## 4 Analysis of Experimental Results
For the damaged sample, the peak central frequency (grey) is at 5.1 MHz. The chirp signal helps in exciting the natural frequency of health as well as crack sensors, irrespective of the IDT design. We observed that the resonating frequency of the damage sensors shifts towards the left with respect to the reference sensors. The wavelength of the guided wave is in close proximity to the thickness of the piezoelectric sensor. During the propagation, the wave interacts with the thickness and the energy content of the elastic wave is dispersed throughout the thickness and length of the media. The energy of the guided waves is scattered, in the presence of the surface anomalies such as crack, inter-laminar debonding, and delamination. The scattering phenomena are complex to understand in the absence of full wave-field imaging. The scattering further influences the dispersion of elastic waves, which is evident in the frequency domain of the signal (Figure 5). Based on the experimental results it is evident that there exists a damage (preferably on the surface) which is manifested through the difference in TOF and the shift in the frequency spectrum acquired through IDT. Figure 6 shows the time domain signal for the reference and damage state acquired using SAM at 100 MHz. The time domain signal in the reference state (Figure 6a) is normalized to the unit using the peak reference voltage of 220 millivolts. Similarly, the time domain signal in the damage state (Figure 6b) is normalized with respect to peak amplitude in the reference state. Figure 6a,b show  
Figure 4: The transient signal of the ultrasonic wave (**a**) in reference and (**b**) damaged sensors excited using chirp signal. The shaded section indicates the selected window of the first arrival wave packet. Figure 5: The frequency spectrum of guided wave in reference and damaged sensors using chirp excitation. three distinct arrival peaks due to excitation of longitudinal waves in the Y cut LiNbO\({}_{3}\). The first and second arrival peak corresponds to the reflection of longitudinal waves from the top and bottom surface of the crystal. The third arrival peak is the surface reflection of longitudinal waves from the printed circuit board (PCB). It is observed that the peak amplitude is dramatically reduced from unity to half, corresponding to reference state to damage state, respectively. Reduction in the amplitude of the reflected signal is due to the surface damage. The longitudinal waves are dispersed due to surface detection, as visible from the time domain signal (Figures 6b and 7b). To accurately quantify the frequency content of reference and damage state, we performed a Fourier transform on Figure 6a,b. The normalized frequency domain signal in the reference state shows two distinct peak amplitudes at 107 MHz and 113 MHz at a unit amplitude and 0.88 (arbitrary units) amplitude, respectively (Figure 7a). Due to damage, the energy of the signal is dispersed, scattered, and attenuated, resulting in a reduction of peak amplitude to 0.38. For the damage signal, the mode one peak amplitude is reduced to 0.22 at a frequency of 103 MHz and mode two peak amplitude is reduced to 0.38 at a frequency of 109 MHz, respectively. In conventional methods, these observational changes in frequencies have been identified as a measure of damage. However, the consistent and reliable quantification of damage is rather difficult. Several experimental investigations have been conducted to explore the utility of guided waves for crack detection. The pitch-catch active sensing method has been employed for crack detection  
Figure 6: The transient signal of the longitudinal wave (**a**) in reference and (**b**) damaged sensors excited using scanning acoustic microscopy at 100 MHz. Figure 7: The frequency spectrum of the longitudinal wave (**a**) in reference and (**b**) damaged sensors excited using scanning acoustic microscopy at 100 MHz. in aerospace structures using guided ultrasonic waves [46].