shown in Table 2. The distances between identified positions and the actual centers of delamination are listed. In this table, the negative values denote that the identified positions are located on the left side of the delamination centers, and positive values represent that the identified positions are on the right side of the delamination centers. In this table, it is clear that the delamination positions in almost all laminates have been successfully detected compared with the beam length. For the laminate with [010//906/906/010] and 20 mm delamination, the reflection is not very clear due to strong noises in experiments. As a result, only numerical estimation is presented in this table. The similar case can be found in the laminate with [012/04//04/012] and 30 mm delamination, as shown in this table.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 101

**Figure 10.** Comparison of numerical and experimental results for [010/906//906/010]

 Experimental Numerical

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Reflection from delamination

Time (ms)



0.0

Voltage (V)

0.5

1.0

**Figure 11.** Comparison between experimental and numerical results for a 10 mm delamination

Unit (mm) [0/90//90/0] [0//90/90/0] [0/0//0/0] [0//0/0/0] 30mm Num. -11.0 4.0 0.0 18.0

20mm Num. -12.0 -5.0 -14.0 18.0

10mm Num. 2.0 2.0 1.0 1.0

**Table 2.** Identified delamination positions for various cases



0.0

Voltage (V)

0.5

1.0

Exp. -19.0 39.0 -11.0

Reflection from delamination

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Time (ms)

(a) delamination length: 30 mm (b) delamination length: 10 mm



0.0

Voltage (V)

0.5

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Reflection from delamination

 Experimental Numerical

Time (ms)

 Experimental Numerical

Exp. 9.0 -11.0 4.0

Exp. 2.0 -2.0 10.0 -17.0

([012/04//04/012])

In summary, unlike S0 mode, A0 mode can be used for different applications. However, the amplitude of reflection from delamination may not depend on the length of the delamination. The amplitude of reflection from short delamination is higher than that from long delamination due to overlapping of reflections from the ends of short delamination. The disadvantage of A0 mode is the short travelling distance due to its high attenuation in CFRP materials. Therefore, if possible, both the actuators and sensors should be placed close to the potential damage sites.

**Figure 8.** FEM numerical results for [010//906/906/010] (delamination length: 30 mm)

**Figure 9.** Comparison of numerical and experimental results for [010//906/906/010]

**Figure 10.** Comparison of numerical and experimental results for [010/906//906/010]

to the potential damage sites.




0.0

Voltage (V)

0.5

1.0


0.0

Voltage (V)

0.5

1.0

successfully detected compared with the beam length. For the laminate with [010//906/906/010] and 20 mm delamination, the reflection is not very clear due to strong noises in experiments. As a result, only numerical estimation is presented in this table. The similar case can be found

In summary, unlike S0 mode, A0 mode can be used for different applications. However, the amplitude of reflection from delamination may not depend on the length of the delamination. The amplitude of reflection from short delamination is higher than that from long delamination due to overlapping of reflections from the ends of short delamination. The disadvantage of A0 mode is the short travelling distance due to its high attenuation in CFRP materials. Therefore, if possible, both the actuators and sensors should be placed close

Upper sensor

in the laminate with [012/04//04/012] and 30 mm delamination, as shown in this table.

**Figure 8.** FEM numerical results for [010//906/906/010] (delamination length: 30 mm)

 Experimental Numerical

Lower sensor S0 wave

0.0 0.1 0.2 0.3 0.4 0.5 0.6

A0 wave A0 wave

Reflections from boundary

Reflections from delamination

Time (ms)

(a) signals of upper and lower sensors (b) difference of signals of

(a) delamination length: 30 mm (b) delamination length: 10 mm



0.0

Voltage (V)

0.5

1.0



0.0

Voltage (V)

0.5

1.0

upper and lower sensors

 Experimental Numerical

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Reflection from delamination

Time (ms)

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Time (ms)

**Figure 9.** Comparison of numerical and experimental results for [010//906/906/010]

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Reflection from delamination

Time (ms)

**Figure 11.** Comparison between experimental and numerical results for a 10 mm delamination ([012/04//04/012])


**Table 2.** Identified delamination positions for various cases

## **3. Interaction between Lamb waves and delamination**

### **3.1. S0 mode**

To gain a better understanding of this complex interaction, a beam with a length of 1500 mm and a 200 mm long delamination damage was examined using the numerical simulation under a condition of *f*=100 kHz and *N*=5. The very long delamination was chosen for obtaining the separated reflected signals from the two ends of the delamination. The transverse deflections at various mesh points on the top surface of the beam were used and the subtraction between the wave signals from the intact and the delaminated beams was done to amplify the scattered wave signals from the delamination. Fig. 12 shows the detail information of the scattered wave signals from the delamination at different time domains. The typical sensor signal for accurately evaluating the arrival times of various waves is shown in Fig. 13.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 103

**Figure 12.** Wave signal differences of intact and delaminated beams(S0 mode)

(c) 236 μs (d) 303 μs

(a) 139 μs (b) 182 μs

At the time domain of 139 μs, the S0 mode arrives at the left end of the delamination and then reflects from or transmits from the left end, as shown in Fig. 12(a). At 182 μs, the S0 mode passes through the right end of the delamination. Fig. 12(b) shows two separate reflected modes, i.e., A0 and S0 from the left end of the delamination. In principle, it is easy to distinguish a S0 from an A0 mode by the propagation speed and wave length. However, the amplitude of the reflected S0 mode is smaller than that of the A0 mode. This can be explained by the fact that the bending deformation in a laminated beam is much higher as compared to the axial deformation. Also, when the S0 mode travels through the left end of delamination, a new transmitted A0 mode is generated as shown in Fig. 12(b). The reflected waves from the right end of the delamination, consisted both A0 and S0 modes, can be also observed in Fig. 12(b). Corresponding to 236 μs, the incident A0 mode from the actuator does not reach the sensor, and the reflected S0 mode from the left side of the delamination has already passed through the sensor. However, at the same time point in Fig. 13, this reflected S0 mode cannot be detected by the sensor due to its small amplitude. Also, the reflected S0 mode from the right end of the delamination, which is surpassing the reflected A0 mode from the left end of the delamination, can be observed, as shown in Fig. 12(c). A new transmitted A0 mode and the transmitted S0 mode from the right end of the delamination can be clearly observed, Fig. 12(c). At 303 μs, the incident A0 mode does not arrive at the sensor. As indicated in Fig. 12(d), the reflected S0 mode from the right end of the delamination completely surpasses the reflected A0 mode from its left end. This reflected S0 mode has already passed through the sensor position. From Fig. 13, the arrival time of the reflected wave from the delamination is identified as around 275 μs. The reflected wave actually arrives at the sensor between the time points shown in Figs. 12(c) and 12(d), i.e., 236 μs and 303 μs. Therefore, the reflected wave signal detected clearly by the sensor is considered to be the reflected S0 mode from the right end rather than the left end of the beam, due to its higher intensity. Based on the numerical simulations above, the very complex interaction between the different signal modes and the boundaries of delamination can be identified. When only a single S0 mode passes through the delamination, four modes will be generated at one end of the delamination, including two transmitted A0 and S0

**3.1. S0 mode** 

shown in Fig. 13.

**3. Interaction between Lamb waves and delamination** 

To gain a better understanding of this complex interaction, a beam with a length of 1500 mm and a 200 mm long delamination damage was examined using the numerical simulation under a condition of *f*=100 kHz and *N*=5. The very long delamination was chosen for obtaining the separated reflected signals from the two ends of the delamination. The transverse deflections at various mesh points on the top surface of the beam were used and the subtraction between the wave signals from the intact and the delaminated beams was done to amplify the scattered wave signals from the delamination. Fig. 12 shows the detail information of the scattered wave signals from the delamination at different time domains. The typical sensor signal for accurately evaluating the arrival times of various waves is

At the time domain of 139 μs, the S0 mode arrives at the left end of the delamination and then reflects from or transmits from the left end, as shown in Fig. 12(a). At 182 μs, the S0 mode passes through the right end of the delamination. Fig. 12(b) shows two separate reflected modes, i.e., A0 and S0 from the left end of the delamination. In principle, it is easy to distinguish a S0 from an A0 mode by the propagation speed and wave length. However, the amplitude of the reflected S0 mode is smaller than that of the A0 mode. This can be explained by the fact that the bending deformation in a laminated beam is much higher as compared to the axial deformation. Also, when the S0 mode travels through the left end of delamination, a new transmitted A0 mode is generated as shown in Fig. 12(b). The reflected waves from the right end of the delamination, consisted both A0 and S0 modes, can be also observed in Fig. 12(b). Corresponding to 236 μs, the incident A0 mode from the actuator does not reach the sensor, and the reflected S0 mode from the left side of the delamination has already passed through the sensor. However, at the same time point in Fig. 13, this reflected S0 mode cannot be detected by the sensor due to its small amplitude. Also, the reflected S0 mode from the right end of the delamination, which is surpassing the reflected A0 mode from the left end of the delamination, can be observed, as shown in Fig. 12(c). A new transmitted A0 mode and the transmitted S0 mode from the right end of the delamination can be clearly observed, Fig. 12(c). At 303 μs, the incident A0 mode does not arrive at the sensor. As indicated in Fig. 12(d), the reflected S0 mode from the right end of the delamination completely surpasses the reflected A0 mode from its left end. This reflected S0 mode has already passed through the sensor position. From Fig. 13, the arrival time of the reflected wave from the delamination is identified as around 275 μs. The reflected wave actually arrives at the sensor between the time points shown in Figs. 12(c) and 12(d), i.e., 236 μs and 303 μs. Therefore, the reflected wave signal detected clearly by the sensor is considered to be the reflected S0 mode from the right end rather than the left end of the beam, due to its higher intensity. Based on the numerical simulations above, the very complex interaction between the different signal modes and the boundaries of delamination can be identified. When only a single S0 mode passes through the delamination, four modes will be generated at one end of the delamination, including two transmitted A0 and S0

**Figure 12.** Wave signal differences of intact and delaminated beams(S0 mode)

modes, and two reflected A0 and S0 modes. Further study is required to understand why the stronger reflected A0 and S0 modes. However, the reflected S0 mode received by a sensor is from the reflections are from the right end of delamination. One possible explanation is that the delaminated region is of lower bending stiffness, which can be considered to be a softer region. When a wave propagates from a harder or intact region into a softer region, the reflections become weak. In contrast, when the wave propagates from a softer region into a harder one, a stronger reflection is expected.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 105

**Figure 14.** Wave signal differences of intact and delaminated beams(A0 mode)

complicated interactions between the A0 mode and the delamination.

distinguish a S0 from an A0 mode by using phase information of signals from the two sensors. At 488 μs, the reflected and transmitted waves at the right end of the delamination separate to each other, as shown in Figs. 14(c). Also, the amplitude of the reflected wave is much higher than that of the reflected wave at the left end of the delamination, confirmed in Fig. 14(b). This phenomenon is similar to that obtained for S0 mode. Fig. 14(d) shows the transmitted and reflected waves at the left end of the delamination when the reflected wave from the right end of the delamination arrives at the left end at 621 μs. It is worthwhile mentioning that although the amplitude of reflected wave from the right end of delamination is very strong, the reflection at another end (left end) of the delamination can significantly reduce the amplitude of the transmitted wave, which is expected to be monitored by the sensors. Therefore, wave mode change and multiple reflections may create

(c) 448 μs (d) 621 μs

(a) 318 μs (b) 433 μs

**Figure 13.** Sensor signal for a delamination of length of 200 mm

## **3.2. A0 mode**

Like S0 mode, a beam with a length of 1200 mm and a 200 mm long delamination case was examined using the numerical simulation under a condition of *f*=50 kHz and *N*=5. The transverse deflections (out-of-plane deformation) at various mesh points on the top surface of the beam were used. Fig. 14 also shows the subtraction between the wave signals from the intact and the delaminated beams, which can amplify the scattered wave signals from the delamination. The typical sensor signal for accurately evaluating the arrival times of various waves is shown in Fig. 15 with an enlarged picture for reflected waves from the delamination.

Firstly, for the laminate with [010//906/906/010], at the time domain of 318 μs, two A0 mode waves were excited by the actuator pair, which propagates to the left and right directions, independently. The A0 mode wave propagating to the right direction arrives at the left end of the delamination and then reflects from or transmits from the left end, as shown in Fig. 14(a), respectively. At 433 μs, the transmitted A0 mode from the left end of delamination passes through the right end. Fig. 14(b) shows the reflected wave from the right end of delamination. By comparing Fig. 14(b) and Fig. 15 at the same time domains, we can identify that the reflected A0 mode from the left end of delamination passes through the sensor. The reflected S0 mode due to mode change can also be identified. In principle, it is easy to

**3.2. A0 mode** 

delamination.

modes, and two reflected A0 and S0 modes. Further study is required to understand why the stronger reflected A0 and S0 modes. However, the reflected S0 mode received by a sensor is from the reflections are from the right end of delamination. One possible explanation is that the delaminated region is of lower bending stiffness, which can be considered to be a softer region. When a wave propagates from a harder or intact region into a softer region, the reflections become weak. In contrast, when the wave propagates from a

Like S0 mode, a beam with a length of 1200 mm and a 200 mm long delamination case was examined using the numerical simulation under a condition of *f*=50 kHz and *N*=5. The transverse deflections (out-of-plane deformation) at various mesh points on the top surface of the beam were used. Fig. 14 also shows the subtraction between the wave signals from the intact and the delaminated beams, which can amplify the scattered wave signals from the delamination. The typical sensor signal for accurately evaluating the arrival times of various waves is shown in Fig. 15 with an enlarged picture for reflected waves from the

Firstly, for the laminate with [010//906/906/010], at the time domain of 318 μs, two A0 mode waves were excited by the actuator pair, which propagates to the left and right directions, independently. The A0 mode wave propagating to the right direction arrives at the left end of the delamination and then reflects from or transmits from the left end, as shown in Fig. 14(a), respectively. At 433 μs, the transmitted A0 mode from the left end of delamination passes through the right end. Fig. 14(b) shows the reflected wave from the right end of delamination. By comparing Fig. 14(b) and Fig. 15 at the same time domains, we can identify that the reflected A0 mode from the left end of delamination passes through the sensor. The reflected S0 mode due to mode change can also be identified. In principle, it is easy to

softer region into a harder one, a stronger reflection is expected.

**Figure 13.** Sensor signal for a delamination of length of 200 mm

**Figure 14.** Wave signal differences of intact and delaminated beams(A0 mode)

distinguish a S0 from an A0 mode by using phase information of signals from the two sensors. At 488 μs, the reflected and transmitted waves at the right end of the delamination separate to each other, as shown in Figs. 14(c). Also, the amplitude of the reflected wave is much higher than that of the reflected wave at the left end of the delamination, confirmed in Fig. 14(b). This phenomenon is similar to that obtained for S0 mode. Fig. 14(d) shows the transmitted and reflected waves at the left end of the delamination when the reflected wave from the right end of the delamination arrives at the left end at 621 μs. It is worthwhile mentioning that although the amplitude of reflected wave from the right end of delamination is very strong, the reflection at another end (left end) of the delamination can significantly reduce the amplitude of the transmitted wave, which is expected to be monitored by the sensors. Therefore, wave mode change and multiple reflections may create complicated interactions between the A0 mode and the delamination.

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 107

 Upper sensor ([0/90//90/0]) Lower sensor ([0/90//90/0])

200 400 600 800 1000

Reflected wave from right end

of del. (*A*0 mode)

Time (s)

400 500 600 700

Time (s)

**Figure 16.** Sensor signal for delamination of 200 mm in length ([010/906//906/010])



0.00

Voltage (V)

0.07

Reflected wave from left end of del. (*A*0 mode)


0.0

Voltage (V)

0.5

1.0

**Figure 15.** Sensor signal for delamination of 200 mm in length ([010//906/906/010])



0.00

Voltage (V)

0.09

Reflected wave from left end of del. (*A*0 mode)


0.0

Voltage (V)

0.5

1.0

**Figure 15.** Sensor signal for delamination of 200 mm in length ([010//906/906/010])

0 200 400 600 800 1000

 Upper sesnor ([0//90/90/0]) Lower sensor ([0//90/90/0])

> Reflected wave from right end of del. (*A*0 mode)

Time (s)

300 400 500 600 700

*S*0 mode due to mode change

Time (s)

**Figure 16.** Sensor signal for delamination of 200 mm in length ([010/906//906/010])

In Fig. 15, the amplitude of the reflected wave from the right end of the delamination is still higher than that of the reflected wave from the left end. Similar results can be observed in the laminate with [012//04/04/012]. In consequence, as shown in Table 2, more identified delamination positions are close to the right end of delamination for the laminates with [010//906/906/010] and [012//04/04/012]. This phenomenon is similar to the result obtained for S0 mode in the case of [010//906/906/010].

Locating Delamination in Composite Laminated Beams Using the Zero-Order Mode of Lamb Waves 109

[1] Su ZQ, Ye L, Lu Y. Guided Lamb waves for identification of damage in composite

[2] Valdes SHD, Soutis C. A structural health monitoring system for laminated composite.

[3] Sohn H, Farrar CR. Damage diagnosis using time series analysis of vibration signals.

[4] Ihn JB, Chang FK. Built-in diagnostics for monitoring crack growth in aircraft structures. *In: Chang Fu-Kuo (ed.), Proceedings of the international workshop on structural* 

[5] Hurlesbaus S, Niethammer M, Jacobs LJ, Valle C. Automated methodology to locate

[6] Su Z, Yang C, Pan N, Ye L, Zhou LM. Assessment of delamination in composite beams

[7] Guo N, Cawley P. The interaction of Lamb waves with delaminations in composite

[8] Wang CH, Rose LRF. Wave reflection and transmission in beams containing

[9] Mahapatra DR, Gopalakrishnan S. A spectral finite element model for analysis of axial– flexural–shear coupled wave propagation in laminated composite beams. *Compos Struct*

[10] Cao YP, Hu N, Lu J, Fukunaga H, Yao ZH. A 3D brick element based on Hu-Washizu variational principle for mesh distorsion. *Int J Numer Meth Engrg* 2002; 53: 2529-

[11] Hu N, Sekine H, Fukunaga H, Yao ZH. Impact analysis of composite laminates with

[12] Hu N, A solution method for dynamics contact problems. *Comput & Struct* 1997; 63:

[13] Li CF, Hu N, Cheng JG, Sekine H, Fukunaga H. Low-velocity impact-induced damage of continuous fiber-reinforced composite laminates. Part II. verification and numerical

[14] Jeong H, Jang YS. Wavelet analysis of plate wave propagation in composite laminates.

[15] Nayfeh AH, Chimenti DE. Elastic wave propagation in fluid-loaded multi-axial

notches with Lamb waves. *Acoustics Research Letters Online* 2001; 2: 97-102.

using shear horizontal (SH) wave mode. *Compos Sci Technol* 2007; 67: 244-51.

delamination and inhomogeneity. *J Sound & Vibration* 2003; 264: 851-72.

multiple delaminations. *Int J Impact Eng* 1999; 22: 633-48.

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anisotropic media. *J Acoust Soc Am* 1991; 89: 542-9.

*Compos Struct* 2000; 49: 443-50.

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**5. References** 

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48.

1053-63.

*School of Manufacturing Science and Engineering,* 

*Southwest University of Science and Technology, Mianyang, P.R.China* 

structures: A review. *J Sound & Vibr* 2006; 295: 753-80.

*Proceedings of DETC, Pittsburgh, PA, USA*; 2001.

*health monitoring. Stanford University, USA;* 2001.

laminates. *J Acoust Soc Am* 1993; 94: 2240-6.

*Smart Mater and Struct* 2001; 10: 1-6.

For the case of [010/906//906/010], the signals of two sensors are plotted in Fig. 16 with the enlarged picture for the reflection from the delamination. In Fig. 16, there is no S0 mode and the mode change does not occur as the delamination is located on the mid-plane of the laminates. In contrast to the cases of [010//906/906/010] and [012//04/04/012], it is interesting to note that the amplitude of the reflected wave from the left end of delamination is higher than that of the right end. Similar results are obtained in the case of [012/04//04/012]. Corresponding to Table 2, more identified delamination positions are close to the left end of the delamination. The physical meaning of why the stronger reflection occurs at the different ends of delamination for different delamination cases in Figs 15 and 16 is not clear. The position of the stronger reflection seems to mainly depend on the following two factors: bending stiffness of upper and lower delaminated layers and their difference, and Lamb wave deformation mode.

## **4. Conclusion**

A technique for identifying a delamination damage in cross-ply laminated composite beams has been developed by using the zero-order mode of Lame waves. The delamination position can be identified if the arrival time of the arrival time of a reflected wave from a delamination and the wave propagation speed of Lame waves are known. One of the advantages of this approach is the baseline data acquired from intact beams are not required.

And, extensive finite element simulations have been conducted to investigate the interaction of Lame waves with various delamination cases. For S0 mode, when a single S0 mode propagates into a delamination, mode change happens and reflected S0 mode with high amplitude is generated from the end point of the delamination where the waves propagate out; for A0 mode, it has been found that the situation is more complex than S0 mode. For the laminates with stack sequence of [010//906/906/010] and [012//04/04/012,], mode change also happens and the reflected wave from the right end of delamination is slight higher than that from the left end. However, for the case of [010/906//06/010] and [012/04//04/012], there is no mode change when a single A0 mode passes through delamination, and the stronger reflection from the delamination occurs at the left end of the delamination.

## **Author details**

Yaolu Liu, Alamusi, Jinhua Li, Huiming Ning, Liangke Wu and Ning Hu *Department of Mechanical Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba, Japan*  Weifeng Yuan and Bin Gu

*School of Manufacturing Science and Engineering, Southwest University of Science and Technology, Mianyang, P.R.China* 

#### **5. References**

108 Composites and Their Applications

wave deformation mode.

**4. Conclusion** 

required.

**Author details** 

mode in the case of [010//906/906/010].

In Fig. 15, the amplitude of the reflected wave from the right end of the delamination is still higher than that of the reflected wave from the left end. Similar results can be observed in the laminate with [012//04/04/012]. In consequence, as shown in Table 2, more identified delamination positions are close to the right end of delamination for the laminates with [010//906/906/010] and [012//04/04/012]. This phenomenon is similar to the result obtained for S0

For the case of [010/906//906/010], the signals of two sensors are plotted in Fig. 16 with the enlarged picture for the reflection from the delamination. In Fig. 16, there is no S0 mode and the mode change does not occur as the delamination is located on the mid-plane of the laminates. In contrast to the cases of [010//906/906/010] and [012//04/04/012], it is interesting to note that the amplitude of the reflected wave from the left end of delamination is higher than that of the right end. Similar results are obtained in the case of [012/04//04/012]. Corresponding to Table 2, more identified delamination positions are close to the left end of the delamination. The physical meaning of why the stronger reflection occurs at the different ends of delamination for different delamination cases in Figs 15 and 16 is not clear. The position of the stronger reflection seems to mainly depend on the following two factors: bending stiffness of upper and lower delaminated layers and their difference, and Lamb

A technique for identifying a delamination damage in cross-ply laminated composite beams has been developed by using the zero-order mode of Lame waves. The delamination position can be identified if the arrival time of the arrival time of a reflected wave from a delamination and the wave propagation speed of Lame waves are known. One of the advantages of this approach is the baseline data acquired from intact beams are not

And, extensive finite element simulations have been conducted to investigate the interaction of Lame waves with various delamination cases. For S0 mode, when a single S0 mode propagates into a delamination, mode change happens and reflected S0 mode with high amplitude is generated from the end point of the delamination where the waves propagate out; for A0 mode, it has been found that the situation is more complex than S0 mode. For the laminates with stack sequence of [010//906/906/010] and [012//04/04/012,], mode change also happens and the reflected wave from the right end of delamination is slight higher than that from the left end. However, for the case of [010/906//06/010] and [012/04//04/012], there is no mode change when a single A0 mode passes through delamination, and the stronger

reflection from the delamination occurs at the left end of the delamination.

Yaolu Liu, Alamusi, Jinhua Li, Huiming Ning, Liangke Wu and Ning Hu

*Department of Mechanical Engineering, Chiba University, Yayoi-cho, Inage-ku, Chiba, Japan* 

	- [16] Toyama N, Noda J, Okabe T. Quantitative damage detection in cross-ply laminates using Lamb wave method. *Compos Sci and Technol* 2003; 63: 1473-79.

**Section 2** 

**Bio-Medical Composites and Their Applications** 

[17] Takeda N, Okabe Y, Kuwahara J, Kojima S, Ogisu T. Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing. *Compos Sci Technol* 2005; 65: 2575-87.

**Bio-Medical Composites and Their Applications** 

110 Composites and Their Applications

[16] Toyama N, Noda J, Okabe T. Quantitative damage detection in cross-ply laminates

[17] Takeda N, Okabe Y, Kuwahara J, Kojima S, Ogisu T. Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave

using Lamb wave method. *Compos Sci and Technol* 2003; 63: 1473-79.

sensing. *Compos Sci Technol* 2005; 65: 2575-87.

**Chapter 6** 

© 2012 Mohamed, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Mohamed, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Biocomposite Materials** 

Additional information is available at the end of the chapter

Composite materials may be restricted to emphasize those materials that contain a continuous matrix constituent that binds together and provides form to an array of a stronger, stiffer reinforcement constituent. The resulting composite material has a balance of structural properties that is superior to either constituent material alone. Combining the advantages of inorganic and organic components such as HA/organic composites that show good biocompatibility and favorable bonding ability with surrounding host tissues inherent from HA. Besides, the problems associated with HA ceramic, such as its intrinsic brittleness, poor formability and migration of HA particles from the implanted sites, can be circumvented by the integration of HA ceramic with biopolymers. In order to achieve controlled bioactivity and biodegradability, polymer-ceramic composites have been proposed. The composites of ceramics with natural degradable polymers have attracted much interest as bone filler. Several particle composites based on degradable biopolymers

such as chitosan and gelatin with inorganic powders, were developed as bone filler.

Hydroxyapatite (HA) (Ca10 (PO4)6(OH)2, is widely used in musculoskeletal procedures due to its chemical and crystallographic similarity to the carbonated apatite in human bones and teeth (Suchanek and Yoshimura, 1998). While sintered HA can be machined and used in pre-fabricated forms, several formulations of calcium phosphate cements can be molded as pastes and harden in situ. HA is the main component of teeth and bones in vertebrates. Good mechanical properties with superior biocompatibility of sintered HA make it well preferred bone and tooth implant material (Kim et al., 2008). Calcium phosphates, especially HA, are excellent candidates for bone repair and regeneration and have been used in bone tissue engineering for two decades. Although HA is bioactive and osteoconductive, its mechanical properties are inadequate, making it unable to be used as a load bearing

Khaled R. Mohamed

http://dx.doi.org/10.5772/48302

**2. Hydroxyapatite (HA)** 

implant.

**1. Introduction** 

**Chapter 6** 

## **Biocomposite Materials**

Khaled R. Mohamed

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48302

## **1. Introduction**

Composite materials may be restricted to emphasize those materials that contain a continuous matrix constituent that binds together and provides form to an array of a stronger, stiffer reinforcement constituent. The resulting composite material has a balance of structural properties that is superior to either constituent material alone. Combining the advantages of inorganic and organic components such as HA/organic composites that show good biocompatibility and favorable bonding ability with surrounding host tissues inherent from HA. Besides, the problems associated with HA ceramic, such as its intrinsic brittleness, poor formability and migration of HA particles from the implanted sites, can be circumvented by the integration of HA ceramic with biopolymers. In order to achieve controlled bioactivity and biodegradability, polymer-ceramic composites have been proposed. The composites of ceramics with natural degradable polymers have attracted much interest as bone filler. Several particle composites based on degradable biopolymers such as chitosan and gelatin with inorganic powders, were developed as bone filler.

## **2. Hydroxyapatite (HA)**

Hydroxyapatite (HA) (Ca10 (PO4)6(OH)2, is widely used in musculoskeletal procedures due to its chemical and crystallographic similarity to the carbonated apatite in human bones and teeth (Suchanek and Yoshimura, 1998). While sintered HA can be machined and used in pre-fabricated forms, several formulations of calcium phosphate cements can be molded as pastes and harden in situ. HA is the main component of teeth and bones in vertebrates. Good mechanical properties with superior biocompatibility of sintered HA make it well preferred bone and tooth implant material (Kim et al., 2008). Calcium phosphates, especially HA, are excellent candidates for bone repair and regeneration and have been used in bone tissue engineering for two decades. Although HA is bioactive and osteoconductive, its mechanical properties are inadequate, making it unable to be used as a load bearing implant.

© 2012 Mohamed, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Mohamed, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **2.1. Properties**

HA power (HAp), a major inorganic component of bone, has been used extensively for biomedical implant applications and bone regeneration due to its bioactive, biodegradable and osteoconductive properties (Gomez-Vega et al., 2000). HA is available in market in many forms like solids blocks, micro-porous blocks and as granules (Murgan and Ramakrishna, 2005). Nano-hydroxyapatite (n-HA) has been proven to be of great biological efficacy. nHA precipitates may have higher solubility and therefore affect the biological responses. It was able to promote the attachment and growth of human osteoblast-like cells (Huang et al., 2004). Clinical trials have shown that HA cement is both biocompatible and resistant to infection and that the HA coating improves the success rate of implants. It has also been demonstrated that HA ceramics support mesenchymal stem cell (MSC) attachment, proliferation, and differentiation (Zhao et al., 2006). Nano scaled HA with extraordinary properties such as high surface area to volume ratio and ultra fine structure similar to that of biological apatite, which is of great effect on cell-biomaterial interaction, has been reported to be used for the treatment of bone defects, and it could bond to living bone in implanted areas (Liuyun et al., 2008). HAp has been used in bone regeneration and as a substitute of bone and teeth because it is a biocompatible, bioactive, non-inflammatory, non-toxic, osteoconductive and non-immunogenic material (Grande et al., 2009).

Biocomposite Materials 115

CPC powder can be mixed with the chitosan liquid to form a paste that can be applied in surgery via minimally invasive techniques such as injection, with fast-setting and anti-

Bone defects that are generated by tumor resection, trauma, and congenital abnormality have been clinically treated by the implantation of bioceramics or autogenous and allogenous bone grafts. Although autografting is a popular procedure for reconstructive surgery, it has several disadvantages, such as the shortage of donor supply, the persistence of pain, the nerve damage, fracture, and cosmetic disability at the donor site. On the other hand, there are no donor site problems for allografting, while allografting has some clinical risks including disease transmission and immunological reaction (Takahashi et al., 2005). HA has been incorporated into a wide variety of biomedical devices including dental implants, coatings on Ti based hip implants, biodegradable scaffolds, and other types of orthopedic implants (Wilson and Hullb, 2007). Synthetic HA, is a bioactive material that is chemically similar to biological apatite HA, has been used as a bioactive phase in the composites, coating on metal implants, and granular filler for direct incorporation into human tissues (Rehman et al., 1995). The characteristics of an ideal ceramic composite for bone tissue engineering should comply to the following parameters:(1) A biodegradability for bone remodeling, (2) A macroporosity for in-growth into the composite, (3) Mechanical stability/ease of handling, (4) Osteoconductivity to guide bone around/inside the implant, (5) Carrier for growth factors/cells. The use of these materials for tissue engineering purposes is still explored. Most researchers are aware that HA has low resorbability of sintered Ca P-ceramics. Because of the positive influence of ceramics on cell differentiation/proliferation, it is not surprising that bone forming cells are introduced into these ceramics to speed-up tissue in-growth. The surface of sintered ceramics is chemically stable and therefore a good substrate for seeding cells. In the field of bone tissue engineering most scaffolds are ceramic or ceramic-derivatives. Especially HA-based calcium phosphate compound is regarded as high-potential scaffolds due to their osteoconductive properties. Next to the scaffold material, in bone tissue engineering, cells and growth factors are also introduced to speed-up tissue in-growth. In most ceramic scaffolds, however, difficulties arise when cells are added, most probably due to a limited supply of nutrition at the inside of the implant or less than optimal cell-cell interactions. Growth factors like bone morphogenic protein-2 (BMP-2), transforming growth factor (TGF-β), basic fibroblast growth factor (b-FGF) and vascular endothelial growth factor (VEGF) are commonly introduced into these scaffolds due to their osteoinductive properties and vascularization (Habrakenb et al., (2007). HA has a composition and structure very close to natural bone mineral and therefore has been considered to be the ideal material to build bone tissue engineering scaffold due to its osteoconductivity and osteoinductivity (Wang et al., 2007). Some in vitro studies demonstrated that nano-phase HA (67 nm grain size) significantly enhanced osteoblast adhesion and strikingly inhibited competitive fibroblast adhesion

washout capabilities to form a scaffold in situ (Moreau and Xu, 2009).

**2.2. Applications** 

Instability of the particulate nHA is often encountered when the particles are mixed with saline or patient's blood and hence migrate from the implanted site into surrounding tissues and causing damage to health tissue (Miyamato and Shikawa, 1998). Also, HA ceramic is difficult to shape in specific forms required for bone substitution, due to its hardness and brittleness. Therefore, a composites of HAp and organic polymers have become of great interest to compensate the weak mechanical points of HAp (Furukawa et al., 2000). Mohamed and Mostafa, (2008) reported that HA has low fracture toughness, hardness and brittleness, therefore, HA cannot serve as a bulk implant material under the high physiological loading conditions traditionally associated with implants.

Since the natural bone is a composite mainly consisted of nano-sized, needle-like HAp crystals and collagen fibers, many efforts have been made to modify HAp by polymers, such as poly-lactic acid (Kasuga et al., 2001), chitosan (Viala et al., 1998), polyethylene (Wang and Bonfield, 2001) to compensate the weak mechanical points of HAp. Yamaguchi et al., (2003) reported that chitosan is flexible and has a high resistance upon heating due to the intramolecular hydrogen bonds formed between hydroxyl and amino groups. A composite biomaterial of HAp and chitosan is, therefore, expected to show an increased osteoconductivity and biodegradation together with a sufficient mechanical strength for orthopedic use. A major disadvantage of current orthopedic implant materials such as sintered hydroxyapatite is that they exist in a hardened form, requiring the surgeon to drill the surgical site around the implant or to carve the graft to the desired shape. This can lead to increases in bone loss, trauma, and surgical time. Hence, the moldable, self-setting calcium phosphate cement (CPC)-chitosan composite is desirable for dental, craniofacial and orthopedic repairs, especially where shaping and contouring for esthetics are needed. The CPC powder can be mixed with the chitosan liquid to form a paste that can be applied in surgery via minimally invasive techniques such as injection, with fast-setting and antiwashout capabilities to form a scaffold in situ (Moreau and Xu, 2009).

#### **2.2. Applications**

114 Composites and Their Applications

HA power (HAp), a major inorganic component of bone, has been used extensively for biomedical implant applications and bone regeneration due to its bioactive, biodegradable and osteoconductive properties (Gomez-Vega et al., 2000). HA is available in market in many forms like solids blocks, micro-porous blocks and as granules (Murgan and Ramakrishna, 2005). Nano-hydroxyapatite (n-HA) has been proven to be of great biological efficacy. nHA precipitates may have higher solubility and therefore affect the biological responses. It was able to promote the attachment and growth of human osteoblast-like cells (Huang et al., 2004). Clinical trials have shown that HA cement is both biocompatible and resistant to infection and that the HA coating improves the success rate of implants. It has also been demonstrated that HA ceramics support mesenchymal stem cell (MSC) attachment, proliferation, and differentiation (Zhao et al., 2006). Nano scaled HA with extraordinary properties such as high surface area to volume ratio and ultra fine structure similar to that of biological apatite, which is of great effect on cell-biomaterial interaction, has been reported to be used for the treatment of bone defects, and it could bond to living bone in implanted areas (Liuyun et al., 2008). HAp has been used in bone regeneration and as a substitute of bone and teeth because it is a biocompatible, bioactive, non-inflammatory,

non-toxic, osteoconductive and non-immunogenic material (Grande et al., 2009).

physiological loading conditions traditionally associated with implants.

Instability of the particulate nHA is often encountered when the particles are mixed with saline or patient's blood and hence migrate from the implanted site into surrounding tissues and causing damage to health tissue (Miyamato and Shikawa, 1998). Also, HA ceramic is difficult to shape in specific forms required for bone substitution, due to its hardness and brittleness. Therefore, a composites of HAp and organic polymers have become of great interest to compensate the weak mechanical points of HAp (Furukawa et al., 2000). Mohamed and Mostafa, (2008) reported that HA has low fracture toughness, hardness and brittleness, therefore, HA cannot serve as a bulk implant material under the high

Since the natural bone is a composite mainly consisted of nano-sized, needle-like HAp crystals and collagen fibers, many efforts have been made to modify HAp by polymers, such as poly-lactic acid (Kasuga et al., 2001), chitosan (Viala et al., 1998), polyethylene (Wang and Bonfield, 2001) to compensate the weak mechanical points of HAp. Yamaguchi et al., (2003) reported that chitosan is flexible and has a high resistance upon heating due to the intramolecular hydrogen bonds formed between hydroxyl and amino groups. A composite biomaterial of HAp and chitosan is, therefore, expected to show an increased osteoconductivity and biodegradation together with a sufficient mechanical strength for orthopedic use. A major disadvantage of current orthopedic implant materials such as sintered hydroxyapatite is that they exist in a hardened form, requiring the surgeon to drill the surgical site around the implant or to carve the graft to the desired shape. This can lead to increases in bone loss, trauma, and surgical time. Hence, the moldable, self-setting calcium phosphate cement (CPC)-chitosan composite is desirable for dental, craniofacial and orthopedic repairs, especially where shaping and contouring for esthetics are needed. The

**2.1. Properties** 

Bone defects that are generated by tumor resection, trauma, and congenital abnormality have been clinically treated by the implantation of bioceramics or autogenous and allogenous bone grafts. Although autografting is a popular procedure for reconstructive surgery, it has several disadvantages, such as the shortage of donor supply, the persistence of pain, the nerve damage, fracture, and cosmetic disability at the donor site. On the other hand, there are no donor site problems for allografting, while allografting has some clinical risks including disease transmission and immunological reaction (Takahashi et al., 2005). HA has been incorporated into a wide variety of biomedical devices including dental implants, coatings on Ti based hip implants, biodegradable scaffolds, and other types of orthopedic implants (Wilson and Hullb, 2007). Synthetic HA, is a bioactive material that is chemically similar to biological apatite HA, has been used as a bioactive phase in the composites, coating on metal implants, and granular filler for direct incorporation into human tissues (Rehman et al., 1995). The characteristics of an ideal ceramic composite for bone tissue engineering should comply to the following parameters:(1) A biodegradability for bone remodeling, (2) A macroporosity for in-growth into the composite, (3) Mechanical stability/ease of handling, (4) Osteoconductivity to guide bone around/inside the implant, (5) Carrier for growth factors/cells. The use of these materials for tissue engineering purposes is still explored. Most researchers are aware that HA has low resorbability of sintered Ca P-ceramics. Because of the positive influence of ceramics on cell differentiation/proliferation, it is not surprising that bone forming cells are introduced into these ceramics to speed-up tissue in-growth. The surface of sintered ceramics is chemically stable and therefore a good substrate for seeding cells. In the field of bone tissue engineering most scaffolds are ceramic or ceramic-derivatives. Especially HA-based calcium phosphate compound is regarded as high-potential scaffolds due to their osteoconductive properties. Next to the scaffold material, in bone tissue engineering, cells and growth factors are also introduced to speed-up tissue in-growth. In most ceramic scaffolds, however, difficulties arise when cells are added, most probably due to a limited supply of nutrition at the inside of the implant or less than optimal cell-cell interactions. Growth factors like bone morphogenic protein-2 (BMP-2), transforming growth factor (TGF-β), basic fibroblast growth factor (b-FGF) and vascular endothelial growth factor (VEGF) are commonly introduced into these scaffolds due to their osteoinductive properties and vascularization (Habrakenb et al., (2007). HA has a composition and structure very close to natural bone mineral and therefore has been considered to be the ideal material to build bone tissue engineering scaffold due to its osteoconductivity and osteoinductivity (Wang et al., 2007). Some in vitro studies demonstrated that nano-phase HA (67 nm grain size) significantly enhanced osteoblast adhesion and strikingly inhibited competitive fibroblast adhesion

compared to conventional, 179 nm grain size HA, after just 4 h of culture. Researchers believe they know why they have elucidated the highest adsorption of vitronectin (a protein well known to promote osteoblast adhesion) on nanophase ceramics, which may explain the subsequent enhanced osteoblast adhesion on these materials. In addition, enhanced osteoclast-like cell functions (such as the synthesis of tartrate-resistant acid phosphatase (TRAP) and the formation of resorption pits) have also been observed on nano-HA compared to conventional HA, nano-porous or nano-fibrous polymer matrices can be fabricated via electrospinning, phase separation, particulate leaching, chemical etching and 3-D printing techniques (Zhang et al., 2008).

Biocomposite Materials 117

**3.3. Properties** 

*3.3.1. Physical and chemical properties* 

The size of chitosan molecule plays an important role in drug delivery system where the release rate was reported to be inversely proportional to the molecular weight of the reservoir. Chitosan is insoluble in water, alkali and many organic solvents but soluble in many dilute aqueous solutions of organic acids, of which most commonly used are formic acid and acetic acid. Tomihata and Ikada, (1997) concluded that molecular weight of chitosan affect the crystal size and morphological character of its cast film. Gorbunoff, (1984) concluded that the chemical interaction between NH3+ and HPO42- control the adsorption of chitosan on octacalcium phosphate (OCP) crystal surface which is similar to the binding of protein to the surface of apatite crystals. Chitosan permeability increased in the amorphous region of the membrane (Brine, 1984). Chitosan forms a polycation in aqueous acid solutions like acetic acid and hydrochloric acid via protonation of amine functions (Muzarelli, 1985).

Blair et al., (1987) reported that chitosan prepared from crab shell chitin has a lower molecular weight than that prepared from prawn shells and tensile strength versus elongation decrease at the break with prolonged treatment in alkali solutions and both increase proportional to higher molecular weight. Also, crystallinity of membrane increased with decreasing molecular weight of chitosan and it is hydrophilic i.e it contains a large number of OH groups which are easy to combine with another group and form new bond (Ogawa et al., 1992). The larger molecular weight of chitosan used, the higher tensile strength and higher tensile elongation of membrane were obtained (Chen and Hwa, 1995). The degree of deacetylation affects the physical properties of chitosan membrane and it does not change during ultrasonic degradation on chitosan. The molecular weight of the

Chitosan is flexible and has a high resistance upon heating due to the intra-molecular hydrogen bonds formed between hydroxyl and amino groups (Lee et al., 1999). The degradation rate is inversely related to the degree of crystallinity which is controlled mainly by the degree of deacetylation (DD). Highly deacetylated forms (85 %) exhibit relatively a low degradation rate and may take several months in vivo, whereas, the forms with lower DD degrade more rapidly. The degradation rates also inherently affect both the mechanical

The cationic nature of chitosan also allows for pH-dependent electrostatic interactions with anionic glycosaminoglycans (GAG) and proteoglycans distributed widely throughout the body and other negatively charged species. This property is one of the important elements for tissue engineering applications because numbers of cytokines/growth factors are known to be bound and modulated by GAG including heparin and heparin sulfate (Nishikawa et al., 2000). Chitosan is easy to handle for its resistive nature to heating due to intra-molecular hydrogen bonds between hydroxyl and amino groups (Itoh et al., 2003). Chitosan is a very versatile biopolymer with film, fiber, and micro/nanoparticle forming properties. It is biocompatible, biodegradable and non-toxic (Yilmaz, 2004). Deacetylation of chitin with a

prepared chitosan depends on the severity of the deacetylation process.

and solubility properties (Kamiyama et al., 1999).

## **3. Chitosan polymer (C)**

## **3.1. Structure**

Brimacombe and Webber, (1964) reported that the chitosan consists of repeating units of beta (1-4) 2-amino-2-deoxy-D-glucopyranon (D-glucosamine) (Fig.1). The primary unit of chitin is 2-acetamido-2-deoxy-D-glucose, while that of chitosan is 2-amino-2-deoxy-Dglucose with beta, 1-4 glucosidic linkages (Muzarelli, 1985). Chitosan is a natural polysaccharide derived from chitin by its deacetylation (Dang and Leong, 2006). Chitosan shares a number of chemical and structural similarities with collagen. These similarities form the basis for the development of HAp/chitosan nano-composites for use in bone tissue engineering (Wilson et al., 2007

**Figure 1.** Structure of chitosan

## **3.2. Origin**

The history of chitosan dates back to the 19 th century, when Rouget, 1859 discussed the deacetylated form of chitosan. Chitin, the source material for chitosan, is one of the most abundant organic materials, being second only to cellulose in the amount produced annually by biosynthesis. It is an important constituent of the exoskeleton in animals, especially in crustacean, molluscs and insects. It is also the principal fibrillar polymer in the cell wall of certain fungi (Eugene and Lee, 2003). Chitosan is one of the most abundant naturally occurring polysaccharides, primarily obtained as a sub-product of seafood, containing amino and hydroxyl groups (Muzarelli, 1985 and Manjubala et al., 2006).

## **3.3. Properties**

116 Composites and Their Applications

3-D printing techniques (Zhang et al., 2008).

**3. Chitosan polymer (C)** 

engineering (Wilson et al., 2007

**Figure 1.** Structure of chitosan

**3.2. Origin** 

**3.1. Structure** 

compared to conventional, 179 nm grain size HA, after just 4 h of culture. Researchers believe they know why they have elucidated the highest adsorption of vitronectin (a protein well known to promote osteoblast adhesion) on nanophase ceramics, which may explain the subsequent enhanced osteoblast adhesion on these materials. In addition, enhanced osteoclast-like cell functions (such as the synthesis of tartrate-resistant acid phosphatase (TRAP) and the formation of resorption pits) have also been observed on nano-HA compared to conventional HA, nano-porous or nano-fibrous polymer matrices can be fabricated via electrospinning, phase separation, particulate leaching, chemical etching and

Brimacombe and Webber, (1964) reported that the chitosan consists of repeating units of beta (1-4) 2-amino-2-deoxy-D-glucopyranon (D-glucosamine) (Fig.1). The primary unit of chitin is 2-acetamido-2-deoxy-D-glucose, while that of chitosan is 2-amino-2-deoxy-Dglucose with beta, 1-4 glucosidic linkages (Muzarelli, 1985). Chitosan is a natural polysaccharide derived from chitin by its deacetylation (Dang and Leong, 2006). Chitosan shares a number of chemical and structural similarities with collagen. These similarities form the basis for the development of HAp/chitosan nano-composites for use in bone tissue

The history of chitosan dates back to the 19 th century, when Rouget, 1859 discussed the deacetylated form of chitosan. Chitin, the source material for chitosan, is one of the most abundant organic materials, being second only to cellulose in the amount produced annually by biosynthesis. It is an important constituent of the exoskeleton in animals, especially in crustacean, molluscs and insects. It is also the principal fibrillar polymer in the cell wall of certain fungi (Eugene and Lee, 2003). Chitosan is one of the most abundant naturally occurring polysaccharides, primarily obtained as a sub-product of seafood,

containing amino and hydroxyl groups (Muzarelli, 1985 and Manjubala et al., 2006).

## *3.3.1. Physical and chemical properties*

The size of chitosan molecule plays an important role in drug delivery system where the release rate was reported to be inversely proportional to the molecular weight of the reservoir. Chitosan is insoluble in water, alkali and many organic solvents but soluble in many dilute aqueous solutions of organic acids, of which most commonly used are formic acid and acetic acid. Tomihata and Ikada, (1997) concluded that molecular weight of chitosan affect the crystal size and morphological character of its cast film. Gorbunoff, (1984) concluded that the chemical interaction between NH3+ and HPO42- control the adsorption of chitosan on octacalcium phosphate (OCP) crystal surface which is similar to the binding of protein to the surface of apatite crystals. Chitosan permeability increased in the amorphous region of the membrane (Brine, 1984). Chitosan forms a polycation in aqueous acid solutions like acetic acid and hydrochloric acid via protonation of amine functions (Muzarelli, 1985).

Blair et al., (1987) reported that chitosan prepared from crab shell chitin has a lower molecular weight than that prepared from prawn shells and tensile strength versus elongation decrease at the break with prolonged treatment in alkali solutions and both increase proportional to higher molecular weight. Also, crystallinity of membrane increased with decreasing molecular weight of chitosan and it is hydrophilic i.e it contains a large number of OH groups which are easy to combine with another group and form new bond (Ogawa et al., 1992). The larger molecular weight of chitosan used, the higher tensile strength and higher tensile elongation of membrane were obtained (Chen and Hwa, 1995). The degree of deacetylation affects the physical properties of chitosan membrane and it does not change during ultrasonic degradation on chitosan. The molecular weight of the prepared chitosan depends on the severity of the deacetylation process.

Chitosan is flexible and has a high resistance upon heating due to the intra-molecular hydrogen bonds formed between hydroxyl and amino groups (Lee et al., 1999). The degradation rate is inversely related to the degree of crystallinity which is controlled mainly by the degree of deacetylation (DD). Highly deacetylated forms (85 %) exhibit relatively a low degradation rate and may take several months in vivo, whereas, the forms with lower DD degrade more rapidly. The degradation rates also inherently affect both the mechanical and solubility properties (Kamiyama et al., 1999).

The cationic nature of chitosan also allows for pH-dependent electrostatic interactions with anionic glycosaminoglycans (GAG) and proteoglycans distributed widely throughout the body and other negatively charged species. This property is one of the important elements for tissue engineering applications because numbers of cytokines/growth factors are known to be bound and modulated by GAG including heparin and heparin sulfate (Nishikawa et al., 2000). Chitosan is easy to handle for its resistive nature to heating due to intra-molecular hydrogen bonds between hydroxyl and amino groups (Itoh et al., 2003). Chitosan is a very versatile biopolymer with film, fiber, and micro/nanoparticle forming properties. It is biocompatible, biodegradable and non-toxic (Yilmaz, 2004). Deacetylation of chitin with a

degree of deacetylation more than 50 % gives chitosan, which is soluble in organic acids such as acetic or formic acid, and has been more widely used than chitin as films, membranes, fibres and particles ( Kang et al., 2006). Chitosan has three types of reactive functional groups, an amino group as well as both primary and secondary hydroxyl groups at the C(2), C(3), and C(6) positions, respectively. These groups allow modification of chitosan like graft copolymerization for specific applications, which can produce various useful scaffolds for tissue engineering applications. The chemical nature of chitosan in turn provides many possibilities for covalent and ionic modifications that allow extensive adjustment of mechanical and biological properties (Kim et al., 2008). When the temperature is 30°C, the crystallinity of chitosan membrane is relatively high and its crystal particles are much small, which makes the mechanical properties poor. When the temperature is as high as 90°C, the temperature will cause the change of chitosan properties and the chitosan membrane is nearly not a crystal structure at this moment, resulting in the fall of mechanical properties (Xianmiao et al., 2009).

Biocomposite Materials 119

et al., 2008). Chitosan has been shown to degrade in vivo, which is mainly by enzymatic hydrolysis. The degradability of a scaffold plays a crucial role on the long-term performance of tissue-engineered cell/material construct because it affect many cellular process, including cell growth, tissue regeneration, and host response. If a scaffold is used for tissue engineering of skeletal system, degradation of the scaffold biomaterial should be relatively slow, as it has to maintain the mechanical strength until tissue regeneration is almost completed. Lysozyme is the primary enzyme responsible for in vivo degradation of

Chitosan of biopolymer have been used as blood coagulant, in artificial kidney membrane, digestive sutures, hypercholesterolemic agents, media for the slow release of drugs and hemostatic agent (Mohamed, 2004). It is a good candidate for biomedical applications such as for wound healing, vaccine delivery, as well as tissue regeneration (Yilmaz, 2004). It has been extensively investigated in biotechnological, biomedical, and environmental fields (Dang and Leong, 2006). Chitosan polymer is used in dentistry, because it prevents the formation of plaque and tooth decay. Since chitosan can regenerate the connective tissue that covers the teeth near the gums, it offers possibilities for treating periodontal diseases

Aiba et al., (1986) used chitosan in membrane separation, chemical engineering, medicine and biotechnology areas. It was also found that the water adsorption and the mechanical properties of fibroin membrane were improved by blending chitosan (Chen et al., 1998).

Muzzarelli et al., (1988) used chitosan as artificial skin substitute and they reported that no adverse effect after implantation in tissue. In general, these materials have been found to evoke a minimal foreign body reaction, with little or no fibrous encapsulation. It observed the typical course of healing with formation of normal granulation tissue, often with accelerated angiogenesis (Suh and Matthew, 2000). Also, chitosan has many advantages for wound healing such as hemostasis, accelerating the tissue regeneration and stimulating the fibroblast synthesis of collagen (Mi et al., 2001). Chitosan possesses the properties favorable for promoting rapid dermal regeneration and accelerate wound healing suitable for applications extending from simple wound coverings to sophisticated artificial skin matrices

Sapelli et al., (1986) used chitosan powder to promote healing of periodontal pockets, palatal wounds and extraction sites. Malette et al., (1986) proved enhanced leg bone regeneration in

chitosan, which appears to target acetylated residues (Kim et al., 2008).

such as gingivitis and periodontitis (Elizalde-Pen et al., 2007).

**3.4. Applications** 

*3.4.1. Membrane* 

*3.4.2. Skin* 

(Kim et al., 2008).

*3.4.3. Bone substitutes* 

#### *3.3.2. Biological properties*

Chitosan potentiates the differentiation of osteoprogenitor cells and may facilitate the formation of bone. Rao and Sharma, (1997) concluded that chitosan is an ideal non-toxic biopolymer and the cell binding and cell activating properties of chitosan play a crucial role in its potential action. Chitosan degrades in the body to non-harmful and non-toxic compounds and has been used in various fields such as nutrition, metal recovery and biomaterials (Muzzarelli et al., 2001). It has gained much attention as a biomaterial in diverse tissue engineering applications due to its low cost, large-scale availability, antimicrobial activity, and biocompatibility (Khora and Limb, 2003). Chitosan was suggested as an alternative polymer for use in orthopedic applications to provide temporary mechanical support to the regeneration of bone cell in-growth due to its good biocompatible (Khora and Limb, 2003), non-toxic, biodegradable and inherent wound healing characteristics (Eugene and Lee 2003). Chitosan had been used in various forms such as zero dimension microsphers, two-dimension membrane, three-dimension pin or rod (Hu et al., 2003). Therefore, much attention has been paid to chitosan-based biomedical materials, for instance, as a drug delivery carrier or a wound-healing agent. Chitosan is structurally similar to glycosaminoglycan (GAG) and has many desirable properties as tissue engineering scaffolds (Kuma, et al., 2004). It was reported as being neither antigenic in mamalian test system nor thrombogenic and chitosan reported to improve hemostasis, decreased fibroplasias with enhanced tissue organization as well as normal bone formation (Mohamed, 2004). Chitosan marginally supports biological activity of diverse cell types (Sarasam and Madihally, 2005).

A number of natural and synthetic polymers have been studied for overcoming the weak points as bone substitutes. Chitosan has been found in a broad spectrum of applications along with unique biological properties including biocompatibility, biodegradability to harmless products, non-toxicity, physiological inertness, remarkable affinity to proteins, antibacterial, haemostatic, fungi-static, anti-tumoral and anti-cholesteremic properties (Kim et al., 2008). Chitosan has been shown to degrade in vivo, which is mainly by enzymatic hydrolysis. The degradability of a scaffold plays a crucial role on the long-term performance of tissue-engineered cell/material construct because it affect many cellular process, including cell growth, tissue regeneration, and host response. If a scaffold is used for tissue engineering of skeletal system, degradation of the scaffold biomaterial should be relatively slow, as it has to maintain the mechanical strength until tissue regeneration is almost completed. Lysozyme is the primary enzyme responsible for in vivo degradation of chitosan, which appears to target acetylated residues (Kim et al., 2008).

## **3.4. Applications**

118 Composites and Their Applications

properties (Xianmiao et al., 2009).

(Sarasam and Madihally, 2005).

*3.3.2. Biological properties* 

degree of deacetylation more than 50 % gives chitosan, which is soluble in organic acids such as acetic or formic acid, and has been more widely used than chitin as films, membranes, fibres and particles ( Kang et al., 2006). Chitosan has three types of reactive functional groups, an amino group as well as both primary and secondary hydroxyl groups at the C(2), C(3), and C(6) positions, respectively. These groups allow modification of chitosan like graft copolymerization for specific applications, which can produce various useful scaffolds for tissue engineering applications. The chemical nature of chitosan in turn provides many possibilities for covalent and ionic modifications that allow extensive adjustment of mechanical and biological properties (Kim et al., 2008). When the temperature is 30°C, the crystallinity of chitosan membrane is relatively high and its crystal particles are much small, which makes the mechanical properties poor. When the temperature is as high as 90°C, the temperature will cause the change of chitosan properties and the chitosan membrane is nearly not a crystal structure at this moment, resulting in the fall of mechanical

Chitosan potentiates the differentiation of osteoprogenitor cells and may facilitate the formation of bone. Rao and Sharma, (1997) concluded that chitosan is an ideal non-toxic biopolymer and the cell binding and cell activating properties of chitosan play a crucial role in its potential action. Chitosan degrades in the body to non-harmful and non-toxic compounds and has been used in various fields such as nutrition, metal recovery and biomaterials (Muzzarelli et al., 2001). It has gained much attention as a biomaterial in diverse tissue engineering applications due to its low cost, large-scale availability, antimicrobial activity, and biocompatibility (Khora and Limb, 2003). Chitosan was suggested as an alternative polymer for use in orthopedic applications to provide temporary mechanical support to the regeneration of bone cell in-growth due to its good biocompatible (Khora and Limb, 2003), non-toxic, biodegradable and inherent wound healing characteristics (Eugene and Lee 2003). Chitosan had been used in various forms such as zero dimension microsphers, two-dimension membrane, three-dimension pin or rod (Hu et al., 2003). Therefore, much attention has been paid to chitosan-based biomedical materials, for instance, as a drug delivery carrier or a wound-healing agent. Chitosan is structurally similar to glycosaminoglycan (GAG) and has many desirable properties as tissue engineering scaffolds (Kuma, et al., 2004). It was reported as being neither antigenic in mamalian test system nor thrombogenic and chitosan reported to improve hemostasis, decreased fibroplasias with enhanced tissue organization as well as normal bone formation (Mohamed, 2004). Chitosan marginally supports biological activity of diverse cell types

A number of natural and synthetic polymers have been studied for overcoming the weak points as bone substitutes. Chitosan has been found in a broad spectrum of applications along with unique biological properties including biocompatibility, biodegradability to harmless products, non-toxicity, physiological inertness, remarkable affinity to proteins, antibacterial, haemostatic, fungi-static, anti-tumoral and anti-cholesteremic properties (Kim Chitosan of biopolymer have been used as blood coagulant, in artificial kidney membrane, digestive sutures, hypercholesterolemic agents, media for the slow release of drugs and hemostatic agent (Mohamed, 2004). It is a good candidate for biomedical applications such as for wound healing, vaccine delivery, as well as tissue regeneration (Yilmaz, 2004). It has been extensively investigated in biotechnological, biomedical, and environmental fields (Dang and Leong, 2006). Chitosan polymer is used in dentistry, because it prevents the formation of plaque and tooth decay. Since chitosan can regenerate the connective tissue that covers the teeth near the gums, it offers possibilities for treating periodontal diseases such as gingivitis and periodontitis (Elizalde-Pen et al., 2007).

## *3.4.1. Membrane*

Aiba et al., (1986) used chitosan in membrane separation, chemical engineering, medicine and biotechnology areas. It was also found that the water adsorption and the mechanical properties of fibroin membrane were improved by blending chitosan (Chen et al., 1998).

## *3.4.2. Skin*

Muzzarelli et al., (1988) used chitosan as artificial skin substitute and they reported that no adverse effect after implantation in tissue. In general, these materials have been found to evoke a minimal foreign body reaction, with little or no fibrous encapsulation. It observed the typical course of healing with formation of normal granulation tissue, often with accelerated angiogenesis (Suh and Matthew, 2000). Also, chitosan has many advantages for wound healing such as hemostasis, accelerating the tissue regeneration and stimulating the fibroblast synthesis of collagen (Mi et al., 2001). Chitosan possesses the properties favorable for promoting rapid dermal regeneration and accelerate wound healing suitable for applications extending from simple wound coverings to sophisticated artificial skin matrices (Kim et al., 2008).

### *3.4.3. Bone substitutes*

Sapelli et al., (1986) used chitosan powder to promote healing of periodontal pockets, palatal wounds and extraction sites. Malette et al., (1986) proved enhanced leg bone regeneration in

dogs using chitosan. It was reported to accelerate wound healing and was applied for bone wound repair in dogs (Borah et al., 1992) as well as bone growth in critical size metacarpal fibular defects. Klokkevold et al., (1992) concluded that chitosan solution may enhance the formation of bone. Later, it was reported to improve osseous healing of defects in femoral coundyl of sheep and stimulated cell proliferation and organized the hystoarchitectural tissue structure (Muzzarelli et al., 1994). Chitosan has been also extensively used in bone tissue engineering since after exploring its capacity to promote growth and mineral rich matrix deposition by osteoblasts in culture. Also, chitosan is biocompatible (additional minimizes local inflammation), biodegradable, and can be molded into porous structures (allows osteoconduction) (Martino et al., 2005 and Kim et al., 2008).

Biocomposite Materials 121

application in bone tissue engineering. The incorporation of chitosan into a collagen scaffold increases the mechanical strength of the scaffold and reduces the biodegradation rate against

**Figure 2.** Cell viability of osteoblasts cultured for 21 days on the sponges (Arpornmaeklong et al., 2007).

The collagen, chitosan and chitosan–collagen sponges were biocompatible. All sponges supported growth of cells on three-dimensional structures in a similar manner. Chitosan sponges had a tendency to promote growth of cells to a greater extent than the other groups (Fig.2). It is postulated that strong attraction between positive charges on the chitosan surface and negative charges on the cell surface enhanced the metabolic activity of cells on chitosan sponges (Mi et al., 2001). In vivo, chitosan enhances angiogenesis and wound

a) Blood vessels:The chitosan fibers offer the potential of being fabricated into blood vessels and their blood compatibility results demonstrated for applications where hemocompatibility is required (Khora and Limb, 2003). Kim et al., (2008) reported that the effort has made to overcome both incomplete endothelialization and smooth muscle cell hyperplasia, which are two of the problems contributing to the poor performance of existing small-diameter (4 mm) vascular grafts, through complexation of GAGs with porous chitosan scaffolds. GAG-based material should promise because of their growth inhibitory effects on vascular smooth muscle cells and their anti-coagulant activity. However, few data regarding chitosan as a scaffold of tissue engineered blood vessels have been reported. Chitosan itself

healing, and supports growth and differentiation of osteoblasts (Lee et al., 2004).

collagenase (Arpornmaeklong et al., 2007).

*3.4.7. Other applications* 

## *3.4.4. Drug delivery system (DDS)*

Chitosan as an inert and hydrophilic material, its gel is suitable for application as matrices for enzyme/cell immobilization (Roberts, 1992) and for separation processes (Li et al., 1992). Felt et al., (1998) discussed the use of chitosan to manufacture sustained release systems deliverable by other routes such as nasal, ophthalmic, transdermal and implantable devices. Tarsi et al., (1998) suggested that low molecular weight chitosan may be very interesting as potential antidental caries agents. Chitosan has been reported to enhance drug delivery across the nasal or mucosal layer without damage (van der Lubben et al., 2001). The selectivity of membrane is a critical parameter in membrane separation and several factors are affecting its selectivity such as pore size, thus it is very suitable for the use as DDS and in artificial kidney (Mohamed, 2004). The cationic properties of chitosan offer valuable properties for drug delivery systems, gene delivery systems, and tissue engineering, that is, the formation of ion complexes between chitosan and anionic drugs or DNA can be used as a delivery vehicle (Kim et al., 2007).

## *3.4.5. Anti-bacterial*

The experiments of antibacterial activity of chitosan-graft-polyethylene terephthalate (PET) against S. aureus showed a high growth inhibition in the range of 75–86% and still maintained a 48–58% bacterial growth inhibition after laundering (Hu et al., 2003). Chitosan and chitooligosaccharides grafted membranes showed antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, methicilin-resistant Staphylococcus aureus (MRSA), and S. aureus (Hu et al., 2003). Chitosan is a biomaterial with antiseptic property and the influence of the release or positive migration of protonated glucosamine fractions from the biopolymer into the microbial culture is the responsible event for the antimicrobial performance of the biopolymer (Beherei et al., 2009).

#### *3.4.6. In-vitro application*

Chitosan and collagen have intrinsic properties that support growth and differentiation of osteoblasts. Collagen was combined with chitosan and cross-linked to improve the biological stability and strength of chitosan–collagen composite sponges to reach the demand of an application in bone tissue engineering. The incorporation of chitosan into a collagen scaffold increases the mechanical strength of the scaffold and reduces the biodegradation rate against collagenase (Arpornmaeklong et al., 2007).

**Figure 2.** Cell viability of osteoblasts cultured for 21 days on the sponges (Arpornmaeklong et al., 2007).

The collagen, chitosan and chitosan–collagen sponges were biocompatible. All sponges supported growth of cells on three-dimensional structures in a similar manner. Chitosan sponges had a tendency to promote growth of cells to a greater extent than the other groups (Fig.2). It is postulated that strong attraction between positive charges on the chitosan surface and negative charges on the cell surface enhanced the metabolic activity of cells on chitosan sponges (Mi et al., 2001). In vivo, chitosan enhances angiogenesis and wound healing, and supports growth and differentiation of osteoblasts (Lee et al., 2004).

## *3.4.7. Other applications*

120 Composites and Their Applications

*3.4.4. Drug delivery system (DDS)* 

a delivery vehicle (Kim et al., 2007).

biopolymer (Beherei et al., 2009).

*3.4.6. In-vitro application* 

*3.4.5. Anti-bacterial* 

dogs using chitosan. It was reported to accelerate wound healing and was applied for bone wound repair in dogs (Borah et al., 1992) as well as bone growth in critical size metacarpal fibular defects. Klokkevold et al., (1992) concluded that chitosan solution may enhance the formation of bone. Later, it was reported to improve osseous healing of defects in femoral coundyl of sheep and stimulated cell proliferation and organized the hystoarchitectural tissue structure (Muzzarelli et al., 1994). Chitosan has been also extensively used in bone tissue engineering since after exploring its capacity to promote growth and mineral rich matrix deposition by osteoblasts in culture. Also, chitosan is biocompatible (additional minimizes local inflammation), biodegradable, and can be molded into porous structures

Chitosan as an inert and hydrophilic material, its gel is suitable for application as matrices for enzyme/cell immobilization (Roberts, 1992) and for separation processes (Li et al., 1992). Felt et al., (1998) discussed the use of chitosan to manufacture sustained release systems deliverable by other routes such as nasal, ophthalmic, transdermal and implantable devices. Tarsi et al., (1998) suggested that low molecular weight chitosan may be very interesting as potential antidental caries agents. Chitosan has been reported to enhance drug delivery across the nasal or mucosal layer without damage (van der Lubben et al., 2001). The selectivity of membrane is a critical parameter in membrane separation and several factors are affecting its selectivity such as pore size, thus it is very suitable for the use as DDS and in artificial kidney (Mohamed, 2004). The cationic properties of chitosan offer valuable properties for drug delivery systems, gene delivery systems, and tissue engineering, that is, the formation of ion complexes between chitosan and anionic drugs or DNA can be used as

The experiments of antibacterial activity of chitosan-graft-polyethylene terephthalate (PET) against S. aureus showed a high growth inhibition in the range of 75–86% and still maintained a 48–58% bacterial growth inhibition after laundering (Hu et al., 2003). Chitosan and chitooligosaccharides grafted membranes showed antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, methicilin-resistant Staphylococcus aureus (MRSA), and S. aureus (Hu et al., 2003). Chitosan is a biomaterial with antiseptic property and the influence of the release or positive migration of protonated glucosamine fractions from the biopolymer into the microbial culture is the responsible event for the antimicrobial performance of the

Chitosan and collagen have intrinsic properties that support growth and differentiation of osteoblasts. Collagen was combined with chitosan and cross-linked to improve the biological stability and strength of chitosan–collagen composite sponges to reach the demand of an

(allows osteoconduction) (Martino et al., 2005 and Kim et al., 2008).

a) Blood vessels:The chitosan fibers offer the potential of being fabricated into blood vessels and their blood compatibility results demonstrated for applications where hemocompatibility is required (Khora and Limb, 2003). Kim et al., (2008) reported that the effort has made to overcome both incomplete endothelialization and smooth muscle cell hyperplasia, which are two of the problems contributing to the poor performance of existing small-diameter (4 mm) vascular grafts, through complexation of GAGs with porous chitosan scaffolds. GAG-based material should promise because of their growth inhibitory effects on vascular smooth muscle cells and their anti-coagulant activity. However, few data regarding chitosan as a scaffold of tissue engineered blood vessels have been reported. Chitosan itself was documented to promote migration of endothelial cells and fibroblasts so as to accelerate wound healing. b) Nerve: Chitosan has been studied as a candidate material for nerve regeneration due to its properties such as antitumor, antibacterial activity, biodegradability and biocompatibility. Neurons that were cultured on the chitosan membrane can grow well and that chitosan tube can greatly promote the repair of the peripheral nervous system, also the chitosan fibers supported the adhesion, migration and proliferation of Schwann cells (SCs), which provide a similar guide for regenerating axons to Büngner bands in the nervous system (Yuan et al., 2004). c) Liver: Chitosan as a promising biomaterial can be applied in liver tissue engineering due to its various properties such as its structure that is similar to glycosamineglycans (GAGs), which are components of the liver extracellular matrix (ECM) (Li et al., 2003). d) Cartilages: Chitosan is one of the most abundant polysaccharides and thus shares some bioactivities with various glycosaminoglycans and hyaluronic acid present in articular cartilage (Suh and Matthew, 2000). Lu et al., 1999 has demonstrated that the chitosan solution injected into the knee articular cavity of rats lead to a significant increase in the density of chondrocytes in the knee articular cartilage, indicating that chitosan could be potentially beneficial to the wound healing of articular cartilage (Lee et al., 2004).

Biocomposite Materials 123

dissolve first the calcium and then the organic matrix of the bone. Osteoblasts arise from mesenchymal cells and are found layered over the bone. They deposit calcium into the matrix that is building up cortical bone and produce collagen and other proteins to synthesize the bone matrix. Osteoclasts that are in close proximity can increase sensitivity of osteoblasts to growth factors. When a bone is broken there is usually bleeding into the space between the bone and the periosteum. This produces a hematoma, swelling due to blood. The osteoblasts, bone-producing cells, near the hematoma invade it along with small blood vessels from the bone. In a short time, the hematoma is replaced by bone tissue produced by the osteoblasts. In general, this bone is cancellous, but the added bone makes the broken junction much thicker than it was. The swollen bone is called the callus. A remarkable process follows. Where there is strain, the newly formed cancellous bone condenses into compact bone. Where strain is absent, the cancellous bone disappears through the activity of osteoclasts, bone destroying cells. This process seems to determine the development of the

skeleton in normal growth (Roodman, 2004).

**Figure 3.** Bone structure

**5. Hydroxyapatite/chitosan biocomposites: Introduction** 

For the increase of bioactivity and mechanical property, some composites of polymer and bioactive ceramics have been developed for bone tissue engineering. These composites fulfill the mechanical properties required for their function as skeleton, teeth and cells of organisms. Among these composites, HAp/polymer composites have attracted much attention since such composites may have osteconductivity due to the presence of HAp, which has a similar chemical composition and structure to the mineral phase of human bones and hard tissues. Thus, HA/polymer composite scaffolds are of interest for biomedical applications (Jin et al.,

## **4. Bone structure**

Bone is a specialized tissue comprising mineral substances, organic tissue, and water (Otto et al., 1997). Cortical bone is largely a composite of collagen, fiber and biological apatite (Fricain et al., 1998). The inorganic component of bone (bone mineral) is calcium phosphate that contains up to 8 wt% carbonate. Substitution of carbonate or other ions HA can occur in two distinct atomic sites in the lattice (Suchanek et al., 2002). These ions can partially substituted in the lattice for hydroxyl ions (OH), known as the A site, and/or for phosphate ions, known as the B site (Gibson and Bonfield, 2002). Skeletal bone is of two types: cortical bone and trabecular bone. Cortical bone is the outermost mineralized cortex. It is compact, strong, and densely packed as an intricate calcium matrix. Cortical bone comprises 85% of the skeleton, specifically 75% in the femoral neck, 75% in the distal radius, and 95% in the midradius. Cortical bone has no contact with marrow. Trabecular bone is the inner spongy structure composed of the sturdy collagen matrix. It comprises 15% of the skeletal mass and has structural rigidity and elasticity to withstand mechanical stress. Trabecular bone contains hematopoietic tissue in its central cavity. The bone of diaphysis consists of cancellous bone covered with a shell of cortical bone (Fig.3). The flat bones of the skull have a middle layer of cancellous bone sandwiched between two relatively thick layers of cortical bone (Liu et al., 2009).

Bone remodeling occurs continuously throughout the lifetime, although the process slows with age. The balance of osteoclastic and osteoblastic activity results in breakdown and reconstruction, which ensures skeletal integrity and maintains mineral homeostasis. Osteoclasts and osteoblasts are interconnected and influence the activity of each other. Osteoclasts are large multinuclear cells that develop from monocyte-macrophage precursors. They are imbedded in the bone matrix at or near the site of bone resorption and dissolve first the calcium and then the organic matrix of the bone. Osteoblasts arise from mesenchymal cells and are found layered over the bone. They deposit calcium into the matrix that is building up cortical bone and produce collagen and other proteins to synthesize the bone matrix. Osteoclasts that are in close proximity can increase sensitivity of osteoblasts to growth factors. When a bone is broken there is usually bleeding into the space between the bone and the periosteum. This produces a hematoma, swelling due to blood. The osteoblasts, bone-producing cells, near the hematoma invade it along with small blood vessels from the bone. In a short time, the hematoma is replaced by bone tissue produced by the osteoblasts. In general, this bone is cancellous, but the added bone makes the broken junction much thicker than it was. The swollen bone is called the callus. A remarkable process follows. Where there is strain, the newly formed cancellous bone condenses into compact bone. Where strain is absent, the cancellous bone disappears through the activity of osteoclasts, bone destroying cells. This process seems to determine the development of the skeleton in normal growth (Roodman, 2004).

**Figure 3.** Bone structure

122 Composites and Their Applications

et al., 2004).

**4. Bone structure** 

bone (Liu et al., 2009).

was documented to promote migration of endothelial cells and fibroblasts so as to accelerate wound healing. b) Nerve: Chitosan has been studied as a candidate material for nerve regeneration due to its properties such as antitumor, antibacterial activity, biodegradability and biocompatibility. Neurons that were cultured on the chitosan membrane can grow well and that chitosan tube can greatly promote the repair of the peripheral nervous system, also the chitosan fibers supported the adhesion, migration and proliferation of Schwann cells (SCs), which provide a similar guide for regenerating axons to Büngner bands in the nervous system (Yuan et al., 2004). c) Liver: Chitosan as a promising biomaterial can be applied in liver tissue engineering due to its various properties such as its structure that is similar to glycosamineglycans (GAGs), which are components of the liver extracellular matrix (ECM) (Li et al., 2003). d) Cartilages: Chitosan is one of the most abundant polysaccharides and thus shares some bioactivities with various glycosaminoglycans and hyaluronic acid present in articular cartilage (Suh and Matthew, 2000). Lu et al., 1999 has demonstrated that the chitosan solution injected into the knee articular cavity of rats lead to a significant increase in the density of chondrocytes in the knee articular cartilage, indicating that chitosan could be potentially beneficial to the wound healing of articular cartilage (Lee

Bone is a specialized tissue comprising mineral substances, organic tissue, and water (Otto et al., 1997). Cortical bone is largely a composite of collagen, fiber and biological apatite (Fricain et al., 1998). The inorganic component of bone (bone mineral) is calcium phosphate that contains up to 8 wt% carbonate. Substitution of carbonate or other ions HA can occur in two distinct atomic sites in the lattice (Suchanek et al., 2002). These ions can partially substituted in the lattice for hydroxyl ions (OH), known as the A site, and/or for phosphate ions, known as the B site (Gibson and Bonfield, 2002). Skeletal bone is of two types: cortical bone and trabecular bone. Cortical bone is the outermost mineralized cortex. It is compact, strong, and densely packed as an intricate calcium matrix. Cortical bone comprises 85% of the skeleton, specifically 75% in the femoral neck, 75% in the distal radius, and 95% in the midradius. Cortical bone has no contact with marrow. Trabecular bone is the inner spongy structure composed of the sturdy collagen matrix. It comprises 15% of the skeletal mass and has structural rigidity and elasticity to withstand mechanical stress. Trabecular bone contains hematopoietic tissue in its central cavity. The bone of diaphysis consists of cancellous bone covered with a shell of cortical bone (Fig.3). The flat bones of the skull have a middle layer of cancellous bone sandwiched between two relatively thick layers of cortical

Bone remodeling occurs continuously throughout the lifetime, although the process slows with age. The balance of osteoclastic and osteoblastic activity results in breakdown and reconstruction, which ensures skeletal integrity and maintains mineral homeostasis. Osteoclasts and osteoblasts are interconnected and influence the activity of each other. Osteoclasts are large multinuclear cells that develop from monocyte-macrophage precursors. They are imbedded in the bone matrix at or near the site of bone resorption and

## **5. Hydroxyapatite/chitosan biocomposites: Introduction**

For the increase of bioactivity and mechanical property, some composites of polymer and bioactive ceramics have been developed for bone tissue engineering. These composites fulfill the mechanical properties required for their function as skeleton, teeth and cells of organisms. Among these composites, HAp/polymer composites have attracted much attention since such composites may have osteconductivity due to the presence of HAp, which has a similar chemical composition and structure to the mineral phase of human bones and hard tissues. Thus, HA/polymer composite scaffolds are of interest for biomedical applications (Jin et al., 2008). Kikuchi et al., (2004) prepared a self organized HA-collagen nano-composite by a biomimetic co-precipitation method. It was reported that the composite had similar microstructure to native bone and showed osteoclastic resorption and good osteoconductivity. However, a major concern over HA-collagen composite is the high cost of collagen, which limits its clinical application in healing bone defect to its insouciant formability and flexibility. Polymers such as chitosan have a higher degradation rate than bioceramics. Incorporation of HA into a chitosan polymer matrix has been shown to increase osteoconductivity and biodegradability with significant enhancement of mechanical strength (Yamaguchi et al., 2001). Chitosan's primary attractive features including its biocompatibility, biodegradability, flexibility, adhesiveness and anti-infectivity, make it as a feasible wound healing agent and an ideal polymeric matrix for HA ceramic (Rusu et al., 2005).

Biocomposite Materials 125

the structure of bone. Thus, the use of a hybrid composite that makes up of chitosan and calcium phosphate resembles the morphology and properties of natural bone. This may be one-way to solve the problem of calcium phosphates brittleness, besides possessing good biocompatibility, high bioactivity and great bone-bonding properties (Ding, 2007). The mechanical strength of the chitosan/calcium phosphate composite fiber with core-shell structure increased with an increased concentration of chitosan solution (Matsuda et al., 2004). Among the composites studied, the 30/70 chitosan/n HA exhibits the maximum value of compressive strength, about 120 MPa, which is strong enough to be used in load-bearing sites of bone tissue. In contrast the compressive strength of pure HA compact prepared by the similar method has been reported as 6.5 MPa about one twentieth of the maximum value of the composite. In general, the proper stress transfer occurring between the reinforcement and the matrix governs the mechanical characteristics of filled polymers. The chemical and mechanical interlocking between n-HA and chitosan are accounts for the efficient stresstransfer in the composite system. Besides, the interactions such as hydrogen bonding and chelation between the two phases, also contribute to the good mechanical properties of chitosan/n-HA composite (Zhang et al., 2005). The biodegradable composites based on chitosan and calcium phosphate have been prepared using a simple mixing and heating method. The detrimental effects of the simulated physiological environment on mechanical properties of the hybrid composites resulted in the significantly decrease in strength and modulus. The chitosan/calcium phosphate composites containing 10 wt/v % might an optimal material in terms of initial strength and degradation behavior. Although susceptibility to solution attack, this type of chitosan/calcium phosphate composites with

high initial strength might be acceptable for use in bone tissue repair (Ding, 2007).

Three-dimensional biodegradable chitosan/nHA composite scaffolds were characterized by superior mechanical, physicochemical, and biological properties compared to pure chitosan scaffolds for bone tissue engineering. The nanocomposite scaffolds were characterized by a highly porous structure and the pore size was similar for scaffolds with varying n-HA content. The nano-composite scaffolds exhibited greater compression modulus, slower degradation rate and reduced water uptake, but the water retention ability was similar to that of pure chitosan scaffolds. Favorable biological response of pre-osteoblast on nanocomposite scaffolds included improved cell adhesion, higher proliferation, and well spreading morphology in relation to pure chitosan scaffold (Thein-Han and Misra, 2009).

Mechanical properties of biocomposites: hydroxyapatite/chitosan (HA/CS) nano-composite rods were reinforced via a covalently cross-linking method. The bending strength and bending modulus of the cross-linked HA/CS (5/100, wt/wt) rods could arrive at 178 MPa and 5.2 GPa, respectively, increased by 107% and 52.9% compared with uncross-linked HA/CS (5/100, wt/wt) rods (Takagi et al., 2003). The presence of HA-DBM filler into the grafted chitosan copolymer matrix resulted in compressive strength properties are quite close to those of cancellous bone (2–12 MPa). This result is due to effect of the presence of demineralized bone matrix (DBM) powder and pMMA having bone cement formation within this composite (Mohamed et al., 2007). The E-modulus and compressive strength for

### **5.1. Structure**

The free amino groups of chitosan (C-NH2) was protonated to C-NH3+, when chitosan was dissolved in acetic acid (HAc) solution, which was shown as follows:

$$\text{C}-\text{NH}\_2 + \text{HAc} \longrightarrow \text{C}-\text{NH}\_3^+ + \text{Ac} \qquad \text{pH} = 4.2^\circ$$

The presence of calcium and phosphate ions in chitosan solutions leads to formation of HA\chitosan composites through electrostatic interactions between C-NH3+ and Ca2+ and\or PO43- ions to form C-Ca and C-PO4 complexes. There is also an interaction between OH of chitosan and OH of HA via hydrogen bond (Hu et al., 2004).

## **5.2. Properties**

Chitosan can be utilized in combination with other bioactive inorganic ceramics, especially HA to further enhance tissue regenerative efficacy and osteoconductivity. Incorporation of HA with chitosan, the mineral component of bone, could improve the bioactivity and the bone bonding ability of the chitosan/HA composites (Wang et al., 2002). Chitosan just plays in a role of adhesive to dissolve the problem of difficulty of HA specific shape and migration of HA powder when implanted. HA/chitosan nano-composites are prepared by the precipitation. It is proposed that the nano-structure of HA/chitosan composite will have the best biomedical properties in the biomaterials applications (Chen et al., 2002). Also, it has been demonstrated that chitosan-hydroxyapatite composite induces osteoconductivity in osseous defects and could act as drug vehicle. It is important to be able to load these composites with short-time life and controlled action anti-inflammatory to reduce or eliminate undesirable inflammatory processes (Larena et al., 2004).

It is desirable to develop a composite material with favorable properties of chitosan and HA. The designed composites are expected to have an optimal mechanical performance and a controllable degradation rate as well as eminent bioactivity and this will be of great importance for bone remodeling and growth (Zhang et al., 2005). It must be emphasized at this point that the successful design of a bone substitute material requires an appreciation of the structure of bone. Thus, the use of a hybrid composite that makes up of chitosan and calcium phosphate resembles the morphology and properties of natural bone. This may be one-way to solve the problem of calcium phosphates brittleness, besides possessing good biocompatibility, high bioactivity and great bone-bonding properties (Ding, 2007). The mechanical strength of the chitosan/calcium phosphate composite fiber with core-shell structure increased with an increased concentration of chitosan solution (Matsuda et al., 2004). Among the composites studied, the 30/70 chitosan/n HA exhibits the maximum value of compressive strength, about 120 MPa, which is strong enough to be used in load-bearing sites of bone tissue. In contrast the compressive strength of pure HA compact prepared by the similar method has been reported as 6.5 MPa about one twentieth of the maximum value of the composite. In general, the proper stress transfer occurring between the reinforcement and the matrix governs the mechanical characteristics of filled polymers. The chemical and mechanical interlocking between n-HA and chitosan are accounts for the efficient stresstransfer in the composite system. Besides, the interactions such as hydrogen bonding and chelation between the two phases, also contribute to the good mechanical properties of chitosan/n-HA composite (Zhang et al., 2005). The biodegradable composites based on chitosan and calcium phosphate have been prepared using a simple mixing and heating method. The detrimental effects of the simulated physiological environment on mechanical properties of the hybrid composites resulted in the significantly decrease in strength and modulus. The chitosan/calcium phosphate composites containing 10 wt/v % might an optimal material in terms of initial strength and degradation behavior. Although susceptibility to solution attack, this type of chitosan/calcium phosphate composites with high initial strength might be acceptable for use in bone tissue repair (Ding, 2007).

124 Composites and Their Applications

**5.1. Structure**

**5.2. Properties** 

ideal polymeric matrix for HA ceramic (Rusu et al., 2005).

dissolved in acetic acid (HAc) solution, which was shown as follows:

OH of chitosan and OH of HA via hydrogen bond (Hu et al., 2004).

eliminate undesirable inflammatory processes (Larena et al., 2004).

2008). Kikuchi et al., (2004) prepared a self organized HA-collagen nano-composite by a biomimetic co-precipitation method. It was reported that the composite had similar microstructure to native bone and showed osteoclastic resorption and good osteoconductivity. However, a major concern over HA-collagen composite is the high cost of collagen, which limits its clinical application in healing bone defect to its insouciant formability and flexibility. Polymers such as chitosan have a higher degradation rate than bioceramics. Incorporation of HA into a chitosan polymer matrix has been shown to increase osteoconductivity and biodegradability with significant enhancement of mechanical strength (Yamaguchi et al., 2001). Chitosan's primary attractive features including its biocompatibility, biodegradability, flexibility, adhesiveness and anti-infectivity, make it as a feasible wound healing agent and an

The free amino groups of chitosan (C-NH2) was protonated to C-NH3+, when chitosan was

C NH HAc C NH +Ac pH 4.2 2 3 

The presence of calcium and phosphate ions in chitosan solutions leads to formation of HA\chitosan composites through electrostatic interactions between C-NH3+ and Ca2+ and\or PO43- ions to form C-Ca and C-PO4 complexes. There is also an interaction between

Chitosan can be utilized in combination with other bioactive inorganic ceramics, especially HA to further enhance tissue regenerative efficacy and osteoconductivity. Incorporation of HA with chitosan, the mineral component of bone, could improve the bioactivity and the bone bonding ability of the chitosan/HA composites (Wang et al., 2002). Chitosan just plays in a role of adhesive to dissolve the problem of difficulty of HA specific shape and migration of HA powder when implanted. HA/chitosan nano-composites are prepared by the precipitation. It is proposed that the nano-structure of HA/chitosan composite will have the best biomedical properties in the biomaterials applications (Chen et al., 2002). Also, it has been demonstrated that chitosan-hydroxyapatite composite induces osteoconductivity in osseous defects and could act as drug vehicle. It is important to be able to load these composites with short-time life and controlled action anti-inflammatory to reduce or

It is desirable to develop a composite material with favorable properties of chitosan and HA. The designed composites are expected to have an optimal mechanical performance and a controllable degradation rate as well as eminent bioactivity and this will be of great importance for bone remodeling and growth (Zhang et al., 2005). It must be emphasized at this point that the successful design of a bone substitute material requires an appreciation of Three-dimensional biodegradable chitosan/nHA composite scaffolds were characterized by superior mechanical, physicochemical, and biological properties compared to pure chitosan scaffolds for bone tissue engineering. The nanocomposite scaffolds were characterized by a highly porous structure and the pore size was similar for scaffolds with varying n-HA content. The nano-composite scaffolds exhibited greater compression modulus, slower degradation rate and reduced water uptake, but the water retention ability was similar to that of pure chitosan scaffolds. Favorable biological response of pre-osteoblast on nanocomposite scaffolds included improved cell adhesion, higher proliferation, and well spreading morphology in relation to pure chitosan scaffold (Thein-Han and Misra, 2009).

Mechanical properties of biocomposites: hydroxyapatite/chitosan (HA/CS) nano-composite rods were reinforced via a covalently cross-linking method. The bending strength and bending modulus of the cross-linked HA/CS (5/100, wt/wt) rods could arrive at 178 MPa and 5.2 GPa, respectively, increased by 107% and 52.9% compared with uncross-linked HA/CS (5/100, wt/wt) rods (Takagi et al., 2003). The presence of HA-DBM filler into the grafted chitosan copolymer matrix resulted in compressive strength properties are quite close to those of cancellous bone (2–12 MPa). This result is due to effect of the presence of demineralized bone matrix (DBM) powder and pMMA having bone cement formation within this composite (Mohamed et al., 2007). The E-modulus and compressive strength for the three composites HA/, 90%HA-10Ti/, and 70%HA-30%Ti/grafted chitosan copolymer composites recorded comparable values compared to the cancellous bone. Therefore, the presence of HA filler or HA filler containing titania content up to 30% into the copolymer resulted in compressive strength properties that are quite close to those of cancellous bone (2–12 MPa) (Mohamed et a., 2008).

Biocomposite Materials 127

chitosan sheet, coating of HAp crystals onto a tendon chitosan (Yamaguchi et al., 2003) and a co-precipitation method. The chitosan/HAp hybrid fiber has also been reported by Chung and Korean, (2002). However, these materials were shown in the macroscopically homogeneous. The conventional method to fabricate chitosan/HA composite is that HA powder was mixed with chitosan, dissolved in 2% acetic acid solution, then the mixture was impressed into mold, finally was freezing-dried to make sponge composite. Surface modification and polyblend methods can be used to change the physicochemical properties of chitosan–gelatin membranes or scaffolds by incorporating hyaluronic acid. Adding hyaluronic acid can improve the mechanical, biological and anti-degradation properties of

Hydroxyapatite (HAp) was prepared by precipitation method, while the biphasic hydroxyapatite/tricalcium phosphate (HA/B-TCP) was prepared by heating the prepared HAp at 900°C for 5 hours in air. To improve bioactivity both HAp and HAp/TCP fillers were loaded onto chitosan grafted with two monomers, hydroxyethylmethacrylate (HEMA) and methylmethacrylate (MMA) during copolymerization process (Hashem and Mohamed, 2007). Also, biocomposites containing HA-DBM mixture powder loaded onto the copolymer matrix containing the grafted chitosan with poly methylmethacrylate (pMMA) and its

The chitosan mineralization in the case of using a stepwise co-precipitation approach involves the following stages: First, chitosan chains change their conformation as a function of environmental parameters, such as pH. Starting from extended conformations such as wormlike, as seen in Fig.4 (1), adopted at low pH (until 3–3.5), chitosan turns to the more compact conformations such as extended random coil Fig.4(2) and even to more compacted random coil conformations, at higher pH values (from 3.5 until 6). Between 5.5 (the pH at which brushite is precipitated) and 6.5–6.7 (the pH at which chitosan is precipitated) is the pH range that leads to formation of an interconnected three-dimensional net work between chitosan and brushite that can be approximated as a dendritic-like structure, as shown in Fig.4(3). This structural model is characterized by highly dense irregular shaped cores which are linked by the chitosan bridging segments. In the dendritic core, the chitosan chains are randomly packed as amorphous regions while in the bridging regions more extended conformations are found, in some parts parallel oriented chains domains are presented. Since the dendritic core is in the range between 200 and 600nm hence it is formed by the many compact random coil chitosan units that are approaching one another chitosan chain segments can interact with the CaPs phases (i.e. seeds of HAp already formed at pH 5.5 and identified by its XRD pattern, brushite and some other ACP). In this way, they achieve of so called ''anchoring regions'' by different specific interactions such as ion-dipole or/and through the complexation of Ca with chitosan. Considering that a dendritic-like structure is formed in the earlier stage of composite formation, one can explain the complex bimodal distribution of HAp nano-crystallites in the chitosan matrix. Inside the dendritic core, the chitosan chains density is much greater than outside of them, shown as inter-connection blob regions. We assume that the probability to find a certain number of HAp crystallite seeds per a chitosan chain unit is the same, inside and outside the dendritic core regions. Since the chitosan chain density is much greater inside the

derivative during copolymerization were fabricated (Mohamed et al, 2007).

the membranes or the scaffolds (Mao et al., 2003).

Collagen/apatite composite membranes exhibited significantly improved mechanical properties compared with their pure collagen equivalent; their mechanical properties were still lower than those of natural bone (Teng et al., 2009). It was notified that hardness of calcium pyrophosphate (CPP)/ chitosan composite (66.80) was increased compared to chitosan copolymer (60.88) and the hardness of CPP/chitosan-grafted composite (68.23) was also increased compared to chitosan–gelatin copolymer (84.12) proving the polymer/filler interaction and adhesion. Also, the compressive strength of CPP/chitosan composite (6.53 MPa) was increased compared to chitosan copolymer (5.11 MPa). These values of compressive strength were comparable to those of human cancellous bone (Kokubo et al., 2003). As a result, CPP filler powder into the chitosan copolymer matrix containing chitosan or chitosan–gelatin polymer resulted in more effective reinforcement of the composite, then, stiffer composite (El-Kady et al., 2009). The CPC–chitosan composites were more stable in water than conventional calcium phosphate cement (CPC). They did not disintegrate even when placed in water immediately after mixing. The CPC–chitosan paste hardened within 10 min in all cases. The authors demonstrated that CPC–chitosan composites are stable in a wet environment and have acceptable mechanical strengths for clinical applications (Wang et al., 2010).

## **5.3. Preparation**

Although powder ceramics remain the form of choice for filling small irregular defects, the therapeutic effect of the filling implant was lost by migration of particles from the defect site. Furthermore, it was difficult to be handled and kept in place compactly for convenient fabrication and operation of block-type ceramics (Lin et al., 1998). Thus, it is necessary to mix a suitable binder with the granular material to overcome these problems. Presently, the approaches to obtain chitosan/hydroxyapatite (HAp) composite materials are based either on mixing or co-precipitation methods. Yamaguchi et al., (2001) have developed one of the co-precipitation methods that lead to a type of chitosan/HAp composites. In this approach, the composite was co-precipitated in one step, by dropping a chitosan solution containing phosphoric acid into a calcium hydroxide suspension. Other approaches employ either the biomineralization of chitosan in a solid form (especially as membranes) in simulated body fluids (SBF) (Beppu and Santana, 2001) or by mixing of a chitosan solution with different calcium phosphate fillers followed by their precipitation as hydrogel composite. The use of these approaches leads to incorporation of inorganic fillers into the structure of composites (Schwarz and Epple, 1998), either as nano-sized or micro-sized particles.

Different preparation methods of HAp/chitosan composites have been reported, such as mechanical mixing of HAp powder in a chitosan solution, coating of HAp particles onto a chitosan sheet, coating of HAp crystals onto a tendon chitosan (Yamaguchi et al., 2003) and a co-precipitation method. The chitosan/HAp hybrid fiber has also been reported by Chung and Korean, (2002). However, these materials were shown in the macroscopically homogeneous. The conventional method to fabricate chitosan/HA composite is that HA powder was mixed with chitosan, dissolved in 2% acetic acid solution, then the mixture was impressed into mold, finally was freezing-dried to make sponge composite. Surface modification and polyblend methods can be used to change the physicochemical properties of chitosan–gelatin membranes or scaffolds by incorporating hyaluronic acid. Adding hyaluronic acid can improve the mechanical, biological and anti-degradation properties of the membranes or the scaffolds (Mao et al., 2003).

126 Composites and Their Applications

et al., 2010).

**5.3. Preparation** 

(2–12 MPa) (Mohamed et a., 2008).

the three composites HA/, 90%HA-10Ti/, and 70%HA-30%Ti/grafted chitosan copolymer composites recorded comparable values compared to the cancellous bone. Therefore, the presence of HA filler or HA filler containing titania content up to 30% into the copolymer resulted in compressive strength properties that are quite close to those of cancellous bone

Collagen/apatite composite membranes exhibited significantly improved mechanical properties compared with their pure collagen equivalent; their mechanical properties were still lower than those of natural bone (Teng et al., 2009). It was notified that hardness of calcium pyrophosphate (CPP)/ chitosan composite (66.80) was increased compared to chitosan copolymer (60.88) and the hardness of CPP/chitosan-grafted composite (68.23) was also increased compared to chitosan–gelatin copolymer (84.12) proving the polymer/filler interaction and adhesion. Also, the compressive strength of CPP/chitosan composite (6.53 MPa) was increased compared to chitosan copolymer (5.11 MPa). These values of compressive strength were comparable to those of human cancellous bone (Kokubo et al., 2003). As a result, CPP filler powder into the chitosan copolymer matrix containing chitosan or chitosan–gelatin polymer resulted in more effective reinforcement of the composite, then, stiffer composite (El-Kady et al., 2009). The CPC–chitosan composites were more stable in water than conventional calcium phosphate cement (CPC). They did not disintegrate even when placed in water immediately after mixing. The CPC–chitosan paste hardened within 10 min in all cases. The authors demonstrated that CPC–chitosan composites are stable in a wet environment and have acceptable mechanical strengths for clinical applications (Wang

Although powder ceramics remain the form of choice for filling small irregular defects, the therapeutic effect of the filling implant was lost by migration of particles from the defect site. Furthermore, it was difficult to be handled and kept in place compactly for convenient fabrication and operation of block-type ceramics (Lin et al., 1998). Thus, it is necessary to mix a suitable binder with the granular material to overcome these problems. Presently, the approaches to obtain chitosan/hydroxyapatite (HAp) composite materials are based either on mixing or co-precipitation methods. Yamaguchi et al., (2001) have developed one of the co-precipitation methods that lead to a type of chitosan/HAp composites. In this approach, the composite was co-precipitated in one step, by dropping a chitosan solution containing phosphoric acid into a calcium hydroxide suspension. Other approaches employ either the biomineralization of chitosan in a solid form (especially as membranes) in simulated body fluids (SBF) (Beppu and Santana, 2001) or by mixing of a chitosan solution with different calcium phosphate fillers followed by their precipitation as hydrogel composite. The use of these approaches leads to incorporation of inorganic fillers into the structure of composites

Different preparation methods of HAp/chitosan composites have been reported, such as mechanical mixing of HAp powder in a chitosan solution, coating of HAp particles onto a

(Schwarz and Epple, 1998), either as nano-sized or micro-sized particles.

Hydroxyapatite (HAp) was prepared by precipitation method, while the biphasic hydroxyapatite/tricalcium phosphate (HA/B-TCP) was prepared by heating the prepared HAp at 900°C for 5 hours in air. To improve bioactivity both HAp and HAp/TCP fillers were loaded onto chitosan grafted with two monomers, hydroxyethylmethacrylate (HEMA) and methylmethacrylate (MMA) during copolymerization process (Hashem and Mohamed, 2007). Also, biocomposites containing HA-DBM mixture powder loaded onto the copolymer matrix containing the grafted chitosan with poly methylmethacrylate (pMMA) and its derivative during copolymerization were fabricated (Mohamed et al, 2007).

The chitosan mineralization in the case of using a stepwise co-precipitation approach involves the following stages: First, chitosan chains change their conformation as a function of environmental parameters, such as pH. Starting from extended conformations such as wormlike, as seen in Fig.4 (1), adopted at low pH (until 3–3.5), chitosan turns to the more compact conformations such as extended random coil Fig.4(2) and even to more compacted random coil conformations, at higher pH values (from 3.5 until 6). Between 5.5 (the pH at which brushite is precipitated) and 6.5–6.7 (the pH at which chitosan is precipitated) is the pH range that leads to formation of an interconnected three-dimensional net work between chitosan and brushite that can be approximated as a dendritic-like structure, as shown in Fig.4(3). This structural model is characterized by highly dense irregular shaped cores which are linked by the chitosan bridging segments. In the dendritic core, the chitosan chains are randomly packed as amorphous regions while in the bridging regions more extended conformations are found, in some parts parallel oriented chains domains are presented. Since the dendritic core is in the range between 200 and 600nm hence it is formed by the many compact random coil chitosan units that are approaching one another chitosan chain segments can interact with the CaPs phases (i.e. seeds of HAp already formed at pH 5.5 and identified by its XRD pattern, brushite and some other ACP). In this way, they achieve of so called ''anchoring regions'' by different specific interactions such as ion-dipole or/and through the complexation of Ca with chitosan. Considering that a dendritic-like structure is formed in the earlier stage of composite formation, one can explain the complex bimodal distribution of HAp nano-crystallites in the chitosan matrix. Inside the dendritic core, the chitosan chains density is much greater than outside of them, shown as inter-connection blob regions. We assume that the probability to find a certain number of HAp crystallite seeds per a chitosan chain unit is the same, inside and outside the dendritic core regions. Since the chitosan chain density is much greater inside the cores, this leads to the conclusion that the higher density of HAp crystallite seeds is located inside the dendritic cores. Consequently inside the core favorite the formation of the ''small HAp nanocrystallites (their growth is spatially limited) whereas outside the cores, along the interconnection regions where the space constraint is not that much limited, ''large'' HAp nanocrystallites are favorized to be formed. Finally, the cluster-like and scattered-like size domains are generated in this way, as seen in Fig.4(4) This theoretical model is supported by the experimental data, since we could demonstrate that the amount of chitosan in the composite can be used to control the HAp nano-crystallite size, otherwise no influence should be observed (Rusu et al., 2005).

Biocomposite Materials 129

2008). Biodegradable hydroxyapatite/chitosan-gelatin polymeric biocomposites were fabricated by using HA powder and HA filler containing titania powder (10 and 30%) with a chitosan and gelatin grafted co-polymeric matrix during copolymerization process (Mohamed and Mostafa, 2008). Preparation of a model HAp/CTS (30:70 in mass ratio) nanocomposite nanofibers using a two-step method, which involves firstly preparing HAp/CTS nanocomposites by a co-precipitation synthesis approach and then fabricating the resultant HAp/CTS nanocomposites, aided with a fiber-forming additive–ultrahigh molecular weight poly(ethylene oxide), into nanofibers via the electrospinning process (Zhang et al., 2008).

The FT-IR spectra show that the two characteristic bands of amide I (1655 cm-1) and amide II (1599 cm-1) for chitosan shift to lower wavenumber after being compounded, which suggests that interaction must take place between chitosan and n-HA, including hydrogen bonds between -NH2 and-OH of n-HA as well as the chelation between -NH2 and Ca++. The more shift of these bands to lower wave number, the stronger the hydrogen bonds between these groups and also the stronger the interaction between these molecules (Zhang et al., 2005). The XRD pattern of precipitated HA shows un-differentiated broad peaks with poor crystallinity around the characteristic region. However, the crystallographic structure of precipitated HA nano-crystals is more identical with natural bone mineral (biological apatite). Hence, the prepared HA nano-crystallites in this investigation have more similarity with natural bone mineral in terms of degree of crystallinity and structural morphology. The calcined HA exhibited all the characteristic diffracted peaks of stoichiometric HA with higher degree of crystallinity. Rising in the calcination temperature shapes the diffracted peaks more sharper, which is a good sign for the improvement of crystallinity of precipitated HA. The obtained results did not show any peaks corresponding to calcium carbonate and calcium oxide and hence suggesting that the ingredients were reacted completely and produced a homogeneous HA. The XRD analysis of HA/chitosan composites proved that, a broad peak assigned to chitosan at 20° becomes wider and weaker with increase of n-HA. It suggests that the addition of n-HA obviously affects the crystallinity of chitosan. The characteristic peaks at 25.8° (002) and 39.6° (310) are used to calculate the n-HA crystal sizes (Xianmiao et al., 2009). The TGA mass loss of HA\C composite increased from 3.3–6.5 mass% as the chitosan concentration increased from 0–2.5 mass%. The amount of chitosan that adsorbed on HA was 2.8–3.1 mass% based on Carbon– Hydrogen–Nitrogen (CHN) analysis. The specific surface area of HA increased after aging in chitosan acetate gel solutions and attained a high value of 160 m2/g in comparison to 85

The SEM micrograph of precipitated HA (Fig.5a) exhibited nano-sized crystals with almost uniform particles size. The HA particles prepared were not only stoichiometric but also mono-dispersive and roughly particles were not fused together with other crystals. It can be inferred that majority of the particles were of single crystals, regular shape and cleaner contours with no agglomeration which are highly beneficial for coating of nano HA onto

**5.4. Characterization**

m2/g for untreated HA (Wilson and Hull, 2008).

**Figure 4.** Sketch of chitosan (CTS) mineralization through nanosized HAp. The main stages in the formation of the composite structure are outlined by the theoretical approach, as shown on the left side (A). On the right side (B), we present a TEM micrograph a CTS/HAp 50:50wt% composite, which emphasizes the dendritic-like structure of sample at this stage of formation (in gel-like form). Furthermore, when the excess water is released a solid, rigid composite is obtained.

An interesting approach was reported by Hu et al., (2004) in which the chitosan hydrogel is mineralized via in situ hybridization by the ionic diffusion processes in a controlled manner. It should be mentioned that, each approach of those cited above leads to a particular type of chitosan/HAp composite materials with respect to their structure and properties. In order to prepare such types of composites, we have reported a stepwise co-precipitation method in which the pH of the chitosan solution is gradually increased in a stepwise fashion (Ng et al., 2008). Biodegradable hydroxyapatite/chitosan-gelatin polymeric biocomposites were fabricated by using HA powder and HA filler containing titania powder (10 and 30%) with a chitosan and gelatin grafted co-polymeric matrix during copolymerization process (Mohamed and Mostafa, 2008). Preparation of a model HAp/CTS (30:70 in mass ratio) nanocomposite nanofibers using a two-step method, which involves firstly preparing HAp/CTS nanocomposites by a co-precipitation synthesis approach and then fabricating the resultant HAp/CTS nanocomposites, aided with a fiber-forming additive–ultrahigh molecular weight poly(ethylene oxide), into nanofibers via the electrospinning process (Zhang et al., 2008).

## **5.4. Characterization**

128 Composites and Their Applications

be observed (Rusu et al., 2005).

cores, this leads to the conclusion that the higher density of HAp crystallite seeds is located inside the dendritic cores. Consequently inside the core favorite the formation of the ''small HAp nanocrystallites (their growth is spatially limited) whereas outside the cores, along the interconnection regions where the space constraint is not that much limited, ''large'' HAp nanocrystallites are favorized to be formed. Finally, the cluster-like and scattered-like size domains are generated in this way, as seen in Fig.4(4) This theoretical model is supported by the experimental data, since we could demonstrate that the amount of chitosan in the composite can be used to control the HAp nano-crystallite size, otherwise no influence should

**Figure 4.** Sketch of chitosan (CTS) mineralization through nanosized HAp. The main stages in the formation of the composite structure are outlined by the theoretical approach, as shown on the left side (A). On the right side (B), we present a TEM micrograph a CTS/HAp 50:50wt% composite, which emphasizes the dendritic-like structure of sample at this stage of formation (in gel-like form).

An interesting approach was reported by Hu et al., (2004) in which the chitosan hydrogel is mineralized via in situ hybridization by the ionic diffusion processes in a controlled manner. It should be mentioned that, each approach of those cited above leads to a particular type of chitosan/HAp composite materials with respect to their structure and properties. In order to prepare such types of composites, we have reported a stepwise co-precipitation method in which the pH of the chitosan solution is gradually increased in a stepwise fashion (Ng et al.,

Furthermore, when the excess water is released a solid, rigid composite is obtained.

The FT-IR spectra show that the two characteristic bands of amide I (1655 cm-1) and amide II (1599 cm-1) for chitosan shift to lower wavenumber after being compounded, which suggests that interaction must take place between chitosan and n-HA, including hydrogen bonds between -NH2 and-OH of n-HA as well as the chelation between -NH2 and Ca++. The more shift of these bands to lower wave number, the stronger the hydrogen bonds between these groups and also the stronger the interaction between these molecules (Zhang et al., 2005). The XRD pattern of precipitated HA shows un-differentiated broad peaks with poor crystallinity around the characteristic region. However, the crystallographic structure of precipitated HA nano-crystals is more identical with natural bone mineral (biological apatite). Hence, the prepared HA nano-crystallites in this investigation have more similarity with natural bone mineral in terms of degree of crystallinity and structural morphology. The calcined HA exhibited all the characteristic diffracted peaks of stoichiometric HA with higher degree of crystallinity. Rising in the calcination temperature shapes the diffracted peaks more sharper, which is a good sign for the improvement of crystallinity of precipitated HA. The obtained results did not show any peaks corresponding to calcium carbonate and calcium oxide and hence suggesting that the ingredients were reacted completely and produced a homogeneous HA. The XRD analysis of HA/chitosan composites proved that, a broad peak assigned to chitosan at 20° becomes wider and weaker with increase of n-HA. It suggests that the addition of n-HA obviously affects the crystallinity of chitosan. The characteristic peaks at 25.8° (002) and 39.6° (310) are used to calculate the n-HA crystal sizes (Xianmiao et al., 2009). The TGA mass loss of HA\C composite increased from 3.3–6.5 mass% as the chitosan concentration increased from 0–2.5 mass%. The amount of chitosan that adsorbed on HA was 2.8–3.1 mass% based on Carbon– Hydrogen–Nitrogen (CHN) analysis. The specific surface area of HA increased after aging in chitosan acetate gel solutions and attained a high value of 160 m2/g in comparison to 85 m2/g for untreated HA (Wilson and Hull, 2008).

The SEM micrograph of precipitated HA (Fig.5a) exhibited nano-sized crystals with almost uniform particles size. The HA particles prepared were not only stoichiometric but also mono-dispersive and roughly particles were not fused together with other crystals. It can be inferred that majority of the particles were of single crystals, regular shape and cleaner contours with no agglomeration which are highly beneficial for coating of nano HA onto

biomedical implants. On the other hand, composites bone paste (Figs.5b and c) showed heterogeneous phases with complete fusion of HA crystallites into chitosan matrix. Major changes in the crystal size of composites were monitored as compared to single phase HA due to the presence of chitosan macromolecules. The physical appearances of the composites are quite different from the starting material and also apparent that ultra-fine particles of HA are found to aggregate into large clusters and precipitate in the chitosan matrix. One of the possible reason may be due to some of the HA nano-particles might have partially dissolved in the acidic chitosan solution that permitting the HA particles more easily to penetrate into the chitosan matrix. The particles of composites showed a high tendency to agglomerate and hence it can have capability to prevent the particle mobilization after post-implantation. Both the SEM pictures (b and c) of composites exhibited porous surfaces, but the pores were not uniform. The average pore size was found to 105 and 80mm for 5 and 10 wt chitosan composites, respectively. The composites containing porous structure on their surfaces will be more beneficial for tissue in-growth (Murugan and Ramakrishna, 2004). Also, the TEM micrographs of HA\chitosan =100\5 (wt\wt) proved that the HA particle size was 100 nm in length and 20-50 nm in width which dispersed well in chitosan matrix homogenously (Hu et al., 2004)

Biocomposite Materials 131

features dictating the biomaterial to matrix was progressively resorbed. Therefore, HA or other calcium containing materials incorporated into chitosan has been a primary research area where orthopedic or bone substitution and periodontal applications were the focus

Maruyama and Ito, 1996 reported that the strength of chitosan-HA hardened composite was comparable to that the cancellous bone derived from tibial eminentia. Pal et al., (1997) prepared different varieties of HA in conjunction with chitosan, as binder, to know its unique biological behavior in bone bonding. These hybrid materials displayed good blood compatibility (Chen et al., 1998). Apatite cement (AC) was almost completely surrounded by mature bone at eight weeks. No promotion or production of osteoconductivity was observed by chitosan even though it is considered to promote bone formation. Then, they

Chitosan and silk fobroin (SF) together as a complex organic matrix for HA granules were employed attempting to obtain a novel composite HA/chitosan–silk fobroin (HA/CTS–SF) with good osteoconductivity, enhanced mechanical strength and sufficient formability and flexibility. Additionally, chitosan and SF are easily derived from naturally abundant chitin and silk cocoon, respectively, which offers a great promise for the potential use of HA/CTS– SF composite as bone scaffold material. HA/CTS–SF composite was obtained via a simple co-precipitation method at room temperature with chitosan and SF serving as a complex organic matrix. The inorganic component in the composite is identified as mono-phase poorly crystalline HA containing carbonate ions. The chemical interactions between the inorganic and organic constituents in the composite, probably take place via the chemical bonding between Ca2+ and the amino group of chitosan or the amide bands of SF. The involvement of chitosan and SF endows the composite with higher compressive strengths compared to pure HA. These findings suggest that HA/ CTS–SF composite may be a

promising biomaterial for bone in-growth and implant fixation (Wang and Li, 2007).

regeneration (GBR) technique (Xianmiao et al., 2009).

A novel bone repair material can be obtained by incorporating carboxymethyl cellulose (CMC) into n-HA/CS system. Not only did it compound uniformity by chemical interactions and resembled natural bone apatite in composition morphology and size, but also it improved the compressive strength compared with n-HA/CS composite and had controllable degradation rate via adjusting the CS/CMC weight ratio (Liuyun et al., 2008 ). HA can promote the formation of bone-like apatite on its surface. Polymers combined with HA are capable of promoting osteoblast adhesion, migration, differentiation and proliferation, especially useful for potential applications in bone repair and regeneration. HA particles have been incorporated into chitosan matrices to enhance the bioactivity of tissue engineering scaffolds for hard tissue regeneration. Therefore, composite membrane of HA and chitosan is expected to be a good degradable barrier membrane for guided bone

concluded that there is enhancement of bone formation (Takechi et al., 2001).

(Khor and Lim, 2003).

*5.5.1. Bone substitutes* 

**Figure 5.** SEM photographs of (a) precipitated nano HA; (b) composite with 5% chitosan sol; and (c) composite with 10% chitosan sol.

#### **5.5. Applications**

One of the present trends in implantable applications requires materials that are derived from nature. The impetus is twofold. First, such ''natural'' materials have been shown to better promote healing at a faster rate and are expected to exhibit greater compatibility with humans. Second, new concepts in implantable medical devices especially tissue engineering derived from a combination of biomaterial onto which cells are seeded, require ''temporal'' features dictating the biomaterial to matrix was progressively resorbed. Therefore, HA or other calcium containing materials incorporated into chitosan has been a primary research area where orthopedic or bone substitution and periodontal applications were the focus (Khor and Lim, 2003).

## *5.5.1. Bone substitutes*

130 Composites and Their Applications

composite with 10% chitosan sol.

**5.5. Applications** 

biomedical implants. On the other hand, composites bone paste (Figs.5b and c) showed heterogeneous phases with complete fusion of HA crystallites into chitosan matrix. Major changes in the crystal size of composites were monitored as compared to single phase HA due to the presence of chitosan macromolecules. The physical appearances of the composites are quite different from the starting material and also apparent that ultra-fine particles of HA are found to aggregate into large clusters and precipitate in the chitosan matrix. One of the possible reason may be due to some of the HA nano-particles might have partially dissolved in the acidic chitosan solution that permitting the HA particles more easily to penetrate into the chitosan matrix. The particles of composites showed a high tendency to agglomerate and hence it can have capability to prevent the particle mobilization after post-implantation. Both the SEM pictures (b and c) of composites exhibited porous surfaces, but the pores were not uniform. The average pore size was found to 105 and 80mm for 5 and 10 wt chitosan composites, respectively. The composites containing porous structure on their surfaces will be more beneficial for tissue in-growth (Murugan and Ramakrishna, 2004). Also, the TEM micrographs of HA\chitosan =100\5 (wt\wt) proved that the HA particle size was 100 nm in length and 20-50 nm in width which

dispersed well in chitosan matrix homogenously (Hu et al., 2004)

**Figure 5.** SEM photographs of (a) precipitated nano HA; (b) composite with 5% chitosan sol; and (c)

One of the present trends in implantable applications requires materials that are derived from nature. The impetus is twofold. First, such ''natural'' materials have been shown to better promote healing at a faster rate and are expected to exhibit greater compatibility with humans. Second, new concepts in implantable medical devices especially tissue engineering derived from a combination of biomaterial onto which cells are seeded, require ''temporal'' Maruyama and Ito, 1996 reported that the strength of chitosan-HA hardened composite was comparable to that the cancellous bone derived from tibial eminentia. Pal et al., (1997) prepared different varieties of HA in conjunction with chitosan, as binder, to know its unique biological behavior in bone bonding. These hybrid materials displayed good blood compatibility (Chen et al., 1998). Apatite cement (AC) was almost completely surrounded by mature bone at eight weeks. No promotion or production of osteoconductivity was observed by chitosan even though it is considered to promote bone formation. Then, they concluded that there is enhancement of bone formation (Takechi et al., 2001).

Chitosan and silk fobroin (SF) together as a complex organic matrix for HA granules were employed attempting to obtain a novel composite HA/chitosan–silk fobroin (HA/CTS–SF) with good osteoconductivity, enhanced mechanical strength and sufficient formability and flexibility. Additionally, chitosan and SF are easily derived from naturally abundant chitin and silk cocoon, respectively, which offers a great promise for the potential use of HA/CTS– SF composite as bone scaffold material. HA/CTS–SF composite was obtained via a simple co-precipitation method at room temperature with chitosan and SF serving as a complex organic matrix. The inorganic component in the composite is identified as mono-phase poorly crystalline HA containing carbonate ions. The chemical interactions between the inorganic and organic constituents in the composite, probably take place via the chemical bonding between Ca2+ and the amino group of chitosan or the amide bands of SF. The involvement of chitosan and SF endows the composite with higher compressive strengths compared to pure HA. These findings suggest that HA/ CTS–SF composite may be a promising biomaterial for bone in-growth and implant fixation (Wang and Li, 2007).

A novel bone repair material can be obtained by incorporating carboxymethyl cellulose (CMC) into n-HA/CS system. Not only did it compound uniformity by chemical interactions and resembled natural bone apatite in composition morphology and size, but also it improved the compressive strength compared with n-HA/CS composite and had controllable degradation rate via adjusting the CS/CMC weight ratio (Liuyun et al., 2008 ). HA can promote the formation of bone-like apatite on its surface. Polymers combined with HA are capable of promoting osteoblast adhesion, migration, differentiation and proliferation, especially useful for potential applications in bone repair and regeneration. HA particles have been incorporated into chitosan matrices to enhance the bioactivity of tissue engineering scaffolds for hard tissue regeneration. Therefore, composite membrane of HA and chitosan is expected to be a good degradable barrier membrane for guided bone regeneration (GBR) technique (Xianmiao et al., 2009).

## *5.5.2. Bone tissue engineering*

The scaffold is a key component of tissue engineering (Langer and Vacanti, 1993). The study of inorganic crystal assembly in or on an organic polymer matrix is an important focus of bio-mineralization to produce nano composites, which can mimic natural bone. The 3D macro porous scaffolds play an important role in the formation of new tissues and provide a temporary scaffold to guide new tissue in-growth and regeneration (Nikalson and Langer, 1997). Chitosan has been proposed to serve as a non-protein matrix for 3D tissue growth. Chitosan could provide the biological primer for cell-tissue proliferation and reconstruction. One of the most promising features of chitosan is its excellent ability to be processed into porous structures for use in cell transplantation and tissue regeneration. In tissue engineering, the porous structure of chitosan provides a scaffold for bone cells to grow in and seed new bone regeneration. For rapid cell growth, the scaffold must have optimal micro architecture such as pore size, shape and specific surface area (Madihally and Matthew, 1999).

Biocomposite Materials 133

their functions by using laboratory-grown tissues, materials and artificial implants. For regeneration of failed tissues, this biomedical engineering utilizes three fundamental tools: living cell, signal molecules, and scaffold. The choice of chitosan as a tissue support material is governed among others by multiple ways by which its biological, physical and chemical properties can be controlled and engineered under mild conditions (Krajewska, 2005). The 3D macro porous scaffolds play an important role in the formation of new tissues and provide a temporary scaffold to guide new tissue in-growth and regeneration. The fabrication of biodegradable and osteoconductive scaffolds with a 3D interconnected porous network has been a formidable challenge. The feasibility of producing cost effective organic–inorganic scaffolds for tissue engineering to mimic bone by the diffusion method was performed. The porous structure of chitosan scaffold was homogeneously mineralized using this technique of apatite formation at room temperature. The mineralized scaffolds were found to be non-cytotoxic and better for cell proliferation and growth, as indicated by the enzyme activity and protein levels, than un-mineralized scaffold. This suggested that it could be used for further osteoconductivity studies. A biodegradable matrix with sufficient mechanical strength, optimized architecture and suitable degradation rate, which could finally be replaced by newly formed bone, is most desirable (Manjubala et al., 2006). Although the chitosan based composite biomaterials need to improve their mechanical properties for bone tissue engineering, no doubt that chitosan is a promising candidate scaffold material in clinical practice due to the worthiest ability to bind anionic molecules such as growth factors, GAG and DNA. Especially, the ability to link chitosan to DNA may render this material a good potential as a substrate for gene activated matrices in gene

Shen et al., (2007) performed that with the increase of pH after the addition of ammonia, carboxyl groups of citric acid may begin to act as nucleation center for calcium phosphate formation. These negatively charged carboxyls in the reaction system can bond Ca2+ strongly and thus forms a large scale of local super saturation microenvironment, and strong electric field resulted from high concentration of negatively charged carboxyls are favor of the interaction that with the most positively charged crystalline plane, so there are many nucleation sites in the network of hydrogel template, each point of nucleation can result in microcrystal. Here, to our attention, biocompatible citric acid took the place of acetic acid in this work because three carboxyl of citric acid could provide more nucleation sites which were appropriate to formation of ultra fine nano-sized carbonate apatite. And it has been conjectured that appropriate increase of citrate ions can benefit the bone resorption and ossification through the formation of dissociated calcium citrate complexes in the surrounding body fluid (Rhee and Tanaka, 1999). Each citric acid molecule can provide three negatively charged carboxyls which act as nucleation center for calcium phosphate formation. Increase of nucleation center can be appropriate to fine crystallites. Furthermore, cross-linking chitosan hydrogel was provided with three dimensional network microstructures, its compartment effect limited the growth of inorganic mineral particles, so the inorganic nano-particles were limited to aggregate in the compartment of the chitosan hydrogel template according to orientation of preferential growth of crystal plane. This

therapy application in orthopedics (Kim et al., 2008).

In bone tissue engineering, the biodegradable substitutes act as a temporary skeleton inserted into the defective sites of skeleton or lost bone sites, in order to support and stimulate bone tissue regeneration while they gradually degrade and are replaced by new bone tissue (Service, 2000). Chitosan-based scaffolds possess some special properties for use in tissue engineering. The major goal in fabricating scaffolds for bone tissue engineering is to accurately control pore size and porosity. Porous chitosan structures can be formed by freezing and lyophilizing chitosan–acetic acid solutions in suitable moulds (Chow and Khor, 2000). Bone regeneration research needed to deal with various clinical bone diseases such as bone infections, bone tumors and bone loss by trauma (Braddock et al., 2001). To combine the osteoconductivity of calcium phosphate and good biodegradability of polymers, composites have been developed for bone tissue engineering either by directly mixing the components or by a biomimetic approach (Wei and Ma, 2004). Polymer-ceramic composite scaffolds are expected to mimic natural bone, in the way that natural bone is also a composite of inorganic compounds (calcium phosphates especially substituted carbonated hydroxyapatite) and organic compounds (collagen, protein matrix, etc.). The hydroxyapatite–chitosan–alginate porous network has been reported and demonstrated to be suitable for bone tissue engineering applications using osteoblast cells (Zhao et al., 2003).

Research advances in bone regeneration in tissue engineering have focused on the development of three dimensional (3D) porous scaffolds that can serve as a support, reinforce and in some cases organize the tissue regeneration or replacement in a natural way (Sachlos et al., 2003). Several studies have been focused on chitosan–calcium phosphates (CP) composites for this purpose in bone tissue engineering. Beta-tricalcium phosphate (β-TCP) and hydroxyapatite (HA) of CP bioceramics are excellent candidates for bone repair and regeneration because of their similarity in chemical composition with inorganic components of bone (Zhang et al., 2003). Tissue engineering is regarded as an ultimately ideal medical treatment for diseases that have been too difficult to be cured by existing methods. This biomedical engineering is designed to repair injured body parts and restore their functions by using laboratory-grown tissues, materials and artificial implants. For regeneration of failed tissues, this biomedical engineering utilizes three fundamental tools: living cell, signal molecules, and scaffold. The choice of chitosan as a tissue support material is governed among others by multiple ways by which its biological, physical and chemical properties can be controlled and engineered under mild conditions (Krajewska, 2005). The 3D macro porous scaffolds play an important role in the formation of new tissues and provide a temporary scaffold to guide new tissue in-growth and regeneration. The fabrication of biodegradable and osteoconductive scaffolds with a 3D interconnected porous network has been a formidable challenge. The feasibility of producing cost effective organic–inorganic scaffolds for tissue engineering to mimic bone by the diffusion method was performed. The porous structure of chitosan scaffold was homogeneously mineralized using this technique of apatite formation at room temperature. The mineralized scaffolds were found to be non-cytotoxic and better for cell proliferation and growth, as indicated by the enzyme activity and protein levels, than un-mineralized scaffold. This suggested that it could be used for further osteoconductivity studies. A biodegradable matrix with sufficient mechanical strength, optimized architecture and suitable degradation rate, which could finally be replaced by newly formed bone, is most desirable (Manjubala et al., 2006). Although the chitosan based composite biomaterials need to improve their mechanical properties for bone tissue engineering, no doubt that chitosan is a promising candidate scaffold material in clinical practice due to the worthiest ability to bind anionic molecules such as growth factors, GAG and DNA. Especially, the ability to link chitosan to DNA may render this material a good potential as a substrate for gene activated matrices in gene therapy application in orthopedics (Kim et al., 2008).

132 Composites and Their Applications

Matthew, 1999).

*5.5.2. Bone tissue engineering* 

The scaffold is a key component of tissue engineering (Langer and Vacanti, 1993). The study of inorganic crystal assembly in or on an organic polymer matrix is an important focus of bio-mineralization to produce nano composites, which can mimic natural bone. The 3D macro porous scaffolds play an important role in the formation of new tissues and provide a temporary scaffold to guide new tissue in-growth and regeneration (Nikalson and Langer, 1997). Chitosan has been proposed to serve as a non-protein matrix for 3D tissue growth. Chitosan could provide the biological primer for cell-tissue proliferation and reconstruction. One of the most promising features of chitosan is its excellent ability to be processed into porous structures for use in cell transplantation and tissue regeneration. In tissue engineering, the porous structure of chitosan provides a scaffold for bone cells to grow in and seed new bone regeneration. For rapid cell growth, the scaffold must have optimal micro architecture such as pore size, shape and specific surface area (Madihally and

In bone tissue engineering, the biodegradable substitutes act as a temporary skeleton inserted into the defective sites of skeleton or lost bone sites, in order to support and stimulate bone tissue regeneration while they gradually degrade and are replaced by new bone tissue (Service, 2000). Chitosan-based scaffolds possess some special properties for use in tissue engineering. The major goal in fabricating scaffolds for bone tissue engineering is to accurately control pore size and porosity. Porous chitosan structures can be formed by freezing and lyophilizing chitosan–acetic acid solutions in suitable moulds (Chow and Khor, 2000). Bone regeneration research needed to deal with various clinical bone diseases such as bone infections, bone tumors and bone loss by trauma (Braddock et al., 2001). To combine the osteoconductivity of calcium phosphate and good biodegradability of polymers, composites have been developed for bone tissue engineering either by directly mixing the components or by a biomimetic approach (Wei and Ma, 2004). Polymer-ceramic composite scaffolds are expected to mimic natural bone, in the way that natural bone is also a composite of inorganic compounds (calcium phosphates especially substituted carbonated hydroxyapatite) and organic compounds (collagen, protein matrix, etc.). The hydroxyapatite–chitosan–alginate porous network has been reported and demonstrated to be suitable for bone tissue

Research advances in bone regeneration in tissue engineering have focused on the development of three dimensional (3D) porous scaffolds that can serve as a support, reinforce and in some cases organize the tissue regeneration or replacement in a natural way (Sachlos et al., 2003). Several studies have been focused on chitosan–calcium phosphates (CP) composites for this purpose in bone tissue engineering. Beta-tricalcium phosphate (β-TCP) and hydroxyapatite (HA) of CP bioceramics are excellent candidates for bone repair and regeneration because of their similarity in chemical composition with inorganic components of bone (Zhang et al., 2003). Tissue engineering is regarded as an ultimately ideal medical treatment for diseases that have been too difficult to be cured by existing methods. This biomedical engineering is designed to repair injured body parts and restore

engineering applications using osteoblast cells (Zhao et al., 2003).

Shen et al., (2007) performed that with the increase of pH after the addition of ammonia, carboxyl groups of citric acid may begin to act as nucleation center for calcium phosphate formation. These negatively charged carboxyls in the reaction system can bond Ca2+ strongly and thus forms a large scale of local super saturation microenvironment, and strong electric field resulted from high concentration of negatively charged carboxyls are favor of the interaction that with the most positively charged crystalline plane, so there are many nucleation sites in the network of hydrogel template, each point of nucleation can result in microcrystal. Here, to our attention, biocompatible citric acid took the place of acetic acid in this work because three carboxyl of citric acid could provide more nucleation sites which were appropriate to formation of ultra fine nano-sized carbonate apatite. And it has been conjectured that appropriate increase of citrate ions can benefit the bone resorption and ossification through the formation of dissociated calcium citrate complexes in the surrounding body fluid (Rhee and Tanaka, 1999). Each citric acid molecule can provide three negatively charged carboxyls which act as nucleation center for calcium phosphate formation. Increase of nucleation center can be appropriate to fine crystallites. Furthermore, cross-linking chitosan hydrogel was provided with three dimensional network microstructures, its compartment effect limited the growth of inorganic mineral particles, so the inorganic nano-particles were limited to aggregate in the compartment of the chitosan hydrogel template according to orientation of preferential growth of crystal plane. This

multiple-order template effect based on multiple-point nucleation of citric acid and compartment of hydrogel network had a very obvious mediation in the formation process of homogeneous composites (Shen et al., 2007) (Fig.6). In bone tissue engineering, the biodegradable scaffold is a temporary template introduced at the defective site or lost bone to initiate bone tissue regeneration, while it gradually degrades and is replaced by newly formed bone tissue. Finally, an ideal scaffold is characterized by excellent biocompatibility, controllable biodegradability, cytocompatibility, suitable microstructure (pore size and porosity) and mechanical properties. Additionally, it must be capable of promoting cell adhesion and retaining the metabolic functions of attached cells (Thein-Han and Misra, 2009).

Biocomposite Materials 135

was able to have apatite form on its surface in SBF, and consequently has apatite produced on its surface in the living body, and bonds to living bone through this apatite layer. The other type was directly bond to living bone without the formation of apatite on their surfaces, so examination of apatite formation on the surface of a material in SBF is useful for predicting the in vivo bone bioactivity of the material, not only qualitatively but also

From SEM photos (Fig.7), it can be known that chitosan in the chitosan /nHA composite gradually degraded during the soaking in SBF solution, which resulted in plenty of macroand micropores on the surface of and inside the specimens. At the same time, a lot of tiny apatite crystals deposited on the surface of the specimens, and till the 8th week, a thin layer of bone-like apatite, being highly bioactive was formed. At the first 4 weeks, the degradation rate of chitosan was higher than the deposition rate of apatite on the surface of specimens, which corresponding to a continuous increase of the rate of weight loss. After that, the deposition of apatite is prior to the degradation of chitosan, so the rate of weight loss decreased. This was also confirmed by the rate of water adsorption with the degradation of chitosan during the specimen's soaking in SBF solution, a more sponge-like structure was formed, which can hold more water. However, with more apatite crystals deposition, some of these pores were filled or covered, so water adsorption decreased (Zhang et al., 2005). Kong et al., (2006) reported that chitosan/nano-hydroxyapatite composite scaffolds analysis showed that after incubation in simulated body fluid on both of the scaffolds (the apatite-coated composite scaffolds and apatite-coated chitosan scaffolds), carbonate hydroxyapatite was formed. With increasing nano-hydroxyapatite content in the composite, the quantity of the apatite formed on the scaffolds increased. Compared with pure chitosan, the composite with nano-hydroxyapatite could form apatite more readily during the biomimetic process, which suggests that the composite possessed better mineralization activity (Kong et al., 2006).

**Figure 7.** The SEM images of chitosan/nHA composites after soaking in SBF solutions for (a) 0 week,

The swelling properties and degradation behavior proved the stability of hydroxyapatitetitania/chitosan-gelatin polymeric biocomposites into the media. In-vitro test behavior confirmed that the prepared composite enhanced the deposition of Ca++ and P ions onto the surface that is in the favor of the formation of apatite layer. FT-IR and SEM-EDAX of

quantitatively (Kokubo and Takadama, 2006).

(b) 1week, (c) 4 weeks and (d) 8 weeks, Magn. × 400.

*5.5.3.1. In simulated body fluid (SBF)* 

**Figure 6.** The scheme of formation of homogeneous chitosan/carbonate apatite composite and 3D nanocomposite scaffold (Shen et al., 2007).

#### *5.5.3. In vitro applications*

The process of apatite formation on the bioactive materials in living body could be reproduced in simulated body fluid (SBF), which means that in vivo bone bioactivity of a material can be predicted by assessing apatite formation on its surface in SBF. They confirmed that there are two types of material which inserted into living body. One of them was able to have apatite form on its surface in SBF, and consequently has apatite produced on its surface in the living body, and bonds to living bone through this apatite layer. The other type was directly bond to living bone without the formation of apatite on their surfaces, so examination of apatite formation on the surface of a material in SBF is useful for predicting the in vivo bone bioactivity of the material, not only qualitatively but also quantitatively (Kokubo and Takadama, 2006).

#### *5.5.3.1. In simulated body fluid (SBF)*

134 Composites and Their Applications

2009).

multiple-order template effect based on multiple-point nucleation of citric acid and compartment of hydrogel network had a very obvious mediation in the formation process of homogeneous composites (Shen et al., 2007) (Fig.6). In bone tissue engineering, the biodegradable scaffold is a temporary template introduced at the defective site or lost bone to initiate bone tissue regeneration, while it gradually degrades and is replaced by newly formed bone tissue. Finally, an ideal scaffold is characterized by excellent biocompatibility, controllable biodegradability, cytocompatibility, suitable microstructure (pore size and porosity) and mechanical properties. Additionally, it must be capable of promoting cell adhesion and retaining the metabolic functions of attached cells (Thein-Han and Misra,

**Figure 6.** The scheme of formation of homogeneous chitosan/carbonate apatite composite and 3D

The process of apatite formation on the bioactive materials in living body could be reproduced in simulated body fluid (SBF), which means that in vivo bone bioactivity of a material can be predicted by assessing apatite formation on its surface in SBF. They confirmed that there are two types of material which inserted into living body. One of them

nanocomposite scaffold (Shen et al., 2007).

*5.5.3. In vitro applications* 

From SEM photos (Fig.7), it can be known that chitosan in the chitosan /nHA composite gradually degraded during the soaking in SBF solution, which resulted in plenty of macroand micropores on the surface of and inside the specimens. At the same time, a lot of tiny apatite crystals deposited on the surface of the specimens, and till the 8th week, a thin layer of bone-like apatite, being highly bioactive was formed. At the first 4 weeks, the degradation rate of chitosan was higher than the deposition rate of apatite on the surface of specimens, which corresponding to a continuous increase of the rate of weight loss. After that, the deposition of apatite is prior to the degradation of chitosan, so the rate of weight loss decreased. This was also confirmed by the rate of water adsorption with the degradation of chitosan during the specimen's soaking in SBF solution, a more sponge-like structure was formed, which can hold more water. However, with more apatite crystals deposition, some of these pores were filled or covered, so water adsorption decreased (Zhang et al., 2005). Kong et al., (2006) reported that chitosan/nano-hydroxyapatite composite scaffolds analysis showed that after incubation in simulated body fluid on both of the scaffolds (the apatite-coated composite scaffolds and apatite-coated chitosan scaffolds), carbonate hydroxyapatite was formed. With increasing nano-hydroxyapatite content in the composite, the quantity of the apatite formed on the scaffolds increased. Compared with pure chitosan, the composite with nano-hydroxyapatite could form apatite more readily during the biomimetic process, which suggests that the composite possessed better mineralization activity (Kong et al., 2006).

**Figure 7.** The SEM images of chitosan/nHA composites after soaking in SBF solutions for (a) 0 week, (b) 1week, (c) 4 weeks and (d) 8 weeks, Magn. × 400.

The swelling properties and degradation behavior proved the stability of hydroxyapatitetitania/chitosan-gelatin polymeric biocomposites into the media. In-vitro test behavior confirmed that the prepared composite enhanced the deposition of Ca++ and P ions onto the surface that is in the favor of the formation of apatite layer. FT-IR and SEM-EDAX of

copolymer and three composites post-immersion verified the formation of spherical apatite particles onto the copolymer surface; therefore, it was expected to enhance the apatite nucleation onto the filler composite surface especially hydroxyapatite-titania/chitosangelatin (AK1) composite containing 10% content of titania (Mohamed and Mostafa, 2008).

Biocomposite Materials 137

The morphology and behavior of bone marrow stem cell (BMSCs) cultured in-vitro with the n-HA/chitosan (CS) composite membranes are observed under phase-contrast microscope. Fig.9 shows representative phase-contrast micrographs of cell attachment on the membrane with a n-HA/CS ratio of 4:6 after culture for 1 day, 7 days and 11 days. At the first day, only a few BMSCs are present with the elongated fusing form shape. At 7 days, a large amount of cells proliferate and form cell colony. At 11 days, the population of cells increases manifestly and cells fully attach to the membrane. Obviously, the n-HA/CS composite membrane has no negative effect on the cell morphology, viability and proliferation (Xianmiao et al., 2009).

**Figure 9.** Phase-contrast micrographs of the BMSCs (denoted as C) attached to n-HA/CS (4:6)

In chitosan–nHA scaffolds, the presence of extensive filopodia, flat morphology, and excellent spreading in and around the interconnected porous structure, indicated strong cellular adhesion and growth (Fig.10e–h). Furthermore, cell density, cell–cell contact, sheet-like structure, and formation of extracellular matrix and cytoplasmic extensions were more pronounced on the chitosan–nHA surface than on pure chitosan. They believe based on Fig.10e–h that the steps involved in the development of sheet-like morphology involves clustering of cells and bridge formation between the pore walls with consequent formation of a multilayer structure. These steps occurred during early stages in the nano-composite constructs in relation to chitosan scaffolds, suggesting that pre-osteoblasts have high affinity to the surface of chitosan–nHA composite, which is attributed to its increase surface area and composition. Chitosan–nanocrystalline calcium phosphate scaffolds characterized by a relatively rough surface and approximately 20 times greater area/unit mass than chitosan scaffold indicated increase adsorption of fibronectin and improved cell attachment (Thein-Han and Misra, 2009).

The extracellular matrix (ECM) is a powerful regulator of cell adhesion and indeed cells respond to the ECM by means of integrins, which couple the component of the ECM with the actin cytoskeleton. This structure thus mediates adhesion to the ECM and therefore to the implant material. In this respect, the possibility of bonding osteoinductive polymers such as modified chitosan to the ceramic substrate could enhance cell proliferation and consequently anchorage to the implant (Mattiolibelmonte et al., 1998). Sections from chitosan coated HA implants exhibited an evident mesenchymal reaction between bone and implant with several features of osteoinduction. Bone trabeculae penetrating the HA implant were also observed.

membrane (denoted as M) after in vitro culture for 1d (a), 7d (b) and 11 d (c).

*5.5.4. In-vivo application* 

#### *5.5.3.2. Bone Tissue engineering (Scaffold)*

Anti-washout scaffold paste could be directly applied to fit complex shapes of bone defects, without involving machining as in the case of sintered hydroxyapatite. The synergistic use of a reinforcing agent (e.g., chitosan) and a pore-forming agent (e.g., mannitol) in a bone graft may be applicable to other tissue engineering materials. In developing strong and macro-porous calcium phosphate cement (CPC) scaffolds by incorporating chitosan and water-soluble mannitol. The new CPC–chitosan formulation was biocompatible and supported the adhesion, spreading, proliferation and viability of osteoblast cells. The cells were observed to infiltrate into the pores of the scaffold and establish cell–cell interactions. The increased strength and macroporosity of the new apatite scaffold may help facilitate bone ingrowth, implant fixation, and more rapid new bone formation (Fig.8) (Hockin et al., 2005).

**Figure 8.** The SEM of cell attachment on (A) CPC control and (B) CPC chitosan composite. The cells developed cytoplasmic processes with lengths ranging from approximately 20 to 50 mm, and the materials exhibited similar cell attachment and cytoplasmic processes development (Hockin et al., 2005).

Kong et al., (2006) reported that pre-osteoblast cells cultured on the apatite-coated scaffolds showed different behavior. On the apatite-coated chitosan/nano-hydroxyapatite composite scaffolds cells presented better proliferation than on apatite-coated chitosan scaffolds. The cells on composite scaffolds showed a higher alkaline phosphatase activity which suggested a higher differentiation level. The results indicated that the addition of nano-hydroxyapatite improved the bioactivity of chitosan/nano-hydroxyapatite composite scaffolds. MSCs do not appear to be rejected by the immune system, allowing for large-scale production, appropriate characterization and testing, and the subsequent ready availability of allogeneic tissue repair enhancing cellular therapeutics. Overall it can be said that, for now, MSCs present more advantages than other cells and have already been widely used in bone tissue engineering. All the superiorities of MSCs encourage us to introduce MSCs into n-HA composite scaffolds for tissue engineering application (Wang et al., 2007).

The morphology and behavior of bone marrow stem cell (BMSCs) cultured in-vitro with the n-HA/chitosan (CS) composite membranes are observed under phase-contrast microscope. Fig.9 shows representative phase-contrast micrographs of cell attachment on the membrane with a n-HA/CS ratio of 4:6 after culture for 1 day, 7 days and 11 days. At the first day, only a few BMSCs are present with the elongated fusing form shape. At 7 days, a large amount of cells proliferate and form cell colony. At 11 days, the population of cells increases manifestly and cells fully attach to the membrane. Obviously, the n-HA/CS composite membrane has no negative effect on the cell morphology, viability and proliferation (Xianmiao et al., 2009).

**Figure 9.** Phase-contrast micrographs of the BMSCs (denoted as C) attached to n-HA/CS (4:6) membrane (denoted as M) after in vitro culture for 1d (a), 7d (b) and 11 d (c).

In chitosan–nHA scaffolds, the presence of extensive filopodia, flat morphology, and excellent spreading in and around the interconnected porous structure, indicated strong cellular adhesion and growth (Fig.10e–h). Furthermore, cell density, cell–cell contact, sheet-like structure, and formation of extracellular matrix and cytoplasmic extensions were more pronounced on the chitosan–nHA surface than on pure chitosan. They believe based on Fig.10e–h that the steps involved in the development of sheet-like morphology involves clustering of cells and bridge formation between the pore walls with consequent formation of a multilayer structure. These steps occurred during early stages in the nano-composite constructs in relation to chitosan scaffolds, suggesting that pre-osteoblasts have high affinity to the surface of chitosan–nHA composite, which is attributed to its increase surface area and composition. Chitosan–nanocrystalline calcium phosphate scaffolds characterized by a relatively rough surface and approximately 20 times greater area/unit mass than chitosan scaffold indicated increase adsorption of fibronectin and improved cell attachment (Thein-Han and Misra, 2009).

#### *5.5.4. In-vivo application*

136 Composites and Their Applications

2005).

*5.5.3.2. Bone Tissue engineering (Scaffold)* 

copolymer and three composites post-immersion verified the formation of spherical apatite particles onto the copolymer surface; therefore, it was expected to enhance the apatite nucleation onto the filler composite surface especially hydroxyapatite-titania/chitosangelatin (AK1) composite containing 10% content of titania (Mohamed and Mostafa, 2008).

Anti-washout scaffold paste could be directly applied to fit complex shapes of bone defects, without involving machining as in the case of sintered hydroxyapatite. The synergistic use of a reinforcing agent (e.g., chitosan) and a pore-forming agent (e.g., mannitol) in a bone graft may be applicable to other tissue engineering materials. In developing strong and macro-porous calcium phosphate cement (CPC) scaffolds by incorporating chitosan and water-soluble mannitol. The new CPC–chitosan formulation was biocompatible and supported the adhesion, spreading, proliferation and viability of osteoblast cells. The cells were observed to infiltrate into the pores of the scaffold and establish cell–cell interactions. The increased strength and macroporosity of the new apatite scaffold may help facilitate bone ingrowth, implant fixation, and more rapid new bone formation (Fig.8) (Hockin et al.,

**Figure 8.** The SEM of cell attachment on (A) CPC control and (B) CPC chitosan composite. The cells developed cytoplasmic processes with lengths ranging from approximately 20 to 50 mm, and the materials exhibited similar cell attachment and cytoplasmic processes development (Hockin et al., 2005).

Kong et al., (2006) reported that pre-osteoblast cells cultured on the apatite-coated scaffolds showed different behavior. On the apatite-coated chitosan/nano-hydroxyapatite composite scaffolds cells presented better proliferation than on apatite-coated chitosan scaffolds. The cells on composite scaffolds showed a higher alkaline phosphatase activity which suggested a higher differentiation level. The results indicated that the addition of nano-hydroxyapatite improved the bioactivity of chitosan/nano-hydroxyapatite composite scaffolds. MSCs do not appear to be rejected by the immune system, allowing for large-scale production, appropriate characterization and testing, and the subsequent ready availability of allogeneic tissue repair enhancing cellular therapeutics. Overall it can be said that, for now, MSCs present more advantages than other cells and have already been widely used in bone tissue engineering. All the superiorities of MSCs encourage us to introduce MSCs into n-HA

composite scaffolds for tissue engineering application (Wang et al., 2007).

The extracellular matrix (ECM) is a powerful regulator of cell adhesion and indeed cells respond to the ECM by means of integrins, which couple the component of the ECM with the actin cytoskeleton. This structure thus mediates adhesion to the ECM and therefore to the implant material. In this respect, the possibility of bonding osteoinductive polymers such as modified chitosan to the ceramic substrate could enhance cell proliferation and consequently anchorage to the implant (Mattiolibelmonte et al., 1998). Sections from chitosan coated HA implants exhibited an evident mesenchymal reaction between bone and implant with several features of osteoinduction. Bone trabeculae penetrating the HA implant were also observed.

Biocomposite Materials 139

**Figure 11.** (A) SEM of cell infiltration into a macropore. (B) Cell attachment (arrows) to the bottom of a pore. (C) Cells inside a large pore near an opening at the bottom of the pore. (D) Cell-cell interactions

The development of suitable three-dimensional scaffold for the maintenance of cellular viability and differentiation is critical for applications in periodontal tissue engineering. The different ratios of porous nanohydroxyapatite/chitosan (HA/chitosan) scaffolds were prepared through a freeze-drying process. The results indicated that the porosity and pore diameter of the HA/chitosan scaffolds were lower than those of pure chitosan scaffold. The HA/chitosan scaffold containing 1% HA exhibited better cytocompatibility than the pure chitosan scaffold. These scaffolds are evaluated in vitro by the analysis of microscopic structure, porosity, and cytocompatibility. The expression of type I collagen and alkaline phosphatase (ALP) activity are detected with real-time polymerase chain reaction (RT-PCR). Human periodontal ligament cells (HPLCs) transfected with enhanced green fluorescence protein (EGFP) are seeded onto the scaffolds, and then these scaffolds are implanted subcutaneously into athymic mice after implanted in vivo, EGFP transfected with HPLCs) not only proliferate but also recruit surrounding tissue to grow in the scaffold. The degradation of the scaffold significantly decreased in the presence of HA. This study demonstrated the potential of HA/ chitosan scaffold as a good substrate candidate in

inside a pore (arrow indicates a cell-cell junction).

periodontal tissue engineering (Zhang et al., 2007).

*Biomaterials Department, National Research Centre, Cairo, Egypt* 

**Author details** 

Khaled R. Mohamed

**Figure 10.** Scanning electron micrographs illustrating morphology of pre-osteoblasts seeded on high-MW chitosan (CH) and chitosan–nHA (CH1) scaffolds (Saggital section). Pre-osteoblasts on chitosan surface after (a) day 1, (b) day 3, (c) day 7 and (d) day 21 of cell culture; and on chitosan–nHA surface after (e) day 1, (f) day 3, (g) day 7 and (h) day 21 of cell culture. EDS spectra for the boxed region in (c) and (g) are presented in (i) and (j), respectively, showing the presence of Ca and P. The P peak is merged with the Au peak, which is due to conductive gold coating on the sample.

Sunny et al., (2002) have reported the preparation of HA-chitosan microspheres as potential bone and periodontal filling materials. HA powder was mixed with chitosan solution followed by paraffin oil, hexane and a surfactant and the microsphere production process commenced. Subsequently, glutaraldehyde was added to crosslink chitosan to give spherical particles ranging from 125 to 1000 mm. When the chitosan/n-HA composite implanted in body using as tissue scaffold, the degradation of chitosan makes room for the growth of new bone and then is substituted by new bone completely. It has been reported that chitosan can promote nucleation and growth of apatite and calcite crystals as well. Moreover, the surface of chitosan is hydrophilic, which can facilitate cell adhesion, proliferation and differentiation (Fig. 11). So, the chitosan/n-HA composite, used as bone substitutes, are hopeful to activate the regeneration and remodeling of bone tissue (Zhang et al., 2005).

**Figure 11.** (A) SEM of cell infiltration into a macropore. (B) Cell attachment (arrows) to the bottom of a pore. (C) Cells inside a large pore near an opening at the bottom of the pore. (D) Cell-cell interactions inside a pore (arrow indicates a cell-cell junction).

The development of suitable three-dimensional scaffold for the maintenance of cellular viability and differentiation is critical for applications in periodontal tissue engineering. The different ratios of porous nanohydroxyapatite/chitosan (HA/chitosan) scaffolds were prepared through a freeze-drying process. The results indicated that the porosity and pore diameter of the HA/chitosan scaffolds were lower than those of pure chitosan scaffold. The HA/chitosan scaffold containing 1% HA exhibited better cytocompatibility than the pure chitosan scaffold. These scaffolds are evaluated in vitro by the analysis of microscopic structure, porosity, and cytocompatibility. The expression of type I collagen and alkaline phosphatase (ALP) activity are detected with real-time polymerase chain reaction (RT-PCR). Human periodontal ligament cells (HPLCs) transfected with enhanced green fluorescence protein (EGFP) are seeded onto the scaffolds, and then these scaffolds are implanted subcutaneously into athymic mice after implanted in vivo, EGFP transfected with HPLCs) not only proliferate but also recruit surrounding tissue to grow in the scaffold. The degradation of the scaffold significantly decreased in the presence of HA. This study demonstrated the potential of HA/ chitosan scaffold as a good substrate candidate in periodontal tissue engineering (Zhang et al., 2007).

## **Author details**

138 Composites and Their Applications

**Figure 10.** Scanning electron micrographs illustrating morphology of pre-osteoblasts seeded on high-MW chitosan (CH) and chitosan–nHA (CH1) scaffolds (Saggital section). Pre-osteoblasts on chitosan surface after (a) day 1, (b) day 3, (c) day 7 and (d) day 21 of cell culture; and on chitosan–nHA surface after (e) day 1, (f) day 3, (g) day 7 and (h) day 21 of cell culture. EDS spectra for the boxed region in (c) and (g) are presented in (i) and (j), respectively, showing the presence of Ca and P. The P peak is

Sunny et al., (2002) have reported the preparation of HA-chitosan microspheres as potential bone and periodontal filling materials. HA powder was mixed with chitosan solution followed by paraffin oil, hexane and a surfactant and the microsphere production process commenced. Subsequently, glutaraldehyde was added to crosslink chitosan to give spherical particles ranging from 125 to 1000 mm. When the chitosan/n-HA composite implanted in body using as tissue scaffold, the degradation of chitosan makes room for the growth of new bone and then is substituted by new bone completely. It has been reported that chitosan can promote nucleation and growth of apatite and calcite crystals as well. Moreover, the surface of chitosan is hydrophilic, which can facilitate cell adhesion, proliferation and differentiation (Fig. 11). So, the chitosan/n-HA composite, used as bone substitutes, are hopeful to activate

merged with the Au peak, which is due to conductive gold coating on the sample.

the regeneration and remodeling of bone tissue (Zhang et al., 2005).

Khaled R. Mohamed *Biomaterials Department, National Research Centre, Cairo, Egypt* 

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**Chapter 7** 

© 2012 Li et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Li et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Non-Destructive Examination of Interfacial** 

**Using Acoustic Emission** 

Haiyan Li, Jianying Li, Xiaozhou Liu and Alex Fok

Additional information is available at the end of the chapter

causes for replacement of composite restorations [2-4].

techniques, thermal and mechanical loading, etc. [5-8].

http://dx.doi.org/10.5772/51369

**1. Introduction** 

**Debonding in Dental Composite Restorations** 

Light-cured, dental resin composites are widely used to repair decayed or damaged teeth because of their superior esthetics, ease of use and ability to bond to tooth tissues. However, during setting or polymerization, the resin composite shrinks, producing shrinkage stresses within the tooth and composite [1]. If the internal shrinkage stress is high enough, debonding between the tooth and restoration will occur, causing problems such as reduced fracture resistance and increased micro-leakage. The latter will ultimately lead to secondary caries. Clinical studies have identified the loss of interfacial integrity as one of the main

Unfortunately, interfacial debonding caused by the shrinkage of composite resins can be hard to avoid. Fig. 1 shows evidence of interfacial debonding by comparing the Micro-CT images of a restored tooth before and after curing the composite. It can be seen that an interfacial crack of a significant length appeared after curing the composite along the initially intact interface. On the other hand, in the example shown in Fig. 2, where a different resin composite was used, no clear interfacial debonding was observed from the Micro-CT images. Therefore, whether interfacial debonding will occur depends on the restorative composite material, specifically the level of shrinkage stress it produces. There are other factors that may affect the initial quality and subsequent degradation of bonding at the tooth-restoration interface, e.g. the adhesive/bonding materials, cavity geometry, restorative

It is very difficult to predict whether interfacial debonding will take place in a particular composite restoration system. This is because the shrinkage stress within a real tooth restoration is difficult to predict or measure due to the small but complicated geometry and


## **Non-Destructive Examination of Interfacial Debonding in Dental Composite Restorations Using Acoustic Emission**

Haiyan Li, Jianying Li, Xiaozhou Liu and Alex Fok

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51369

## **1. Introduction**

146 Composites and Their Applications

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Light-cured, dental resin composites are widely used to repair decayed or damaged teeth because of their superior esthetics, ease of use and ability to bond to tooth tissues. However, during setting or polymerization, the resin composite shrinks, producing shrinkage stresses within the tooth and composite [1]. If the internal shrinkage stress is high enough, debonding between the tooth and restoration will occur, causing problems such as reduced fracture resistance and increased micro-leakage. The latter will ultimately lead to secondary caries. Clinical studies have identified the loss of interfacial integrity as one of the main causes for replacement of composite restorations [2-4].

Unfortunately, interfacial debonding caused by the shrinkage of composite resins can be hard to avoid. Fig. 1 shows evidence of interfacial debonding by comparing the Micro-CT images of a restored tooth before and after curing the composite. It can be seen that an interfacial crack of a significant length appeared after curing the composite along the initially intact interface. On the other hand, in the example shown in Fig. 2, where a different resin composite was used, no clear interfacial debonding was observed from the Micro-CT images. Therefore, whether interfacial debonding will occur depends on the restorative composite material, specifically the level of shrinkage stress it produces. There are other factors that may affect the initial quality and subsequent degradation of bonding at the tooth-restoration interface, e.g. the adhesive/bonding materials, cavity geometry, restorative techniques, thermal and mechanical loading, etc. [5-8].

It is very difficult to predict whether interfacial debonding will take place in a particular composite restoration system. This is because the shrinkage stress within a real tooth restoration is difficult to predict or measure due to the small but complicated geometry and

rapidly changing material properties during curing. At the same time, the bond strength between the tooth and restoration is difficult to determine. Tensile and shear tests have been widely used for bond strength testing. However, the results are highly variable, being dependent on the test devices and specimen size used [9-11]. Therefore, they are not very predictive of the actual clinical performance.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 149

**Figure 2.** Micro-CT images of a tooth specimen restored with P90: (a) horizontal cross-section before curing, (b) horizontal cross-section after curing, (c) vertical cross-section before curing and (d) vertical

cross-section after curing. *(Note: E-enamal, D-Dentin, C-composite.)* 

**Figure 1.** Micro-CT images of a tooth specimen restored with Z100: (a) horizontal cross-section before curing, (b) horizontal cross-section after curing, (c) vertical cross-section before curing and (d) vertical cross-section after curing. *(E-enamal, D-Dentin, C-composite. Lines 1-4 in (c) schematically indicate the locations of micro-hardness measurement described in Section 3.2.5 and Section 4.1. The arrows in (b) and (d) point out the debonding positions at the interface. )*

Non-Destructive Examination of Interfacial Debonding in Dental Composite Restorations Using Acoustic Emission 149

148 Composites and Their Applications

predictive of the actual clinical performance.

*point out the debonding positions at the interface. )*

rapidly changing material properties during curing. At the same time, the bond strength between the tooth and restoration is difficult to determine. Tensile and shear tests have been widely used for bond strength testing. However, the results are highly variable, being dependent on the test devices and specimen size used [9-11]. Therefore, they are not very

**Figure 1.** Micro-CT images of a tooth specimen restored with Z100: (a) horizontal cross-section before curing, (b) horizontal cross-section after curing, (c) vertical cross-section before curing and (d) vertical cross-section after curing. *(E-enamal, D-Dentin, C-composite. Lines 1-4 in (c) schematically indicate the locations of micro-hardness measurement described in Section 3.2.5 and Section 4.1. The arrows in (b) and (d)* 

**Figure 2.** Micro-CT images of a tooth specimen restored with P90: (a) horizontal cross-section before curing, (b) horizontal cross-section after curing, (c) vertical cross-section before curing and (d) vertical cross-section after curing. *(Note: E-enamal, D-Dentin, C-composite.)* 

Further details on the development of shrinkage stress in composite restorations and some current methods for assessing the resulting interfacial debonding are described in Section 2. The aim of this study is to develop a new method to evaluate the interfacial debonding of dental composite restorations. A non-destructive method, the acoustic emission (AE) technique, will be used to monitor *in-situ* the interfacial debonding of composite restorations during polymerization. The AE technique and the system used for monitoring interfacial debonding will be described in Section 3, where the capability of this new evaluation method will be verified by different tests. Then, in Section 4, the AE technique will be used to study the influences of several factors, including the composite material, C-factor (ratio between bonded and unbonded surfaces) and filling technique, on the debonding behavior of composite restorations.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 151

Interfacial debonding has been identified as one of the main causes for replacement of

To date, there is no reliable tool to detect or monitor debonding between the composite and tooth during the curing process. Traditional methods for studying the interfacial integrity of dental restorations include optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [5-8, 17, 18]. The major disadvantage of these methods is that they are limited to essentially surface examination; internal debonding cannot be detected. To see whether debonding has occurred inside the restoration, these methods require destructive sectioning of the specimens which may introduce more uncertainties to the results due to possible machining damage. As an alternative, nondestructive methods, such as X-ray micro-computed tomography (micro-CT), have received much attention in recent years in the study of interfacial bonding/debonding of composite restorations [19]. Micro-CT is a computer-aided, 3D reconstruction of a structure or material that can be sliced virtually along any direction to gain accurate information on their internal geometric properties and structural parameters. Figures 1 and 2 show Micro-CT images used to study the interfacial debonding of composite restorations. Although Micro-CT can provide 3D examinations of the whole structure, its lower resolution means that it cannot detect debonding at a submicron level. Most of all, none of these imaging techniques can be

**3. AE technique and its application to interfacial debonding examination** 

The acoustic emission (AE) measurement technique is also a non-destructive method. It is normally used to monitor the integrity of structures by providing real-time information of the fracture or damage process. It uses transducers or sensors to detect the high-frequency sound waves produced as a result of the sudden strain energy released within a material following fracture. Figure 3 shows schematically how an AE test system works. First, the cracking event within a material is captured by the AE sensor attached on the surface of the component. The raw signal is then passed through a preamplifier for pre-amplification and then to the acquisition system for acquisition and storage. Finally, the AE data can be

AE technology has been widely used in research and industry to monitor the development of crack growth, wear, fiber-matrix debonding in composites, phase transformation, etc [20- 22]. It has also been used to detect the fracture of different dental structures. For example, Ereifej et al. [23] used the AE technique to detect the initial fracture of ceramic crowns; Vallittu [24] used it to study the fracture of a composite veneer reinforced by woven glass fibres; and Kim and Okuno [25] used it to study the micro-fracture behavior of composite

The interfacial debonding in composite restorations, either adhesive or cohesive, is actually a kind of fracture within the restored tooth structure. Therefore, it is possible to use the AE

composite restorations [2-4].

used to monitor debonding as it happens.

**3.1. The acoustic emission (AE) measurement technique** 

displayed and analyzed by specially designed software.

resins containing irregular-shaped fillers.

## **2. Shrinkage stress and interfacial debonding in dental composite restorations**

The dental resin composite is a mixture of organic monomer systems and inorganic filler particles. The monomer systems, which act as a matrix for the dental composite, are flowable before curing so that the composite can be easily packed into the tooth cavity and achieve good marginal adaptation with the tooth tissues. When polymerization is activated, the resin matrix will solidify, with its mechanical properties (Young's modulus, viscosity, hardness, strength, etc.) changing significantly and rapidly in the process. Volumetric shrinkage also occurs during polymerization due to the reduction of intermolecular separations in the monomers [12]. This is a very quick process which normally takes place within the first 30-50 seconds. The volumetric contraction that accompanies polymerization is typically on the order of 1.5–5% [1].

For most dental composite restorations, a bonding agent (adhesive) is used to create a strong bond at the tooth-composite interface. Thus, during the polymerization process, because the composite is constrained along the interfaces, shrinkage stresses are built up within the composite and tooth structure. These shrinkage stresses are difficult to predict or measure because of the small but complicated tooth geometry and rapidly changing material properties. Some experimental methods using simple specimens have been developed to estimate the polymerization shrinkage stress that develops in dental composites [1, 13, 14]. However, the measured shrinkage stresses are sensitive to the test configuration and procedures, e.g. the instrument's compliance, direction of curing light application and specimen shape. Also, because the material parameters (shrinkage strain, viscosity, Young's modulus and Poisson's ratio) of the composite that control the development of shrinkage stresses all change rapidly with time during polymerization, even with a suitable mathematical model, the accurate prediction of the shrinkage stresses is not trivial [15, 16].

If the shrinkage stress within a composite restoration is high enough, interfacial debonding or, more generally, failure between the tooth and restoration will occur. Interfacial failure can be adhesive, in which case failure occurs right in the adhesive layer; or it can be cohesive, in which case failure occurs in the tooth or composite materials near the interface. Interfacial debonding has been identified as one of the main causes for replacement of composite restorations [2-4].

To date, there is no reliable tool to detect or monitor debonding between the composite and tooth during the curing process. Traditional methods for studying the interfacial integrity of dental restorations include optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [5-8, 17, 18]. The major disadvantage of these methods is that they are limited to essentially surface examination; internal debonding cannot be detected. To see whether debonding has occurred inside the restoration, these methods require destructive sectioning of the specimens which may introduce more uncertainties to the results due to possible machining damage. As an alternative, nondestructive methods, such as X-ray micro-computed tomography (micro-CT), have received much attention in recent years in the study of interfacial bonding/debonding of composite restorations [19]. Micro-CT is a computer-aided, 3D reconstruction of a structure or material that can be sliced virtually along any direction to gain accurate information on their internal geometric properties and structural parameters. Figures 1 and 2 show Micro-CT images used to study the interfacial debonding of composite restorations. Although Micro-CT can provide 3D examinations of the whole structure, its lower resolution means that it cannot detect debonding at a submicron level. Most of all, none of these imaging techniques can be used to monitor debonding as it happens.

## **3. AE technique and its application to interfacial debonding examination**

## **3.1. The acoustic emission (AE) measurement technique**

150 Composites and Their Applications

of composite restorations.

is typically on the order of 1.5–5% [1].

**restorations** 

Further details on the development of shrinkage stress in composite restorations and some current methods for assessing the resulting interfacial debonding are described in Section 2. The aim of this study is to develop a new method to evaluate the interfacial debonding of dental composite restorations. A non-destructive method, the acoustic emission (AE) technique, will be used to monitor *in-situ* the interfacial debonding of composite restorations during polymerization. The AE technique and the system used for monitoring interfacial debonding will be described in Section 3, where the capability of this new evaluation method will be verified by different tests. Then, in Section 4, the AE technique will be used to study the influences of several factors, including the composite material, C-factor (ratio between bonded and unbonded surfaces) and filling technique, on the debonding behavior

**2. Shrinkage stress and interfacial debonding in dental composite** 

The dental resin composite is a mixture of organic monomer systems and inorganic filler particles. The monomer systems, which act as a matrix for the dental composite, are flowable before curing so that the composite can be easily packed into the tooth cavity and achieve good marginal adaptation with the tooth tissues. When polymerization is activated, the resin matrix will solidify, with its mechanical properties (Young's modulus, viscosity, hardness, strength, etc.) changing significantly and rapidly in the process. Volumetric shrinkage also occurs during polymerization due to the reduction of intermolecular separations in the monomers [12]. This is a very quick process which normally takes place within the first 30-50 seconds. The volumetric contraction that accompanies polymerization

For most dental composite restorations, a bonding agent (adhesive) is used to create a strong bond at the tooth-composite interface. Thus, during the polymerization process, because the composite is constrained along the interfaces, shrinkage stresses are built up within the composite and tooth structure. These shrinkage stresses are difficult to predict or measure because of the small but complicated tooth geometry and rapidly changing material properties. Some experimental methods using simple specimens have been developed to estimate the polymerization shrinkage stress that develops in dental composites [1, 13, 14]. However, the measured shrinkage stresses are sensitive to the test configuration and procedures, e.g. the instrument's compliance, direction of curing light application and specimen shape. Also, because the material parameters (shrinkage strain, viscosity, Young's modulus and Poisson's ratio) of the composite that control the development of shrinkage stresses all change rapidly with time during polymerization, even with a suitable mathematical model, the accurate prediction of the shrinkage stresses is not trivial [15, 16].

If the shrinkage stress within a composite restoration is high enough, interfacial debonding or, more generally, failure between the tooth and restoration will occur. Interfacial failure can be adhesive, in which case failure occurs right in the adhesive layer; or it can be cohesive, in which case failure occurs in the tooth or composite materials near the interface. The acoustic emission (AE) measurement technique is also a non-destructive method. It is normally used to monitor the integrity of structures by providing real-time information of the fracture or damage process. It uses transducers or sensors to detect the high-frequency sound waves produced as a result of the sudden strain energy released within a material following fracture. Figure 3 shows schematically how an AE test system works. First, the cracking event within a material is captured by the AE sensor attached on the surface of the component. The raw signal is then passed through a preamplifier for pre-amplification and then to the acquisition system for acquisition and storage. Finally, the AE data can be displayed and analyzed by specially designed software.

AE technology has been widely used in research and industry to monitor the development of crack growth, wear, fiber-matrix debonding in composites, phase transformation, etc [20- 22]. It has also been used to detect the fracture of different dental structures. For example, Ereifej et al. [23] used the AE technique to detect the initial fracture of ceramic crowns; Vallittu [24] used it to study the fracture of a composite veneer reinforced by woven glass fibres; and Kim and Okuno [25] used it to study the micro-fracture behavior of composite resins containing irregular-shaped fillers.

The interfacial debonding in composite restorations, either adhesive or cohesive, is actually a kind of fracture within the restored tooth structure. Therefore, it is possible to use the AE

technique to capture the cracking events. Unlike some of the imaging methods mentioned above, the AE method does not need destructive sample preparation and is not affected by the curing light. Therefore, it can be used to measure interfacial debonding in real time during the curing process, which cannot be achieved by micro-CT despite it being a nondestructive method as well.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 153

**Figure 4.** Samples: (a) pea-size composite specimen directly attached to the AE sensor, (b) rings

The four ring specimens in Group b were cut from the root of a single bovine tooth. They were, therefore, similar in material properties and structural anatomy; see Figure 4(b). The central holes of the specimens were originally the root canal, which was enlarged to 3mm in diameter with a high-speed handpiece. Compared with the whole tooth specimens in Group c, the composites in the ring specimens had larger free surface areas. This allowed the effect of the so-called C-factor (ratio between the bonded and unbonded surfaces) on interfacial

For Group c, 4 intact human molars with similar dimensions were selected; they had been extracted and stored in saturated thymol solution at 4C for less than one month. Standard Class-I cavities were prepared on these teeth by a single operator following clinical procedures with a high-speed handpiece and dental cutting burs; see Figure 4(c). The whole tooth specimens were considered to be the most representative of those in real clinical

Each of the ring and tooth specimens was first treated with a bonding agent (Adper™ Scotchbond™ SE Self-Etch, 3M ESPE, US) to the cavity surface and then restored with the composite resin Z100TM (3M ESPE, US). Again, the composite was cured with a blue light

To explain the different levels of interfacial debonding measured by the AE method for different composite materials, the development of shrinkage stress for the composite materials during curing were measured using a tensometer (American Dental Association Foundation) [27]. This device is based on the basic engineering cantilever beam bending theory. The tensile force generated by the shrinking composite was calculated from the beam deflection using a previously obtained calibration constant. Further details of the

prepared from the root of a bovine tooth and (c) human teeth with Class-I cavities

debonding to be investigated.

(Elipar TriLight, 3M ESPE, US) at 550mW/cm2 for 40s.

*3.2.2. Shrinkage stress measurement* 

situations.

**Figure 3.** Schematic diagram of the AE test system

## **3.2. Application of AE technique to debonding measurement [26]**

When using the AE technique to study interfacial debonding of dental restorations, it is necessary to prove that the captured AE signals indeed represent the cracking/microcracking events caused by interfacial debonding during curing of the composite. To this end, three groups of tests with different boundary constraints were designed and conducted, as described in this sub-section.

## *3.2.1. Specimens and materials*

Figure 4 shows the specimens used in the three groups of tests: (a) free standing pea-size specimens of composite placed directly on the AE sensor, (b) ring specimens prepared from the root of a single bovine tooth and (c) intact human molars with a Class-I restoration. The composites in Group a were constrained least while those in Group c were constrained most. Each group had 4 specimens and Z100TM (3M ESPE) was the composite material used. More details of the specimens are given below.

The free-standing pea-size specimens of Group a were about 5mm in diameter. They were directly placed on the AE sensor without using any adhesive material, as shown in Figure 4(a). Each of these specimens was cured with a blue light (Elipar TriLight, 3M ESPE, US) at an intensity of 550mW/cm2 for 40s. Testing with these specimens helped to verify that free shrinkage of the composite resin itself did not induce any AE event.

#### Non-Destructive Examination of Interfacial Debonding in Dental Composite Restorations Using Acoustic Emission 153

**Figure 4.** Samples: (a) pea-size composite specimen directly attached to the AE sensor, (b) rings prepared from the root of a bovine tooth and (c) human teeth with Class-I cavities

The four ring specimens in Group b were cut from the root of a single bovine tooth. They were, therefore, similar in material properties and structural anatomy; see Figure 4(b). The central holes of the specimens were originally the root canal, which was enlarged to 3mm in diameter with a high-speed handpiece. Compared with the whole tooth specimens in Group c, the composites in the ring specimens had larger free surface areas. This allowed the effect of the so-called C-factor (ratio between the bonded and unbonded surfaces) on interfacial debonding to be investigated.

For Group c, 4 intact human molars with similar dimensions were selected; they had been extracted and stored in saturated thymol solution at 4C for less than one month. Standard Class-I cavities were prepared on these teeth by a single operator following clinical procedures with a high-speed handpiece and dental cutting burs; see Figure 4(c). The whole tooth specimens were considered to be the most representative of those in real clinical situations.

Each of the ring and tooth specimens was first treated with a bonding agent (Adper™ Scotchbond™ SE Self-Etch, 3M ESPE, US) to the cavity surface and then restored with the composite resin Z100TM (3M ESPE, US). Again, the composite was cured with a blue light (Elipar TriLight, 3M ESPE, US) at 550mW/cm2 for 40s.

#### *3.2.2. Shrinkage stress measurement*

152 Composites and Their Applications

nondestructive method as well.

**Figure 3.** Schematic diagram of the AE test system

conducted, as described in this sub-section.

More details of the specimens are given below.

shrinkage of the composite resin itself did not induce any AE event.

*3.2.1. Specimens and materials* 

**3.2. Application of AE technique to debonding measurement [26]** 

When using the AE technique to study interfacial debonding of dental restorations, it is necessary to prove that the captured AE signals indeed represent the cracking/microcracking events caused by interfacial debonding during curing of the composite. To this end, three groups of tests with different boundary constraints were designed and

Figure 4 shows the specimens used in the three groups of tests: (a) free standing pea-size specimens of composite placed directly on the AE sensor, (b) ring specimens prepared from the root of a single bovine tooth and (c) intact human molars with a Class-I restoration. The composites in Group a were constrained least while those in Group c were constrained most. Each group had 4 specimens and Z100TM (3M ESPE) was the composite material used.

The free-standing pea-size specimens of Group a were about 5mm in diameter. They were directly placed on the AE sensor without using any adhesive material, as shown in Figure 4(a). Each of these specimens was cured with a blue light (Elipar TriLight, 3M ESPE, US) at an intensity of 550mW/cm2 for 40s. Testing with these specimens helped to verify that free

technique to capture the cracking events. Unlike some of the imaging methods mentioned above, the AE method does not need destructive sample preparation and is not affected by the curing light. Therefore, it can be used to measure interfacial debonding in real time during the curing process, which cannot be achieved by micro-CT despite it being a

> To explain the different levels of interfacial debonding measured by the AE method for different composite materials, the development of shrinkage stress for the composite materials during curing were measured using a tensometer (American Dental Association Foundation) [27]. This device is based on the basic engineering cantilever beam bending theory. The tensile force generated by the shrinking composite was calculated from the beam deflection using a previously obtained calibration constant. Further details of the

shrinkage stress measuring method using the tensometer are given in Ref. [27]. Before placing the composite material between the two glass-rod holders at the free end of the cantilever beam, the end surfaces of the glass rods were first polished with 600-grit sandpaper, silanized with a porcelain primer (Bisco Inc., Schaumburg, IL, USA), and then applied with a layer of adhesive (Scotchbond Multi-purpose, 3M, St. Paul, MN, USA). The dimensions of the composite specimens were 6mm in diameter and 2mm in height. The temporal developments of shrinkage stress for Z100TM (3M ESPE, US) and Filtek P90 (3M, St. Paul, MN, USA) are plotted in Figure 5. As can be seen, the shrinkage stresses developed rapidly and reached their maximum values within the first 50s, with Z100 producing a much higher shrinkage stress than Filtek P90.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 155

instantaneous and accumulated AE events against time to study the curing behavior of the different specimens. During the AE tests, the teeth were wrapped by wet paper tissue to

**Figure 6.** Procedure of an AE test on composite debonding: (a) sample preparation; (b) placement of adhesive, restoration, and AE sensor; (c) AE recording (10mins) & curing of composite resin (40s); (d)

Figures 7 and 8 show the cumulative numbers of AE events against time for the ring specimens (Group b) and the human tooth specimens (Group c), respectively. The temporal development of the shrinkage stress of Z100, as shown in Figure 5, was also plotted for comparison. It was found that the temporal developments of the AE events followed roughly those of the shrinkage stress. It can also be seen that while some AE events occurred during the initial rapid polymerization of the composite resin, there were still some late events which took place a few minutes after the completion of light curing. In order to compare these two groups, the average temporal AE developments for Group b and Group c are plotted together in Figure 9, with the standard deviation of the cumulative number of AE events of each group being shown as red bars. No AE events were detected for the freestanding composite blob specimens (Group a) placed directly onto the AE sensor, illustrating that the free shrinkage of composite does not produce any AE event during curing. The mean and standard deviation of the total number of AE events for Group a, Group b and Group c were 0(-), 3.7(2.1) and 9.0(1.6), respectively, as summarized in Table 1.

The integrity of the tooth-restoration interface within the real tooth specimens (Group c) was examined further using a micro-CT machine (XT H 225, X-TEK Systems LTD). Two specimens were selected and they were first scanned immediately after placement of the composites to see how well they had adapted to the cavity walls. After curing and AE measurement, they were scanned for the second time to look for any detachment of the restoration from the cavity walls. In order to ensure the same position and orientation for the two scans to facilitate "same-slice" comparison, each of the specimens was mounted into a Teflon ring with positioning pins using an orthodontic resin (DENTSPLY International Inc., US). During scanning, the teeth were covered by wet paper tissue to avoid cracking

avoid cracking through dehydration.

*3.2.4. Debonding evaluation using Micro-CT* 

through dehydration.

AE data analysis

**Figure 5.** Shrinkage stress against time for Z100 and P90

#### *3.2.3. AE measurement*

A 2-channel AE system (PCI-2, Physical Acoustic Corporation, USA) was used in this study for AE data acquisition and digital signal processing. The AE sensor/transducer used for detecting interfacial debonding was S9225 (Physical Acoustic Corporation, USA), which had a resonance frequency of 250kHz. For the whole tooth and ring specimens, the AE sensor was attached to their outer surfaces using cyanoacrylate adhesive (Super Bond, Staples Inc, USA). The signals acquired with the sensor were amplified by a preamplifier with 20/40/60 dB gains. The parameters selected for the signal acquisition were: a 40dB gain for the preamplifier, a 100kHz-2MHz band pass and a 32dB threshold. These parameters were selected through many trial tests, with the aim of maximizing the system sensitivity while minimizing the background noises.

Figure 6 shows the procedures of a typical AE test on composite restoration debonding: (1) Prepare the cavity on a tooth sample; (2) Apply adhesive (bond agent) to cavity walls, fill the cavity with composite resin, and attach the AE sensor onto the tooth surface; (3) Turn on the AE system and blue curing light simultaneously. Cure the composite resin for 40s and record AE data continuously for 10 minutes. (4) Analyze the AE data. Use the curves of instantaneous and accumulated AE events against time to study the curing behavior of the different specimens. During the AE tests, the teeth were wrapped by wet paper tissue to avoid cracking through dehydration.

**Figure 6.** Procedure of an AE test on composite debonding: (a) sample preparation; (b) placement of adhesive, restoration, and AE sensor; (c) AE recording (10mins) & curing of composite resin (40s); (d) AE data analysis

Figures 7 and 8 show the cumulative numbers of AE events against time for the ring specimens (Group b) and the human tooth specimens (Group c), respectively. The temporal development of the shrinkage stress of Z100, as shown in Figure 5, was also plotted for comparison. It was found that the temporal developments of the AE events followed roughly those of the shrinkage stress. It can also be seen that while some AE events occurred during the initial rapid polymerization of the composite resin, there were still some late events which took place a few minutes after the completion of light curing. In order to compare these two groups, the average temporal AE developments for Group b and Group c are plotted together in Figure 9, with the standard deviation of the cumulative number of AE events of each group being shown as red bars. No AE events were detected for the freestanding composite blob specimens (Group a) placed directly onto the AE sensor, illustrating that the free shrinkage of composite does not produce any AE event during curing. The mean and standard deviation of the total number of AE events for Group a, Group b and Group c were 0(-), 3.7(2.1) and 9.0(1.6), respectively, as summarized in Table 1.

#### *3.2.4. Debonding evaluation using Micro-CT*

154 Composites and Their Applications

much higher shrinkage stress than Filtek P90.

**Figure 5.** Shrinkage stress against time for Z100 and P90

*3.2.3. AE measurement* 

minimizing the background noises.

shrinkage stress measuring method using the tensometer are given in Ref. [27]. Before placing the composite material between the two glass-rod holders at the free end of the cantilever beam, the end surfaces of the glass rods were first polished with 600-grit sandpaper, silanized with a porcelain primer (Bisco Inc., Schaumburg, IL, USA), and then applied with a layer of adhesive (Scotchbond Multi-purpose, 3M, St. Paul, MN, USA). The dimensions of the composite specimens were 6mm in diameter and 2mm in height. The temporal developments of shrinkage stress for Z100TM (3M ESPE, US) and Filtek P90 (3M, St. Paul, MN, USA) are plotted in Figure 5. As can be seen, the shrinkage stresses developed rapidly and reached their maximum values within the first 50s, with Z100 producing a

A 2-channel AE system (PCI-2, Physical Acoustic Corporation, USA) was used in this study for AE data acquisition and digital signal processing. The AE sensor/transducer used for detecting interfacial debonding was S9225 (Physical Acoustic Corporation, USA), which had a resonance frequency of 250kHz. For the whole tooth and ring specimens, the AE sensor was attached to their outer surfaces using cyanoacrylate adhesive (Super Bond, Staples Inc, USA). The signals acquired with the sensor were amplified by a preamplifier with 20/40/60 dB gains. The parameters selected for the signal acquisition were: a 40dB gain for the preamplifier, a 100kHz-2MHz band pass and a 32dB threshold. These parameters were selected through many trial tests, with the aim of maximizing the system sensitivity while

Figure 6 shows the procedures of a typical AE test on composite restoration debonding: (1) Prepare the cavity on a tooth sample; (2) Apply adhesive (bond agent) to cavity walls, fill the cavity with composite resin, and attach the AE sensor onto the tooth surface; (3) Turn on the AE system and blue curing light simultaneously. Cure the composite resin for 40s and record AE data continuously for 10 minutes. (4) Analyze the AE data. Use the curves of The integrity of the tooth-restoration interface within the real tooth specimens (Group c) was examined further using a micro-CT machine (XT H 225, X-TEK Systems LTD). Two specimens were selected and they were first scanned immediately after placement of the composites to see how well they had adapted to the cavity walls. After curing and AE measurement, they were scanned for the second time to look for any detachment of the restoration from the cavity walls. In order to ensure the same position and orientation for the two scans to facilitate "same-slice" comparison, each of the specimens was mounted into a Teflon ring with positioning pins using an orthodontic resin (DENTSPLY International Inc., US). During scanning, the teeth were covered by wet paper tissue to avoid cracking through dehydration.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 157

Group a Group b Group c

Mean 0 3.7 9.0 STD - 2.1 1.6

The cross-sectional images obtained from the 3D Micro-CT reconstructions of one of the human tooth specimens restored with Z100 are shown in Figure 1. The micro-CT images clearly show the internal structures of the three main components of the restored teeth: enamel, dentin and composite. Before curing, as shown in Figures 1a and 1c, the composite can be seen to be perfectly in contact with the surrounding tooth tissues. After curing, however, the specimen showed clear interfacial debonding along the side walls and at the

To ensure that any lack of AE events/debonding was not due to uncured composite, the solidification of the composite within the tooth samples (Group c) after curing was assessed by using mico-hardness tests. After curing and AE measurement, one of the specimens was cut along a vertical central plane with a 102mm Dia. x 0.3mm thick diamond blade (Buehler, USA). The Vickers hardness was then measured with a micro-hardness testing machine (Micromet 5104, Buehler, USA). The indenter load was 100g and the load-holding time was 10s. The measurement points were located along 4 vertical lines within the restoration, which were schematically shown in Figure 1c. The measurement began from the top surface

Figure 10 shows the Vickers hardness of the cured composite (Z100) along its depth. It shows very uniform hardness distributions within the composite restoration, indicating that

**Table 1.** Total number of AE events: mean and standard deviation (STD)

bottom of the cavity.

*3.2.5. Micro-hardness measurement* 

and continued until the bottom interface was reached.

the polymerization of the composite resin was uniform and complete.

**Figure 10.** Vickers hardness along the depth of a Z100 restoration (see Figure 1c)

**Figure 7.** AE results for the ring specimens (Group-b)

**Figure 8.** AE results for the real tooth specimens (Group-c)

**Figure 9.** Average cumulative number of AE events for specimens in Groups b and c, with the standard deviation being plotted with red bars


**Table 1.** Total number of AE events: mean and standard deviation (STD)

The cross-sectional images obtained from the 3D Micro-CT reconstructions of one of the human tooth specimens restored with Z100 are shown in Figure 1. The micro-CT images clearly show the internal structures of the three main components of the restored teeth: enamel, dentin and composite. Before curing, as shown in Figures 1a and 1c, the composite can be seen to be perfectly in contact with the surrounding tooth tissues. After curing, however, the specimen showed clear interfacial debonding along the side walls and at the bottom of the cavity.

### *3.2.5. Micro-hardness measurement*

156 Composites and Their Applications

**Figure 7.** AE results for the ring specimens (Group-b)

**Figure 8.** AE results for the real tooth specimens (Group-c)

deviation being plotted with red bars

**Figure 9.** Average cumulative number of AE events for specimens in Groups b and c, with the standard

To ensure that any lack of AE events/debonding was not due to uncured composite, the solidification of the composite within the tooth samples (Group c) after curing was assessed by using mico-hardness tests. After curing and AE measurement, one of the specimens was cut along a vertical central plane with a 102mm Dia. x 0.3mm thick diamond blade (Buehler, USA). The Vickers hardness was then measured with a micro-hardness testing machine (Micromet 5104, Buehler, USA). The indenter load was 100g and the load-holding time was 10s. The measurement points were located along 4 vertical lines within the restoration, which were schematically shown in Figure 1c. The measurement began from the top surface and continued until the bottom interface was reached.

Figure 10 shows the Vickers hardness of the cured composite (Z100) along its depth. It shows very uniform hardness distributions within the composite restoration, indicating that the polymerization of the composite resin was uniform and complete.

**Figure 10.** Vickers hardness along the depth of a Z100 restoration (see Figure 1c)

## *3.2.6. Discussion on the AE verification tests*

From the AE results shown in Figures 7 and 8, it can be seen that the temporal developments of the AE events followed roughly those of the shrinkage stress. Also, polymerisation of the composite itself did not create any detectable AE events, as demonstrated by the negative results of the freestanding pea-size specimens placed directly on the sensor. All these indicate strongly that the cracking leading to the AE events in the restored samples were caused by the shrinkage stress produced by the polymerization of the composite resin.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 159

The interfacial debonding in composite restorations is mainly caused by the shrinkage stress produced during the polymerization of the composite. The material properties of the composite, i.e. the volume shrinkage, Young's modulus and strength, etc., will affect the internal shrinkage stress level and, thus, the degree of interfacial debonding. To investigate the influence of the composite resin on interfacial debonding, two commercial composite materials: Z100TM (3M ESPE, US) and FiltekTM P90 (3M ESPE, US) were chosen to conduct further AE debonding tests. The shrinkage stress curves for the two materials obtained using a tensometer (see Session 3.2.2) are shown in Figure 5. It can be seen that P90 produces a much lower shrinkage stress than Z100. This is because P90 is a low-shrinkage composite with special molecular structures that can open up to counter the shrinkage during polymerization. 8 intact human molars with similar dimensions were selected and randomly divided into 2 groups of 4. The specimen preparation and testing procedure and test equipments are the same as that in Section 3.2. The bonding agent used for all the specimens was Adper™

Figure 11 shows the average temporal development of AE events during curing for the two groups, with the standard deviation being plotted with red bars. Just as it produced a lower shrinkage stress, Filtek P90 also produced a lower number of AE events than Z100, indicating that there was less interfacial debonding/cracking in the P90 specimens. In fact,

**Figure 11.** Average cumulative number of AE events for specimens restored with Z100 and P90 (the red

The above conclusion is supported by comparing the micro-CT images of the two specimens, as shown in Figures 1 and 2. After curing, the specimen restored with Z100 can be seen to have clear interfacial debonding along the walls and at the bottom of the cavity (Figures 1b and 1d), while that restored with P90 did not show any obvious debonding

two of the P90 specimens did not produce any detectable AE events at all.

**4.1. Influence of the composite material** 

Scotchbond™ SE Self-Etch (3M ESPE, US).

bars show the standard deviation)

(Figures 2b and 2d).

Possible sources of AE events include debonding at the tooth-restoration interface and cohesive cracking in the tooth tissues or composite resin under the action of tensile shrinkage stresses. The only tensile stress in the ring specimens was the radial stress, which was maximum at the tooth-restoration interface. Therefore, cracking, either cohesive or adhesive, at the interface was the most possible reason causing those AE events in the ring specimens. The stress distributions within the restored tooth specimens were more complicated, due to the complex geometry involved. During shrinkage of the composite restoration, tensile stresses could also be induced in the axial direction, which would be maximum on the outer tooth surfaces. However, the micro-CT images of the tooth specimen, shown in Figure 1, confirm again that the AE events recorded were probably induced by adhesive debonding or cohesive cracking at the tooth-restoration interface.

Figure 9 shows that there were more AE events in the whole tooth specimens than there were in the ring specimens. This could be attributed to the larger restoration volume, and thus larger volumetric shrinkage, and/or the higher ratio of bonded-to-nonbonded surface area in the whole tooth specimens, i.e. the so-called C-factor. Also, the compliances of the two samples were different, with the ring specimens having a lower stiffness and thus a lower shrinkage stress, which led to fewer AE events. The more complicated geometry of the Class-I restorations also meant that local stress concentrations were more likely to exist in the tooth specimens which would lead to more debonding or AE events.

These results verified that the non-destructive AE measurement technique is an effective tool to detect and monitor *in-situ* the interfacial debonding of composite restorations during curing. It will allow quantitative studies to be carried out of the effects of factors such as composite material properties, cavity geometries, and restorative techniques on the integrity of the tooth-restoration interface.

## **4. Factors that affect interfacial debonding**

There are many factors that can affect the initial quality and subsequent degradation of bonding at the tooth-restoration interface, e.g. properties of the composite and adhesive materials, cavity geometry, layering techniques, thermal and mechanical loading, etc. [5-8]. In this section, the influences of three factors on interfacial debonding will be studied using the AE technique described above. The three factors studied include: the composite material, the cavity configuration, and the filling technique.

### **4.1. Influence of the composite material**

158 Composites and Their Applications

composite resin.

of the tooth-restoration interface.

**4. Factors that affect interfacial debonding** 

the cavity configuration, and the filling technique.

*3.2.6. Discussion on the AE verification tests* 

From the AE results shown in Figures 7 and 8, it can be seen that the temporal developments of the AE events followed roughly those of the shrinkage stress. Also, polymerisation of the composite itself did not create any detectable AE events, as demonstrated by the negative results of the freestanding pea-size specimens placed directly on the sensor. All these indicate strongly that the cracking leading to the AE events in the restored samples were caused by the shrinkage stress produced by the polymerization of the

Possible sources of AE events include debonding at the tooth-restoration interface and cohesive cracking in the tooth tissues or composite resin under the action of tensile shrinkage stresses. The only tensile stress in the ring specimens was the radial stress, which was maximum at the tooth-restoration interface. Therefore, cracking, either cohesive or adhesive, at the interface was the most possible reason causing those AE events in the ring specimens. The stress distributions within the restored tooth specimens were more complicated, due to the complex geometry involved. During shrinkage of the composite restoration, tensile stresses could also be induced in the axial direction, which would be maximum on the outer tooth surfaces. However, the micro-CT images of the tooth specimen, shown in Figure 1, confirm again that the AE events recorded were probably induced by adhesive debonding or cohesive cracking at the tooth-restoration interface.

Figure 9 shows that there were more AE events in the whole tooth specimens than there were in the ring specimens. This could be attributed to the larger restoration volume, and thus larger volumetric shrinkage, and/or the higher ratio of bonded-to-nonbonded surface area in the whole tooth specimens, i.e. the so-called C-factor. Also, the compliances of the two samples were different, with the ring specimens having a lower stiffness and thus a lower shrinkage stress, which led to fewer AE events. The more complicated geometry of the Class-I restorations also meant that local stress concentrations were more likely to exist

These results verified that the non-destructive AE measurement technique is an effective tool to detect and monitor *in-situ* the interfacial debonding of composite restorations during curing. It will allow quantitative studies to be carried out of the effects of factors such as composite material properties, cavity geometries, and restorative techniques on the integrity

There are many factors that can affect the initial quality and subsequent degradation of bonding at the tooth-restoration interface, e.g. properties of the composite and adhesive materials, cavity geometry, layering techniques, thermal and mechanical loading, etc. [5-8]. In this section, the influences of three factors on interfacial debonding will be studied using the AE technique described above. The three factors studied include: the composite material,

in the tooth specimens which would lead to more debonding or AE events.

The interfacial debonding in composite restorations is mainly caused by the shrinkage stress produced during the polymerization of the composite. The material properties of the composite, i.e. the volume shrinkage, Young's modulus and strength, etc., will affect the internal shrinkage stress level and, thus, the degree of interfacial debonding. To investigate the influence of the composite resin on interfacial debonding, two commercial composite materials: Z100TM (3M ESPE, US) and FiltekTM P90 (3M ESPE, US) were chosen to conduct further AE debonding tests. The shrinkage stress curves for the two materials obtained using a tensometer (see Session 3.2.2) are shown in Figure 5. It can be seen that P90 produces a much lower shrinkage stress than Z100. This is because P90 is a low-shrinkage composite with special molecular structures that can open up to counter the shrinkage during polymerization.

8 intact human molars with similar dimensions were selected and randomly divided into 2 groups of 4. The specimen preparation and testing procedure and test equipments are the same as that in Section 3.2. The bonding agent used for all the specimens was Adper™ Scotchbond™ SE Self-Etch (3M ESPE, US).

Figure 11 shows the average temporal development of AE events during curing for the two groups, with the standard deviation being plotted with red bars. Just as it produced a lower shrinkage stress, Filtek P90 also produced a lower number of AE events than Z100, indicating that there was less interfacial debonding/cracking in the P90 specimens. In fact, two of the P90 specimens did not produce any detectable AE events at all.

**Figure 11.** Average cumulative number of AE events for specimens restored with Z100 and P90 (the red bars show the standard deviation)

The above conclusion is supported by comparing the micro-CT images of the two specimens, as shown in Figures 1 and 2. After curing, the specimen restored with Z100 can be seen to have clear interfacial debonding along the walls and at the bottom of the cavity (Figures 1b and 1d), while that restored with P90 did not show any obvious debonding (Figures 2b and 2d).

Again, to discard the possibility that the low number of AE events in the P90 specimens was caused by incomplete polymerization, mico-hardness tests were also done on one of the specimens to assess the solidification of the composites. Figure 12 shows the Vickers hardness along the depth of the restoration for both Z100 and P90 specimens. It can be seen that, while Z100 was harder than P90, restorations of both materials had very uniform hardness distributions within them, indicating that the polymerization of all specimens were uniform and complete.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 161

C-factor Bond area (mm2)

Volume (mm3)

**Figure 13.** Schematic diagrams of the prepared tooth cavities with dimensions of interest. The shaded

Class I 1 3.94 2.99 2.08 3.45 40.62 24.50

Mean (STD) 3.96(0.13) 3.63(0.39) 2.24(0.35) 3.37(0.34) 48.44(7.78) 30.68(7.72) Class I 2 2.66 2.77 1.40 3.06 22.57 10.32

Mean (STD) 3.03(0.40) 2.82(0.14) 1.36(0.30) 2.88(0.40) 24.45(4.83) 11.66(3.65) Class II 3 2.09 1.85 1.68 2.01 14.00 6.50

Mean (STD) 2.25(0.14) 1.94(0.09) 1.71(0.21) 2.00(0.09) 15.39(1.75) 7.48(1.19) Class II 4 3.75 3.34 2.47 1.89 39.30 30.94

Mean (STD) 4.12(0.42) 3.62(0.50) 2.15(0.47) 1.79(0.18) 40.48(6.57) 31.93(7.26) **Table 2.** Dimensions and geometrical factors of the specimens used in the experimental study on C-

Depth (mm)

3.76 3.68 1.88 3.02 41.81 26.01

4.13 4.06 2.48 3.42 57.39 41.58 4.00 3.79 2.02 3.08 46.63 30.62 3.99 3.61 2.72 3.87 55.75 39.18

3.29 3.04 1.80 3.28 32.79 18.00 2.86 2.87 1.05 2.47 20.24 8.62 3.59 2.70 1.10 2.43 23.53 10.66 2.73 2.70 1.45 3.14 23.12 10.69

2.35 1.92 1.42 1.92 13.91 6.41 2.30 1.91 1.73 2.03 15.66 7.60 2.39 1.94 2.02 2.13 18.21 9.37 2.11 2.10 1.70 1.90 15.18 7.53

4.18 3.39 2.72 1.97 46.13 38.54 4.49 3.36 2.19 1.88 42.11 33.04 3.63 3.51 1.58 1.63 29.76 20.13 4.57 4.50 1.80 1.57 45.12 37.02

Width (mm)

areas are the free, unbonded surface areas

(mm)

Cavity type Group Length

factor

**Figure 12.** Vickers hardness along the depth of restoration (see Figure 1c) for specimens restored with P90. The results of Z100 specimens were plotted for comparison.

#### **4.2. Influence of cavity configuration (C- factor) [28]**

The C-factor of a restoration is defined as the ratio of the bonded areas to the unbonded areas, and it is normally used to characterize the configuration of the cavity [29]. Experimental studies [30-32] have shown that increasing the C-factor would reduce the interfacial bond strength and increase the incidence of interfacial failure. That was attributed to the higher shrinkage stress in restorations with a higher C-factor which had caused more interfacial debonding. In this section, three cavity configurations with different C-factors were used and their interfacial debonding was evaluated with the AE technique.

20 intact human molars with similar dimensions were selected and randomly divided into 4 groups of 5: Group 1 with large Class-I cavities, Group 2 with small Class-I cavities, Group 3 with small Class-II cavities and Group 4 with large Class-II cavities. The specimen preparation, testing procedure and test equipments were similar to those in Section 3.2 and Section 4.1. The bonding agent used for all the specimens was total-etch adhesive Adper™ Single Bond Plus (3M ESPE, USA) and the composite resin was Z100TM (3M ESPE, US). Figure 13 shows schematically the Class-I and Class-II cavities prepared and the dimensions of interest. The dimensions of each specimen were measured with a micrometer (Mitutoyo, Japan) with which the C-factor was calculated. The average C-factors of the four groups were 3.37, 2.90, 2.00 and 1.79, respectively, as summarized in Table 2.

#### Non-Destructive Examination of Interfacial Debonding in Dental Composite Restorations Using Acoustic Emission 161

160 Composites and Their Applications

were uniform and complete.

Again, to discard the possibility that the low number of AE events in the P90 specimens was caused by incomplete polymerization, mico-hardness tests were also done on one of the specimens to assess the solidification of the composites. Figure 12 shows the Vickers hardness along the depth of the restoration for both Z100 and P90 specimens. It can be seen that, while Z100 was harder than P90, restorations of both materials had very uniform hardness distributions within them, indicating that the polymerization of all specimens

**Figure 12.** Vickers hardness along the depth of restoration (see Figure 1c) for specimens restored with

The C-factor of a restoration is defined as the ratio of the bonded areas to the unbonded areas, and it is normally used to characterize the configuration of the cavity [29]. Experimental studies [30-32] have shown that increasing the C-factor would reduce the interfacial bond strength and increase the incidence of interfacial failure. That was attributed to the higher shrinkage stress in restorations with a higher C-factor which had caused more interfacial debonding. In this section, three cavity configurations with different C-factors

20 intact human molars with similar dimensions were selected and randomly divided into 4 groups of 5: Group 1 with large Class-I cavities, Group 2 with small Class-I cavities, Group 3 with small Class-II cavities and Group 4 with large Class-II cavities. The specimen preparation, testing procedure and test equipments were similar to those in Section 3.2 and Section 4.1. The bonding agent used for all the specimens was total-etch adhesive Adper™ Single Bond Plus (3M ESPE, USA) and the composite resin was Z100TM (3M ESPE, US). Figure 13 shows schematically the Class-I and Class-II cavities prepared and the dimensions of interest. The dimensions of each specimen were measured with a micrometer (Mitutoyo, Japan) with which the C-factor was calculated. The average C-factors of the four groups

were used and their interfacial debonding was evaluated with the AE technique.

were 3.37, 2.90, 2.00 and 1.79, respectively, as summarized in Table 2.

P90. The results of Z100 specimens were plotted for comparison.

**4.2. Influence of cavity configuration (C- factor) [28]** 

**Figure 13.** Schematic diagrams of the prepared tooth cavities with dimensions of interest. The shaded areas are the free, unbonded surface areas


**Table 2.** Dimensions and geometrical factors of the specimens used in the experimental study on Cfactor

Figure 14 shows the mean cumulative number of AE events against time for the four test groups, with the standard deviations shown as red bars. AE caused by interfacial debonding was first detected about 20s into the curing of the composite and developed rapidly thereafter. The mean and standard deviation of the total number of AE events for the four groups were 29.6±15.7, 10.0±5.8, 2.6±1.5, and 2.2±1.3 (Table 3), respectively, which showed an increase with an increasing C-factor. Table 4 shows the statistical significance (*p*-value) of the differences in the total number of AE events between the different groups. It can be seen that the differences were significant, except that between Groups 3 and 4 which had very similar C-factors.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 163

**Figure 15.** The total number of AE events as a function of the C-factor: (a) total number, (b) total number per unit bond area (/mm2), and (c) total number per unit composite volume (/mm3)

In Figure 15(a), the total number of AE events for all the specimens were plotted against their individual C-factors. To account for the differences in the bonded area or volume of restorations with similar C-factors, the total number of AE events per unit bonded area and that per unit composite volume are plotted in Figure 15(b) and 15(c), respectively, for comparison. The mean and standard deviation of these normalized numbers for the 4 groups are also listed in Table 3. Despite the large variations in the number of AE events detected among specimens with similar C-factors, it can be clearly seen that the amount of debonding increased with an increase in the C-factor. There appeared to be a critical value for the C-factor associated with Z-100, i.e. 1.5, below which no AE events could be detected. The trend remained the same even when possible influence from the bonded area and restoration volume was taken into account.

**Figure 14.** The cumulative number of AE events against time for the 4 test groups

similar C-factors.

restoration volume was taken into account.

**Figure 14.** The cumulative number of AE events against time for the 4 test groups

Figure 14 shows the mean cumulative number of AE events against time for the four test groups, with the standard deviations shown as red bars. AE caused by interfacial debonding was first detected about 20s into the curing of the composite and developed rapidly thereafter. The mean and standard deviation of the total number of AE events for the four groups were 29.6±15.7, 10.0±5.8, 2.6±1.5, and 2.2±1.3 (Table 3), respectively, which showed an increase with an increasing C-factor. Table 4 shows the statistical significance (*p*-value) of the differences in the total number of AE events between the different groups. It can be seen that the differences were significant, except that between Groups 3 and 4 which had very

In Figure 15(a), the total number of AE events for all the specimens were plotted against their individual C-factors. To account for the differences in the bonded area or volume of restorations with similar C-factors, the total number of AE events per unit bonded area and that per unit composite volume are plotted in Figure 15(b) and 15(c), respectively, for comparison. The mean and standard deviation of these normalized numbers for the 4 groups are also listed in Table 3. Despite the large variations in the number of AE events detected among specimens with similar C-factors, it can be clearly seen that the amount of debonding increased with an increase in the C-factor. There appeared to be a critical value for the C-factor associated with Z-100, i.e. 1.5, below which no AE events could be detected. The trend remained the same even when possible influence from the bonded area and

**Figure 15.** The total number of AE events as a function of the C-factor: (a) total number, (b) total number per unit bond area (/mm2), and (c) total number per unit composite volume (/mm3)


Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 165

**Figure 16.** (a) Cylindrical cavity on a bovine tooth, and (b) the two different filling techniques

**Figure 17.** The total number of AE events for specimens restored with the bulk and incremental

The AE technique, which is widely used for fracture monitoring in many areas, was for the first time introduced here to monitor *in-situ* the interfacial deboning of composite restorations. Its effectiveness was verified by several groups of experiments using specimens with different boundary constraints: freestanding blobs of composite, ring specimens cut from a tooth root and whole human molars. It was also shown from the AE results that interfacial debonding in composite restorations is greatly influenced by the restorative composite material, cavity configuration and filling technique. In general, composites with

**5. Discussion and concluding remarks** 

techniques

**Table 3.** Results from the AE tests: mean (standard deviation)


**Table 4.** Statistical significance (*p*-value) of the difference in the total number of AE events between the groups with different C-factors

### **4.3. Influence of the filling technique**

The composite placement techniques (bulk vs. incremental) have been shown to affect the magnitude of the polymerization shrinkage stresses and cuspal deformations in restored teeth [33-37]. Compared with the bulk filling method, the incremental filling method has been reported to produce lower shrinkage stresses. Therefore, the incremental filling method is expected to produce less interfacial debonding than the bulk filling method. This hypothesis will be tested in this section with the AE technique.

12 incisor bovine teeth with similar geometries were selected and randomly divided into 2 groups of 6. A cylindrical cavity, which was 2mm in depth and 4mm in diameter, was cut into the top surface of each tooth; see Figure 16(a). All samples were prepared by a single operator following typical clinical procedures with a high-speed handpiece and dental cutting burs. Each specimen was then treated with the bonding agent Adper™ Single Bond Plus (3M ESPE, USA) to the cavity surfaces and then restored with the composite resin Z100TM (3M ESPE, US) using either the bulk or incremental filling technique, as shown in Figure 16(b). The AE sensor was attached onto the opposite surface of the tooth. For the group restored with the incremental technique, recording of the AE events was interrupted in between placements of the 2 layers.

Figure 17 shows the total number of AE events for all the specimens. Note that 4 out of 6 of the specimens with the incremental filling method did not produce any AE events at all, while the remaining 2 produced only a small number of AE events. Compared with the incremental-filling group, specimens restored with the bulk-fill method produced much more AE events, indicating more interfacial debonding in those specimens.

**Figure 16.** (a) Cylindrical cavity on a bovine tooth, and (b) the two different filling techniques

**Figure 17.** The total number of AE events for specimens restored with the bulk and incremental techniques

## **5. Discussion and concluding remarks**

164 Composites and Their Applications

groups with different C-factors

**4.3. Influence of the filling technique** 

in between placements of the 2 layers.

hypothesis will be tested in this section with the AE technique.

Group Total number

of AE events

29.6 (15.7) 10.0 (5.8) 2.6 (1.5) 2.2 (1.3)

**Table 3.** Results from the AE tests: mean (standard deviation)

Total number of AE events per area ( /mm2)

> 0.54(0.24) 0.39(0.14) 0.17(0.10) 0.05(0.02)

Group 2 3 4

1 0.031 0.005 0.005 2 0.025 0.019 3 0.667 **Table 4.** Statistical significance (*p*-value) of the difference in the total number of AE events between the

The composite placement techniques (bulk vs. incremental) have been shown to affect the magnitude of the polymerization shrinkage stresses and cuspal deformations in restored teeth [33-37]. Compared with the bulk filling method, the incremental filling method has been reported to produce lower shrinkage stresses. Therefore, the incremental filling method is expected to produce less interfacial debonding than the bulk filling method. This

12 incisor bovine teeth with similar geometries were selected and randomly divided into 2 groups of 6. A cylindrical cavity, which was 2mm in depth and 4mm in diameter, was cut into the top surface of each tooth; see Figure 16(a). All samples were prepared by a single operator following typical clinical procedures with a high-speed handpiece and dental cutting burs. Each specimen was then treated with the bonding agent Adper™ Single Bond Plus (3M ESPE, USA) to the cavity surfaces and then restored with the composite resin Z100TM (3M ESPE, US) using either the bulk or incremental filling technique, as shown in Figure 16(b). The AE sensor was attached onto the opposite surface of the tooth. For the group restored with the incremental technique, recording of the AE events was interrupted

Figure 17 shows the total number of AE events for all the specimens. Note that 4 out of 6 of the specimens with the incremental filling method did not produce any AE events at all, while the remaining 2 produced only a small number of AE events. Compared with the incremental-filling group, specimens restored with the bulk-fill method produced much

more AE events, indicating more interfacial debonding in those specimens.

Total number of AE events per volume ( /mm3)

> 0.89(0.37) 0.81(0.22) 0.36(0.21) 0.07(0.03)

> > The AE technique, which is widely used for fracture monitoring in many areas, was for the first time introduced here to monitor *in-situ* the interfacial deboning of composite restorations. Its effectiveness was verified by several groups of experiments using specimens with different boundary constraints: freestanding blobs of composite, ring specimens cut from a tooth root and whole human molars. It was also shown from the AE results that interfacial debonding in composite restorations is greatly influenced by the restorative composite material, cavity configuration and filling technique. In general, composites with

lower shrinkage, cavities with smaller C-factors and use of the incremental filling method can produce less interfacial debonding. These findings agreed with the results from other researchers using different evaluative methods.

Non-Destructive Examination of Interfacial

Debonding in Dental Composite Restorations Using Acoustic Emission 167

for their contributions to the AE tests. Xiaozhou Liu would like to thank the China Scholarship Council and the MDRCBB for financially supporting her study visit to the

[1] Ferracane, J.L., *Developing a more complete understanding of stresses produced in dental* 

[2] Forss, H. and E. Widstrom, *Reasons for restorative therapy and the longevity of restorations* 

[3] Dauvillier, B.S., M.P. Aarnts, and A.J. Feilzer, *Developments in shrinkage control of adhesive* 

[4] Brunthaler, A., et al., *Longevity of direct resin composite restorations in posterior teeth.*

[5] Yap, A.U.J., et al*., An in vitro microleakage study of three restorative techniques for class II* 

[6] Lopes, G.C., M. Franke, and H.P. Maia, *Effect of finishing time and techniques on marginal sealing ability of two composite restorative materials*. Journal of Prosthetic Dentistry 2002. 88: p. 5. [7] Abdalla, A.I. and C.L. Davidson, *Effect of mechanical load cycling on the marginal integrity of adhesive class I resin composite restorations*. Journal of Dentistry 1996. 24: p. 4. [8] Ausiello, P., et al., *Debonding of adhesively restored deep class II MOD restorations after* 

[9] Shono, Y., et al., *Effects of cross-sectional area on resin-enamel tensile bond strength*. Dental

[10] Goracci, C., et al., *Influence of substrate, shape, and thickness on microtensile specimens' structural integrity and their measured bond strengths*. Dental Materials 2004. 20: p. 12. [11] Phrukkanon, S., M.F. Burrow, and M.J. Tyas, *The influence of crosssectional shape and* 

[15] Li, J., *The Biomaterials And Biomechanics Of Dental Restorations: Property Measurement And* 

[16] Li, J., H. Li, and S.L. Fok, *A mathematical analysis of shrinkage stress development in dental composite restorations during resin polymerization*. Dental materials 2008. 24: p. 9. [17] Kanemura, N., H. Sano, and J. Tagami, *Tensile bond strength to and SEM evaluation of* 

[18] Sano, H., et al., *Comparative SEM and TEM observations of nanoleakage within the hybrid* 

[19] Santis, R.D., et al., *A 3D analysis of mechanically stressed dentin–adhesive–composite* 

[20] Nair, A. and C.S. Cai, *Acoustic emission monitoring of bridges: Review and case studies.*

*composites during polymerization*. Dental Materials 2005. 11: p. 7.

*in adults*. Acta Odontol Scand 2004. 62: p. 5.

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of Dental Reseach, 2008. 87: p. 5.

*Simulation*, 2008, University of Manchester.

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*restoratives.* Jounal of Esthetic Dentistry, 2000. 12: p. 9.

*restorations in posterior teeth*. Biomaterials, 1996. 17: p. 5.

*functional loading*. American Journal of Dentistry, 1999. 12: p. 5.

*surface area on the microtensile bond test*. Dental Materials 1998. 14: p. 10. [12] McMurray, J., *Fundamentals of Organic Chemistry*1986, Monterey, CA: Brooks/Cole. [13] Watts, D.C., A.S. Marouf, and A.M. Al-Hindi, *Photo-polymerization shrinkage-stress kinetics in resin-composites: methods development*. Dental materials 2003. 19: p. 11. [14] Gonçalves, F., et al., *Contraction Stress Determinants in Dimethacrylate Composites.* Journal

*ground and intact enamel surfaces*. Journal of Dentistry, 1999. 27: p. 8.

*interfaces using X-ray micro-CT*. Biomaterials, 2005. 26: p. 14.

Engineering Structures, 2010. 32(6): p. 1704-1714.

MDRCBB.

**6. References** 

The AE technique has several advantages over the other methods in evaluating the interfacial debonding of dental composite restorations: 1) it is non-destructive; 2) it is not affected by the light-curing process and can therefore be used to make *in-situ* measurement during the curing; and 3) it is very sensitive, being able to detect micro-cracking that cannot be seen by micro-CT. However, there are still certain limitations to the AE technique. The AE results are dependent on the operational parameters of the AE system, e.g. the signal threshold, band pass, and the gain of the preamplifier. Therefore, it is important to specify those parameters when comparing AE results from different AE measurements. Also, false signals which have similar frequencies with the interfacial debonding may not be filtered out. Efforts are therefore needed to minimize such false signals, e.g. by keeping the tooth samples wet to avoid enamel cracking caused by dehydration. Finally, unlike the optical methods, the AE technique cannot provide direct visual images of the interfacial debonding.

In the future, more efforts should be made to increase the amount of information the AE technique can provide in evaluating interfacial debonding in dental composite restorations. For example, it may be possible to locate the source of the AE events by using the times-ofarrival captured by several AE sensors placed at different positions on the tooth [38-40]. This will help to establish where the critical regions of debonding are in a restored tooth. Also, the AE technique can be used to study other factors that may influence interfacial debonding, e.g. the adhesive/bonding agent, cavity shapes (with a constant C-factor), environmental challenges such as thermal and occlusal load, etc. Overall, it is a powerful tool which can be used to evaluate the interfacial debonding of composite restorations under more clinically relevant conditions in a more systematic way. This will help to improve the quality of bonding at the tooth-restoration interface and, thus, increase the longevity of composite restorations.

## **Author details**

Haiyan Li, Jianying Li and Alex Fok *Minnesota Dental Research Center for Biomaterials and Biomechanics, School of Dentistry, University of Minnesota, Minneapolis, MN, United States* 

Xiaozhou Liu *China Medical University School and Hospital of Stomatology, Shenyang, PR China* 

## **Acknowledgement**

The authors would like to acknowledge 3M ESPE for providing the restorative materials and the Minnesota Dental Research Center for Biomaterials and Biomechanics (MDRCBB) for providing the test devices. They also acknowledge Mr. Xiaofei Yun and Mr. Max Hofer for their contributions to the AE tests. Xiaozhou Liu would like to thank the China Scholarship Council and the MDRCBB for financially supporting her study visit to the MDRCBB.

## **6. References**

166 Composites and Their Applications

composite restorations.

**Acknowledgement** 

Haiyan Li, Jianying Li and Alex Fok

*University of Minnesota, Minneapolis, MN, United States* 

**Author details** 

Xiaozhou Liu

researchers using different evaluative methods.

lower shrinkage, cavities with smaller C-factors and use of the incremental filling method can produce less interfacial debonding. These findings agreed with the results from other

The AE technique has several advantages over the other methods in evaluating the interfacial debonding of dental composite restorations: 1) it is non-destructive; 2) it is not affected by the light-curing process and can therefore be used to make *in-situ* measurement during the curing; and 3) it is very sensitive, being able to detect micro-cracking that cannot be seen by micro-CT. However, there are still certain limitations to the AE technique. The AE results are dependent on the operational parameters of the AE system, e.g. the signal threshold, band pass, and the gain of the preamplifier. Therefore, it is important to specify those parameters when comparing AE results from different AE measurements. Also, false signals which have similar frequencies with the interfacial debonding may not be filtered out. Efforts are therefore needed to minimize such false signals, e.g. by keeping the tooth samples wet to avoid enamel cracking caused by dehydration. Finally, unlike the optical methods, the

In the future, more efforts should be made to increase the amount of information the AE technique can provide in evaluating interfacial debonding in dental composite restorations. For example, it may be possible to locate the source of the AE events by using the times-ofarrival captured by several AE sensors placed at different positions on the tooth [38-40]. This will help to establish where the critical regions of debonding are in a restored tooth. Also, the AE technique can be used to study other factors that may influence interfacial debonding, e.g. the adhesive/bonding agent, cavity shapes (with a constant C-factor), environmental challenges such as thermal and occlusal load, etc. Overall, it is a powerful tool which can be used to evaluate the interfacial debonding of composite restorations under more clinically relevant conditions in a more systematic way. This will help to improve the quality of bonding at the tooth-restoration interface and, thus, increase the longevity of

AE technique cannot provide direct visual images of the interfacial debonding.

*Minnesota Dental Research Center for Biomaterials and Biomechanics, School of Dentistry,* 

The authors would like to acknowledge 3M ESPE for providing the restorative materials and the Minnesota Dental Research Center for Biomaterials and Biomechanics (MDRCBB) for providing the test devices. They also acknowledge Mr. Xiaofei Yun and Mr. Max Hofer

*China Medical University School and Hospital of Stomatology, Shenyang, PR China* 


[21] Bohse, J., *Acoustic emission characteristics of micro-failure processes in polymer blends and composites*. Composites Science and Technology, 2000. 60(8): p. 1213-1226.

**Section 3** 

**Natural Fiber, Mineral Filler Composite Materials** 


**Natural Fiber, Mineral Filler Composite Materials** 

168 Composites and Their Applications

[21] Bohse, J., *Acoustic emission characteristics of micro-failure processes in polymer blends and* 

[22] Skåre, T. and F. Krantz, *Wear and frictional behaviour of high strength steel in stamping* 

[23] Ereifej, N., N. Silikas, and D.C. Watts, *Initial versus final fracture of metal-free crowns,* 

[24] Vallittu, P.K., *Use of woven glass fibres to reinforce a composite veneer. A fracture resistance* 

[25] Kim, K.H. and O. Okuno, *Microfracture behaviour of composite resins containing irregular-*

[26] Li, H., et al., *Non-destructive examination of interfacial debonding using acoustic emission*.

[27] Lu, H., et al., *Probing the origins and control of shrinkage stress in dental resin-composites: I. Shrinkage stress characterization technique*. Journal of Materials Science: Materials in

[28] Liu, X., et al., *An acoustic emission study on interfacial debonding in composite restorations*.

[29] Braga, R.R., et al., *Influence of cavity dimensions and their derivatives (volume and 'C' factor) on shrinkage stress development and microleakage of composite restorations*. Dental Materials,

[30] Feilzer, A.J., A.J.D. Gee, and C.L. Davidson, *Setting stress in composite resin in relation to* 

[31] Nikolaenko, S.A., et al., *Influence of c-factor and layering technique on microtensile bond* 

[32] Watts, D.C. and J.D. Satterthwaite, *Axial shrinkage-stress depends upon both C-factor and* 

[33] Park, J., et al., *How should composite be layered to reduce shrinkage stress: Incremental or bulk* 

[34] McCullock, A. and B. Smith, *In vitro studies of cuspal movement produced by adhesive* 

[35] Lee, M.-R., et al., *Influence of cavity dimension and restoration methods on the cusp deflection* 

[36] Versluis, A. and W. Douglas, *Does an incremental filling technique reduce polymerization* 

[37] Abbas, G., et al., *Cuspal movement and microleakage in premolar teeth restored with a packable* 

[38] Mavrogordato, M., et al., *Real time monitoring of progressive damage during loading of a simplified total hip stem construct using embedded acoustic emission sensors. Medical* 

[39] Lympertos, E.M. and E.S. Dermatas, *Acoustic emission source location in dispersive media.*

[40] Salinas, V., et al., *Localization algorithm for acoustic emission.* Physics Procedia, 2010. 3(1):

*of premolars in composite restoration*. Dental Materials, 2007. 23(3): p. 288-295.

*composite cured in bulk or in increments*. J Dent Res, 2003. 31: p. 8.

*composites*. Composites Science and Technology, 2000. 60(8): p. 1213-1226.

*monitored by acoustic emission technique*. Wear, 2003. 255(7–12): p. 1471-1479.

*analyzed via acoustic emission.* Dental Materials, 2008. 24(9): p. 1289-1295.

*and acoustic emission study*. Journal of Oral Rehabilitation, 2002. 29: p. 7.

*shaped fillers Journal of Oral Rehabilitation*, 2002. 29: p. 7.

*configuration of the restoration*. J Dent Res, 1987. 66(11): p. 4.

*strength to dentin.* Dental Materials, 2004. 20(6): p. 579-585.

*composite mass.* Dental Materials, 2008. 24(1): p. 1-8.

*filling*? Dental Materials, 2008. 24(11): p. 1501-1505.

*restorative materials.* Br Dent J, 1986. 161: p. 5.

*shrinkage stresses?* . J Dent Res, 1996. 75: p. 8.

*Engineering &amp*; Physics, 2011. 33(4): p. 395-406.

Signal Processing, 2007. 87(12): p. 3218-3225.

p. 863-871.

Dental Materials, 2011. 27: p. 8.

Dental Materials, 2011. 27: p. 8.

Medicine, 2004. 15: p. 7.

2006. 22(9): p. 818-823.

**Chapter 8** 

© 2012 Kaddami et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Kaddami et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**TEMPO-Mediated Oxidation of Lignocellulosic** 

**Fibers from Date Palm Leaves: Effect of the** 

**and Properties of Epoxy Based Composites** 

Adil Sbiai, Abderrahim Maazouz, Etienne Fleury,

Additional information is available at the end of the chapter

Henry Sautereau and Hamid Kaddami

http://dx.doi.org/10.5772/47763

non-wood engineering fibers.

**1. Introduction** 

**Oxidation on the Processing by RTM Process** 

Lignocellulosic fibers display many well-known advantages as compared to their synthetic counterparts, including their being ecologically and toxicologically harmless, biologically degradable, and carbon dioxide (CO2) neutral. Furthermore, natural fibers are characterized by a huge degree of variability and diversity in their properties. As they could be extracted from wood and annual plants, they are available in various forms, give a feeling of warmth to the touch, and have a pleasant appearance. None of these properties are offered by other

Over the last two decades, a great deal of work has been dedicated to composites reinforced with natural fibers. Indeed the use of such natural product for the reinforcement of thermoplastic or thermosetting resins, leads to composites with lower density, higher specific stiffness and strength, together with a better biodegradability (Bledzki et al., 1999; Mishra et al., 2004; Zimmermann et al., 2004; Gandini, 2008). However, only few studies have dealt with polymers reinforced with lignocellulosic fibers obtained from palm trees (Abu-Sharkh and Hamid 2004; Wan Rosli et al. 2004; Kaddami et al. 2006; Bendahou et al. 2008 ; Sbiai et al. 2008; Bendahou et al. 2009). In the previous investigations (Kaddami et al. 2006; Bendahou et al. 2008; Sbiai et al. 2008), the reinforcing capability of palm tree fibers in thermoset or thermoplastic polymer matrices was demonstrated. In the case of epoxy-based composites, expected and strong interactions gave rise to enhanced mechanical and thermal characteristics. An increase in the glass transition temperature and an improvement of the thermo-mechanical properties, bending moduli, stress at break values, and maximum absorbed energies were
