## 4 Results and Discussions
### PAUT Guided Wave Method  
Tables 3 and 4 show the results for the signal response parameters for Al-1 and Al-2 samples. In the dispersion curve of guided wave modes in plates, the smaller values of _\"fd\"_ (i.e., frequency x plate thickness) are more distinctive and, consequently, have a higher probability of detection [31]. In practical application specifically when the thickness of the plate structure is a fixed and known value, only the frequency of inspection can be changed. So, for thicker structures, one should use much lower frequencies, while for thinner structures, the range of possible frequencies will be wider and higher frequencies can be used to increase the resolution. The effect of _\"fd\"_ value in response signals is presented in Tables 3 and 4. We find that, in lower _\"fd\"_ values, the distance from which the signal from the defect is still detectable is longer. The phase velocity for the generated guided wave were calculated based on the theory and properties of the angle wedge. Based on these values, S0, A1, and S1 modes were possible for the Al-1 sample (_fd_ = 3.3 MHz.mm), and A0 and S0 modes are possible for Al-2 sample (_fd_ = 0.96 MHz.mm). The strongest reflection which also has the closest phase velocity value to the theory were identified as the dominate wave modes. In this case it was A1 for Al-1 sample, S0 for Al-2 sample, and S0 for CFRP sample. In Tables 3 and 4, signal parameters from the reflection of the edge of the plate close to the hole, and from the hole are presented. These signal parameters  
Figure 8: Response signal for GFRP: (**a**) SEUT method and (**b**) PAUT method. (Freq. = 1.5 MHz, Thickness = 25 mm, Hole Diameter = 0.8 mm, Hole Depth = 12 mm). include the arrival time and the amplitude of the signal at the edge of the plate and the defect. Figure 9 shows the change of the signal's amplitude over the distance of the PAUT transducer from the edge for Al-1 and Al-2. For Al-1 it was noticed that the hole's signal has larger amplitude at a longer distance compared to Al-2. This is attributed to the interference of the stationary wedge reflection signal and the signal from the hole. Figure 10 shows typical signals for the experiments in Tables 3 and 4 for Al-1 and Al-2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**Experimental Setup Parameters for PAUT Guided Wave Inspection of Al-2 Sample**} \\ \hline \multirow{2}{*}{**Frequency (MHz)**} & \multirow{2}{*}{\
\begin{tabular}{c} **Thickness** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{**Gain (dB)**} & \multirow{2}{*}{**Element Qty.1**} & \multirow{2}{*}{_fd2_} & \multirow{2}{*}{
\begin{tabular}{c} **Element** \\ **Step3** \\ \end{tabular} } \\ \hline
1.5 & 0.635 & 30 & 4 & 0.96 & 1 \\ \hline \hline \end{tabular}  
* Number of active elements at each sequence in phased array ultrasound transducer
* frequency \(\times\) plate thickness (\(MHz.mm\))
* Incremental steps in terms of number of elements at each sequence  
\end{table} 
Table 4: Signal parameters for flaw detection for Al-2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{6}{c}{**Experimental Setup Parameters for PAUT Guided Wave Inspection of Al-1 Sample**} \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} **Frequency (MHz)** \\ \end{tabular} } & \multirow{2}{*}{**Thickness** \\ **(mm)**} & & \multirow{2}{*}{**Gain (dB)**} & \multirow{2}{*}{**Element Qty.1**} & \multirow{2}{*}{_fd2_} & \multirow{2}{*}{
\begin{tabular}{c} **Element** \\ **Step3** \\ \end{tabular} } \\ \hline
1.5 & 2.2 & 30 & 4 & 3.3 & 1 \\ \hline \hline \end{tabular}  
* Number of active elements at each sequence in phased array ultrasound transducer
* frequency \(\times\) plate thickness (\(MHz.mm\))
* Incremental steps in terms of number of elements at each sequence  
\end{table} 
Table 3: Signal parameters for flaw detection in Al-1 sample. Figure 10: Typical PAUT guided wave signal of flaw detection for (**a**) Al-1 sample and (**b**) Al-2 sample. Figure 9: Change of signal amplitude over distance for Al-1 and Al-2 samples.