## 4 Analysis of Experimental Results  
The surface acoustic and longitudinal waves were excited and detected using the IDT. [PERSON] et al. [45] have studied the generation of broadband SAWs using spatial and temporal chirp method. They argued that the impulse excitation (Dirac delta pulse) produce limited SAWs output amplitude due to piezo-electric breakdown voltage. [PERSON] et al. excited the IDT using temporal chirp excitation corresponding to the frequency and duration of the spatial chirp configuration [45]. In that work, they implemented the chirp excitation methods for a wide range from 20 MHz-125 MHz. The excitation of IDT with chirp provides a rapid, accurate estimation of phase velocity and helps in material characterization. For high frequency (>100 MHz) applications, the IDT is designed for the specific center frequency. At high frequencies, the losses due to attenuation and scattering along the comb of IDT are larger. In high frequency and broadband chirp excitation the IDT will distribute the energy throughout the frequency spectra, resulting in greater energy losses. The resonating frequency (\(f_{o}\)) response of the simple transducer with \(N\) pair of fingers is deduced from the impulse response. A frequency domain response of the impulse excitation has a broadband spectrum. A pulse is usually applied to the electrode having a short duration compared to the time associated with the Rayleigh waves to pass between two finger length [39]. As the electric field is reversed at each interval between figures, the transmitted signal has a spatial period of \(2d\). The time duration of response is \(\Theta=\frac{2 dN}{V_{R}}=\frac{N}{f_{0}}\). On the other hand, at low frequency (<10 MHz) application, the resonating frequency is commonly determined using broadband chirp excitation rather than the impulse response. The low resonant frequency IDT was fabricated for excitation and detection of the guided wave for detection of aberration, abnormality and surface cracks. For nondestructive evaluation applications, the resonating frequency of IDT rarely exceeds 20 MHz. In order to determine the resonating frequency of the referenced sensor, the IDT was excited using chirp signals and we evaluated its frequency response. A linear chirp used for excitation of ultrasonic wave is given by \(x(t)=A\sin(2\pi ft)\) of sinusoidal frequency, \(f\). Considering that the active length of IDT is 4 mm, the temporal signal duration \((T_{c})\) is 1.12 \(\upmu\)s. The instantaneous frequency is calculated as \(f=\left(f_{c}-\frac{\Delta f}{2}\right)+\frac{\Delta f}{2T_{c}}t\). Here \(f_{c}\) is the average frequency and \(\Delta f\) is the difference of lower and upper limit of frequency spectrum as obtained from chirp excitation. Initially, the chirp excitation frequency was 10 Hz-20 MHz. Based on the Fourier spectra, it was observed that the dominant mode exists in the frequency range of 4 MHz-7 MHz. Therefore, in the damage state, the frequency of excitation was narrowed down between 1 MHz-10 MHz. The range of excitation frequency was quite large, to account for dispersion and scattering of the wave due to surface cracks. The IDT was excited using linear chirp and the received transient signal of the SAW for the reference sensor is shown in Figure 4a. The signal is normalized to unity using the maximum amplitude of 125 millivolts of the reference state. Similarly, the transient signal for the defective sensors excited using linear chirp signal is shown in Figure 4b. There is a dramatic change in normalized amplitude of the signal from 1 to 0.2 (arbitrary units) corresponding to reference state and damage state, respectively. In Figures 4 and 5, the shaded section indicates the window used for evaluating the Fourier transform of the first arrival wave packet. Figure 5 shows the frequency spectrum of the first arrival wave packet for the reference and damaged sensor. For the reference sensors (black), the significant spectral peak was observed at 5.8 MHz (refer to Figure 5).