Preface

Chapter 9 **Otoacoustic Emissions 203**

**VI** Contents

Giovanna Zimatore, Domenico Stanzial and Maria Patrizia Orlando

Acoustic emission (AE) is a phenomenon in which elastic or stress waves are emitted from rapid, localized change of strain energy in material. The practical application of the AE first emerged in the 1950's, but only in the last 20 years the science, technology and applications of AE have progressed significantly. Currently AE has become one of the most important non-destructive testing techniques, which is widely applied in: fatigue cracks detection and location in pressure vessels and pipelines, partial discharge sources detection and location in power transformers and rotating machinery, damage assessment in fibre-reinforced poly‐ mer-matrix composites, monitoring welding applications and corrosion processes, on-line monitoring of civil-engineering structures (e.g. highway bridges, skyscrapers, dams etc.).

This interdisciplinary book consists of nine chapters, which is a proof of the fact that the AE method is continuously and intensively developing and scientists still find some new areas of its application.

**Chapter 1** presents diverse methods and algorithms for detecting and determining acoustic emission hits in burst waveforms.

**Chapter 2** discusses the application of acoustic emission to detect damage in FRP strength under freeze and thaw cycles. The strengthening performance and evaluation of the damage caused by acoustic emission activities for RC beams strengthened in shear with CFRP are examined under various freezing and thawing cycles with and without shear strengthening.

Number of research proved that the significant increase in acoustic emission activities (event, energy, amplitude, and frequency) is observed at each damage level that corre‐ sponds to the initiation of the crack, propagation of diagonal cracking, de-bonding of CFRP and failure.

**Chapter 3** presents the example of application of the AE method to structural ageing detec‐ tion of a cable-stayed bridge during load-test before its opening to public use. Apart from detailed analysis of the obtained research results, the authors answered many important questions, such as: what parameter should be monitored, which measuring procedure is op‐ timal in bridge monitoring using AE technology or what is the relevant difference between a metal viaduct and a concrete or brick or wood viaduct.

**Chapter 4** focuses on the application of the AE characteristics to detect defects in ball bear‐ ings. Simulated defects were introduced in the balls and inner race. The research results validated the use of AE RMS and AE Counts in detecting bearing defects.

**Chapters 5** and **6** concern the application of the acoustic emission in electrical power engi‐ neering. **Chapter 5** discusses different techniques of defects location in large power trans‐ formers. Additionally, there are presented some examples of implementation of partial

discharge on-line monitoring system based on AE method. In turn, **Chapter 6** discusses the issue of detection and location of partial discharges in power generator coil bars. The au‐ thors used both conventional electrical PD measurement method and non-conventional acoustic emission method to the research. The registered pulses of partial discharges were analysed with the use of the advanced methods of signal processing.

**Chapter 7** shows the possibility of using the AE method to diagnostic purposes of destruc‐ tion due to monitoring materials subjected to drying. The results of the tests obtained from convective and microwave drying of ceramic and wood materials carried out in the labora‐ tory drier equipped with the acoustic emission set-up constitute the illustrative material of this chapter.

**Chapter 8** presents recent developments in area of application of acoustic emission for fruits and vegetables including: methods and devices, results of fruit and vegetables testing under various conditions and comparison with sensory evaluation.

**Chapter 9** presents a very special kind of acoustic emission, coming from inside the cochlea and generated along the basilar membrane by the electro-motile (active) vibrations of outer hair cells of the organ of Corti. They are called OtoAcoustic Emissions (OAE) and are detect‐ ed in the ear canal by means of microphones which are usually assembled as part of ear‐ phone-like probes.

Enjoy the book!

**Wojciech Sikorski, PhD** Poznan University of Technology Poland **Chapter 1**

**Provisional chapter**

**Hit Detection and Determination in AE Bursts**

**Hit Detection and Determination in AE Bursts**

This chapter presents a methodology for detecting and determining Acoustic Emission (AE) hits in AE bursts, i.e. signals with large number of overlapping transients with variable strengths. The methodology is designed to overcome important limitations of threshold-based approaches in determining hits in this type of AE signal; for example, when the signal's amplitude between transients does not fall below the threshold for a predetermined period of time. The threshold-based approach is a special case of the proposed methodology. The methodology, and the associated algorithms, were presented in *Acoustic Emission-Based Fatigue Failure Criterion for CFRP* by Runar Unnthorsson, Thomas P. Runarsson and Magnus T. Jonsson [17] and used in four articles by the same authors [15,

The chapter is organized as follows. Section 2 provides the reader with an overview of Acoustic Emissions, what they are, how they are acquired and the various factors that can affect them from when they are emitted until they are digitized by the AE system. Majority of these factors will change the originally emitted AE waves so that the digitized representation will be different. In Section 3 an overview of the AE processing techniques is given with emphasis on conventional methods of determining AE hits and the corresponding hit parameters. The section also introduces the problem of determining AE hits in bursts. Section 4 then introduces the methodology and presents the algorithms. In section 5 an experimental AE signal is used to demonstrate the methodology. The chapter ends with section 6 which concludes the chapter and provides suggestions for future research into this

Acoustic Emission (AE) is a term used for transient elastic stress waves generated by the energy released when microstructural changes occur in a material [9, 21]. The energy

> ©2012 Unnþórsson, 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. © 2013 Unnþórsson; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Unnþórsson; 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.

distribution, and reproduction in any medium, provided the original work is properly cited.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Rúnar Unnþórsson

Rúnar Unnþórsson

10.5772/54754

**1. Introduction**

16, 18, 19].

topic.

**2. Acoustic Emissions**

http://dx.doi.org/10.5772/54754

### **Chapter 1**

**Provisional chapter**
