Abstract

Composite materials, having better properties than traditional materials, are susceptible to potential damage during operating conditions, and this issue is usually not found until it is too late. Thus, it is important to identify when cracks occur within a structure, to avoid catastrophic failure. The objective of this chapter is to fabricate a new generation of strain sensors in the form of a wire/thread that can be incorporated into a material to detect damage before they become fatal. This microscale strain sensor consists of flexible, untwisted nylon yarn coated with a thin layer of silver using electroless plating process. The electromechanical response of this sensor wire was tested experimentally using tensile loading and then verified numerically with good agreement in results. This flexible strain sensor was then incorporated into a composite specimen to demonstrate the detection of damage initiation before the deformation of structure becomes fatal. The specimens were tested mechanically in a standard tensometer machine, while the electrical response was recorded. The results were very encouraging, and the signal from the sensor was correlated perfectly with the mechanical behavior of the specimen. This showed that these flexible strain sensors can be used for in situ structural health monitoring (SHM) and real-time damage detection applications.

Keywords: composites, structural health monitoring, flexible yarn, strain sensor, conductive film Ag-coating, electromechanical behavior

## 1. Introduction

Composite materials, despite having better physical properties, are not exempt from limitations and drawbacks [1–4]. The mechanisms of damage initiation and propagation leading to ultimate fracture in these materials are very complex but very well established [5–11]. Structural health monitoring (SHM) is a well-known technique to examine the mechanical behavior of the structure during operation and to avoid its sudden failure [12]. In situ real-time SHM has been used frequently for detecting different types of damages in materials such as corrosion, deformation, debonding/delamination, fiber cracking, thermal degradation, intralaminar cracking, etc. to ensure safe and durable service life of the structures [13–18]. So, vast research had been carried out during the past years to develop SHM sensors, and this development took place gradually over time from strain gages, fiber optic

sensors, and piezoelectric sensors to microelectromechanical systems (MEMSs) [19–21]. But they all have some limitations such as strain gauges behave as defects or inclusions, fiber optic sensors require lot of instrumentation and data analysis, brittle material is used in manufacturing piezoelectric sensors, and MEMSs are manufactured at microscale, which makes the manufacturing process difficult [22–25].

reliability, and low cost. Once the experimental results were validated numerically, these microscale flexible sensor wires were incorporated in the composite specimen to demonstrate its SHM application. The specimens were then tested in standard tensometer machine mechanically while the electrical response was recorded, which correlated perfectly with the mechanical behavior of composite specimen. This showed that these flexible yarn wires can be used as piezo-resistive strain

Sensors were developed using untwisted nylon yarn and by depositing a silver (Ag) metal film on the surface of its filaments because even though nylon yarn behaved well mechanically, it was poor in electrical conductance. Thin film coating was applied to overcome the conductance issue without jeopardizing structural integrity of each material. Electroless plating process was used for this purpose, and the sensor specimens were characterized by the following dimensions: length 50 mm, diameter of yarn 225 μm, and coating thickness 1–2 μm. This thickness of the coating film is the best compromise between uniform thickness throughout the yarn and good conductive flexible coating. These dimensions were confirmed when scanning electron microscopy (SEM) of sample wires was performed for the char-

Uniaxial tensile testing machine and oscilloscope were used simultaneously to examine the sensing behavior of this flexible strain sensor wire. Three experimental

Flexible sensor wire was attached with electrodes and paper support, placed in the tensile machine, and

sensors for SHM applications of composite structures.

Nanotechnology and Development of Strain Sensor for Damage Detection

acterization of samples after deposition of silver coating.

3. Experimental test setup for sensor wires

Figure 1.

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electrodes were attached to the data acquisition system.

2. Sensor fabrication and characterization

DOI: http://dx.doi.org/10.5772/intechopen.82871

Another class of sensors, known as textile sensors or flexible sensors, is the new focus of study for many researchers. These are conductive strain sensor wires/ threads in which electrical resistance varies reversibly to applied stress [26–29]. They can be developed at nanoscale or microscale and can overcome the limitation of brittle behavior of conventional piezo-resistive strain gauges. So, it is essential to understand the mechanical behavior of these flexible sensors for better structural integrity and longer service life. Moreover, it is also important to understand the concept of computational modeling of these flexible yarns to model and analyze their behavior numerically. However, very less research has been conducted to use the concept of coated yarn as a flexible piezo-resistive strain sensor for structural health monitoring without jeopardizing the mechanical behavior of core material especially numerically. Different researchers had worked on numerical models and had used finite element analysis (FEA) to predict the mechanical behavior of yarn [30–32]. With the advancement of computer-aided design (CAD) and computeraided engineering (CAE), it is possible to investigate the mechanical behavior of yarn using finite element modeling (FEM) [33]. Many CAD models of filaments, yarns, and fabrics have been developed by researchers with most of them related to geometrical modeling of yarns based on single line yarn path also known as pitch length [34–37]. Some researchers have attempted to overcome difficulties like small- and large-scale deformation, complex material properties, and 3D modeling [38]. Several analytical models had been established for the estimation of mechanical tensile performance of yarns. The tensile behavior of yarn, using force method, was first studied 90 years ago, which was then extended to examine the mechanical behavior of continuous filament yarns [39, 40]. Other than force method, energy method was used to study the continuous filament and to predict the whole stressstrain behavior in Tenasco yarn which was first proposed by Treloar and Riding [41]. Then, Riding and Wilson [42] extended this study and predicted the stressstrain relations for materials such as low-tenacity Terylene, Super Tenasco, and Nylon 6-6. Moreover, energy method was also used to study the tensile and torsional behavior of bulky wool single yarn [43]. Cartraud and Messager [44] studied the model of 1 + 6 (six cylindrical filaments were wrapped around a straight filament at core) stranded fibrous structure under tensile loading. Vassiliadis et al. [38] suggested a computational method to study the mechanical behavior of multifilament twisted yarn from 2 to 1200 filaments based on FEM. However, up to this date and to the best knowledge of the author, very limited or no research has been conducted to experimentally and/or numerically analyze a coated yarn and to study the electromechanical response of coated yarn-based wire models.

In this chapter, the overall objective is to fabricate a conductive wire that functions like a piezo-resistive strain gage while not jeopardizing the structural integrity of the composite and acting as a real-time sensor during the operating condition. This was achieved by using untwisted nylon yarn and depositing silver metal coating on its surface. Initially, experimental tests were conducted to quantify the electromechanical behavior of this detector and analyze its performance. Then, a numerical model was developed to validate this sensor design and confirm the reproducibility of results. Due to their extremely small size and large-scale integration degree, the sensors had the remarkable characteristics of light weight, flexibility in design, low power consumption and noise level, short response time, high

Nanotechnology and Development of Strain Sensor for Damage Detection DOI: http://dx.doi.org/10.5772/intechopen.82871

reliability, and low cost. Once the experimental results were validated numerically, these microscale flexible sensor wires were incorporated in the composite specimen to demonstrate its SHM application. The specimens were then tested in standard tensometer machine mechanically while the electrical response was recorded, which correlated perfectly with the mechanical behavior of composite specimen. This showed that these flexible yarn wires can be used as piezo-resistive strain sensors for SHM applications of composite structures.
