## 2 Experimental Investigation
### Defect Detection Using Surface Acoustic Waves  
The surface acoustic waves were excited using the IDT, which was fabricated as a SAW sensor on a Y cut LiNbO\({}_{3}\) single crystal. Figure 2, shows the experimental set-up for the excitation and defect  
Figure 1: **(a)** Schematic representation of the fabricated interdigital transducers (IDT) marked with all the dimensions and, (b) Experimental setup of the fabricated IDT is a temperature-controlled chamber for excitation and detection of S surface acoustic waves (SAWs) and longitudinal waves.  
detection using IDT. The size of the single electrode was 100 \(\upmu\)m in width and 4000 \(\upmu\)m in length. The employed resonating frequency of the IDT sensor was 5 MHz and also demonstrated in the next section. The distance between transmitting and receiving IDT sensor was 5 mm and also shown in the Figure 1. The applied voltage across the electrodes produces an electrical field due to the accumulation of charges which alternate in polarity between neighboring electrodes.  
The excitation of compression and expansion movement of phonons perpendicular to the electrodes leads to excitation of surface acoustic waves, also known as Rayleigh waves. The employed arbitrary function generator (AFG) has the capability to generate a linear chirp signal 10 Hz to 25 MHz at the voltage of \(\pm\)5 volts. A chirp signal was excited using a universal serial bus (USB) driven by the AFG (Handyscope HS3, Tiepie Engineering, The Netherlands). The AFG could generate 240 MS/s, 14-bit, 128 k sample arbitrary signal which could be digitized at a sampling rate of 500 MS/s. The data acquisition was performed using a mixed domain oscilloscope (MDO 3024, Tektronix, Oregon, USA). The data acquisition system has four analog channels with 200 MHz analog bandwidth with a maximum sampling rate of 2.5 GS/s. The record length of the data acquisition system (DAQ) is 10 million points. The entire experiment was carried out in a temperature-controlled chamber and kept over an expanded polystyrene (EPS) foam so that it can suppress low-frequency vibrations and minimize error due to temperature fluctuations. Figure 3a shows the schematic representation of the data acquisition system of the experiments. Figure 3b shows the experimental setup for conducting excitation and detection of SAW's and longitudinal waves in the LiNbO\({}_{3}\) crystal.  
SAW's have been excited and detected on the reference sample and transient signal was recorded via a transient recorder. Next, a calibrated defect/flaw was created on the surface of the IDT using a high-speed diamond drill of radius 250 \(\upmu\)m and 0.2 \(\upmu\)m deep, considerable representation of a micro-scale defect in the piezoelectric crystal. The microscale damage of depth 0.2 \(\upmu\)m is considered as the precursor to damage. Each experiment of excitation and detection of the ultrasonic wave using IDT was repeated for five times to ensure repeatability. The surface defect/flaw was characterized and  
Figure 3: (**a**) Schematic representation for generation of SAWs and longitudinal waves using IDT sensors and (**b**) experimental setup for conducting excitation and detection of SAWs using IDT sensor in LiNbO\({}_{3}\) crystal.  
Figure 2: A schematic representation of the IDT experiments for damage detection in Lithium Niobate (LiNbO\({}_{3}\)) crystal.  
imaged by scanning acoustic microscopy (SAM). In the next section, the working principle of SAM has already been described somewhere else [28].