*Defect Detection in Delaminated Glass-Fibre/Epoxy Composite Plates Using Local Defect… DOI: http://dx.doi.org/10.5772/intechopen.101178*

integrated nonlinear ultrasound and Vibro-thermography based non-destructive testing was proposed using shared excitation sources to solve low sensitivity and small detection area problems in carbon fibre reinforced polymer structures [11]. The study involved time-domain analysis and fast Fourier transform of image sequences obtained from specimens with different impact loads. In case of visible damages, FFT did not improve the sound to noise ratio although the location and shape of the defect were captured in detail. On the other hand, the barely visible impact damages were only detected using nonlinear ultrasound and vibrothermography technique due to a better sound to noise ratio. Non-destructive testing of CFRP with impact damages using ultrasonic simulation was further implemented using two approaches: low power resonant ultrasonic stimulation and high power stimulation at a fixed frequency [12]. The high power ultrasonic IR thermography using magnetostriction and piezoelectric transmitters proved to be highly informative but at the same time, ultrasonic IR testing needs a higher level of energy consumption in order to induce temperature signatures of desired amplitude. In contrast, low power ultrasonic stimulation using defect resonance allows investigation of complex structured flaws like impact damages while reducing the energy consumption to half when compared with high power ultrasonic thermography. The study was extended by the same group where it is shown that small deviations in the frequency of acoustic signal supplied to transducer from frequency of main resonance leads to a reduction in radiation intensity and thus decreases in temperature at defect location [13]. Hence, the transducer must operate at its resonant frequency for effective ultrasonic stimulated thermography.

Furthermore, a modelling scheme was developed to simulate vibrothermography of structures used in gas turbine engine components in order to reduce mass and enhanced cooling [14]. The framework of the model is comprised of coupled thermoplastic heat generation and various effects due to nonlinear vibration arising from excitation, engagement force on target structure by the ultrasonic horn, and structural boundary conditions. It has been observed from contact dynamic simulations that subharmonics, superharmonics, subsuperharmonic and chaos are all present in the structure even if the exciter is sharply tuned at the resonant frequency. This model is also used for obtaining spatio-temporal temperature distribution in the target structure that can be extended to understand vibro-thermographic characteristics in complex structures with hidden defects. Subsequently, a significant increase in SNR combined with suppression of effects due to non-uniform heating, background reflections and surface non-uniformities can be achieved by a novel adaptive spectral band integration procedure for post-processing of flash thermography data [15]. This procedure integrates spectral information of each individual pixel thus obtaining maximum detectability of defects such as flat bottom holes, BVIDs as well as a stiffened composite panel with production defects. The technique was also able to estimate the accurate size and depth of the defects when compared with pulsed phase thermography. Moreover, the same group of researchers compared the efficiency of time and frequency domain analysis techniques in flash thermography to improve the detectability of defects [16]. Single bin procedures such as thermal signal area and dynamic thermal tomography; and integrated bin procedures such as frequency domain tomography and adaptive spectral band integration are considered for the study. The single bin approach in the frequency domain showed better detectability of defect and higher defect depth estimation as compared to its time domain counterpart. Further, the integrated frequency domain approach shows higher sensitivity to non-uniform heating and the best defect detectability among all.

Subsequently, nonlinear acoustic wave spectroscopy (NAWS) has recently proved to be an efficient tool for detection of small defects in structures [17]. NAWS has an advantage over conventional non-destructive testing (NDT) techniques where the information about reflection of incident vibration waves play a major role in in detection of damages [18]. The interaction between defect and the incident wave leads to generation of higher harmonics which can be better understood using NAWS [19]. Local defect resonance (LDR) phenomenon is one of such NAWS technique that is emerging in recent years. Since the scattering and reflection data of the incident signal in case of smaller defects is not so prominent, NAWS and vibro-thermography technique are combined for detection of defects [20]. LDR based vibro-thermography technique is much advantageous and less time consuming than other NDT techniques for detection and location of small defects even in intricate structures. Moreover, the LDR based frictional heating was introduced which illustrated the rubbing and clapping action at defect site induces an internal heating that can be detected easily using an IR camera [21]. It was observed that the amplitude of vibration is quite high at the defect location when excited with its fundamental defect frequency. Subsequently, an IR camera was used with cell phone attachment for performing lock-in thermography [22]. With the help of image processing, sub-surface defects and dental caries were successfully detected. Also, a LDR based thermal imaging experiment was carried out to characterize various defects using air-coupled ultrasound excitation [23]. Further, defects were successfully activated using sweep excitation for low energy vibro-thermography technique [24]. The post-processing included a Fourier-transform of the temperature response obtained from the defect in case of unknown LDR frequencies. The LDR concept was also used in combination of IR imaging for detecting minute defects using a nonlinear ultrasonic stimulated thermography (NUST) method [25]. It is found that by exciting the structure at a frequency yielding nonlinear response may lead to increase in the temperature rise at defect area. The same research group again used sweep excitation for activating the LDR frequency in order to obtain higher heat generation at a crack surface [20].

The various literatures discussed on vibro-thermography and LDR based vibrothermography suggests that the technique is widely accepted for detection, location and sizing of defects in composite structures. The advantage of such a technique is mainly for detection of small and complicated defects such as delaminations and barely visible impact damages (BVIDs) in composite structures. Hence, this chapter will address the problem of defect detection in glass fibre reinforced polymer (GFRP) composite by implementing LDR based vibro-thermography technique.
