**1. Introduction**

In recent times, composite materials finds a wide variety of applications in several fields of engineering such as automobile, aerospace, building materials, etc. because of their good strength to weight ratio, higher resistance to impact, better fatigue property and lower costs. The composite materials are generally classified into three vast categories according to their matrix material type, namely, polymer matrix composites (PMCs), metal matrix composites (MMC) and ceramic matrix composites (CMCs). The Glass fibre reinforced polymer (GFRP) and carbon fibre

reinforced polymer (CFRP) represents the largest class of polymer matrix composites (PMCs) which find numerous applications in the fabrication of aerospace structures, turbine blades, automobile skin, etc. These structures are continuously subjected to various cyclic loads and impacts during the tenure of their service and hence may lead to development of damages in their sub-surface layers, in form of delamination. So, this chapter will discuss a technique of determining the location of such delaminations by performing vibro-thermography based on local defect resonance of the defect. The vibro-thermography technique is an efficient technique for determining the location of damage in a complex structure by exciting the structure at its defect frequency, known as the local defect resonance frequency. Excitation at the defect frequency leads to high amplitude vibration at the defect site resulting in clapping action of the layers, thus, generating a local heat at the defect area. This temperature gradient in the defect area can be easily detected using an IR-camera.

The vibro-thermography technique was implemented by many researchers in the past for carrying out non-destructive evaluation of defects in structures. Vibrothermography was first introduced by for the detection of subsurface damages in composite structures due to fatigue [1]. They studied the elastic and viscoelastic hysteretic effects by mechanically exciting the specimen and obtaining the thermal patterns. It was observed that the material deformation during the excitation was directly related to the heat generated. The study was further implemented by using resonant vibration to obtain high cyclic stresses [2]. In the year 1996, Rantala et al. excited a sample with mechanical shaker and monitored the sample with an infrared camera to perform a lock-in thermography technique [3]. The high amplitude of vibration led to high temperature signatures at low stress levels which is very good for non-destructive evaluation. Another study used lock-in thermography based on optical heating of defect area for large area inspections [4]. Subsequently, a short pulse sound was used in addition to IR imaging to measure surface temperature as a function of time [5]. This led to efficient detection of subsurface cracks due to enhancement of sonic infrared imaging. Vibro-thermography was found to be an outstanding tool for fast detection of small defects like cracks and delaminations [6]. It was observed that the defect can be detected from any side inspection and not required for all-side inspection. Also, a single excitation location is sufficient to perform a large area scanning and is found very useful for composites with multiple materials having different thermal properties. The vibro-thermography technique was also implemented for determining the size of a defect [7]. In this study, a numerical model was developed using finite difference method where it was observed that the size of defect influences phase angle data, thus affecting the defect depth estimation. In the following year, the lock-in thermography technique was used to differentiate between location, shape and size of defects in case of cracks and corrosion defect [8]. Subsequently, the lock-in thermography was implemented for detection of vertical cracks of arbitrary shape to determine geometry and location of defect [9]. An algorithm is developed to obtain crack shape reconstructions by optimizing the data before entering the algorithm. Deep cracks are precisely detected although the shape of the crack is obtained as rounded and having a slightly over-estimated area. Another group of researchers investigated novel hybrid thermographic techniques in addition to traditional optically excited thermography using external optical radiation such as heaters, flashes and laser systems [10]. Different techniques such as ultrasonic stimulated thermography with ultrasonic waves and damage resonance to enhance the sensitivity of microcracks; microwave thermography that uses electromagnetic radiation at microwave frequency bands; and eddy current stimulated thermography to generate induction heating is used for detection of delaminations and cracks. Subsequently, an
