## 4 Results and Discussions  
### SEUT Versus PAUT Methods  
#### 4.1.1 Focusing Depth Comparison  
Table 2 shows the signal characteristics in terms of signal-to-noise ratio (SNR) for SEUT and PAUT. Data in Table 2 is plotted in Figure 5 and shows the relationship between the thicknesses of the GFRP plates (i.e., wave traveling distance) and travelling time of ultrasound wave. It can be observed that the velocities in the GFRP plate can be calculated as twice the slope of the graph, which are equal to \(2\times 1.57=3.15\) mm/\(\upmu\)s for SEUT and \(2\times 1.59=3.18\) mm/\(\upmu\)s for PAUT. Figure 6 shows an example for typical signals for the back wall reflection in SEUT and PAUT methods in the 12-mm-thick composite plate. As can be seen from the results in Table 2 and considering the form of the ultrasound signal  
Figure 4: Experimental setup for flaw detection in CFRP sample using PAUT guided wave method. Figure 3: Experimental setup for capability and sensitivity evaluation: PAUT (left) and SEUT (right). shown in Figure 6, the features of the signals are clearer and better detectable using PAUT with lower gain values. The wave velocity is important information in determining the depth and location of the defects according to the ultrasound wave's traveling time. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Frequency** & **Method** & **Sample** & \
\begin{tabular}{c} **Thickness** \\ **(mm)** \\ \end{tabular} & **Gain (dB)** & **Time (\(\mu\)s)** &
\begin{tabular}{c} **Signal-to-Noise** \\ **Ratio** \\ \end{tabular} \\ \hline \multirow{4}{*}{0.5 MHz} & \multirow{4}{*}{SEUT} & \multirow{4}{*}{GFRP} & 4 & 42 & 5.01 & 7.95 \\ \cline{3-6}  & & & 10 & 55.8 & 9.87 & 3.99 \\ \cline{3-6}  & & & 12 & 60.5 & 11.01 & 3.07 \\ \cline{3-6}  & & & 18 & 61 & 14.73 & 2.98 \\ \cline{3-6}  & & & 25 & 64.7 & 18.92 & 2.53 \\ \hline \multirow{4}{*}{1 MHz} & \multirow{4}{*}{SEUT} & \multirow{4}{*}{GFRP} & 4 & 18.5 & 4.76 & 5.31 \\ \cline{3-6}  & & & 10 & 36 & 8.56 & 11.93 \\ \cline{3-6}  & & & 12 & 39.6 & 9.80 & 11.75 \\ \cline{3-6}  & & & 18 & 41.2 & 13.60 & 11.10 \\ \cline{3-6}  & & & 25 & 45.9 & 18.35 & 9.57 \\ \hline \multirow{4}{*}{1.5 MHz} & \multirow{4}{*}{SEUT} & \multirow{4}{*}{GFRP} & 4 & 15.5 & 4.67 & 5.14 \\ \cline{3-6}  & & & 10 & 37 & 8.48 & 11.75 \\ \cline{3-6}  & & & 12 & 40.8 & 9.76 & 11.10 \\ \cline{3-6}  & & & 18 & 43 & 13.55 & 9.51 \\ \cline{3-6}  & & & 25 & 46.7 & 18.25 & 9.43 \\ \hline \multirow{4}{*}{1.5 MHz} & \multirow{4}{*}{PAUT} & \multirow{4}{*}{GFRP} & 4 & 13 & 3.64 & 6.10 \\ \cline{3-6}  & & & 10 & 27 & 7.56 & 5.44 \\ \cline{3-6}  & & & 12 & 28.5 & 8.93 & 5.81 \\ \cline{3-6}  & & & 18 & 36 & 12.49 & 3.39 \\ \cline{3-6}  & & & 25 & 40 & 17.41 & 3.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Signal characteristics of back wall reflection for SEUT method with different frequencies and PAUT method. Figure 5: Back wall reflection experiment showing ultrasonic wave velocity evaluation measured by SEUT and PAUT methods. All the measured velocities are very close in value; however, the attenuation (i.e., gain values) is improved for PAUT when compared to SEUT at 1.5 MHz. On the other hand, the quantitative values in Table 2 show that SNR is, on average, two times larger for SEUT when comparing the peak of reflected signal to the background noise. However, it should be mentioned that the resolution of the peak and its location is much lower in SEUT which caused inaccuracy for detection purposes. Higher local value of SNR in SEUT can be attributed to the interference of the signals for each element in PAUT. Qualitatively, PAUT has more uniform and detectable signal with less jitter, specifically at larger thicknesses. Figure 7 shows the gain values (for different inspection frequencies) in order to reach detectable signal in different sample thicknesses. The plot shows that the SNR and signal's attenuation were improved in PAUT technique when compared to SEUT such that 7-20% less gain in value was required to have detectable signal in case of 1.5 MHz transducers.