## 4 Experimental Investigation
### Results and Discussion  
The temperature variations should always be considered in guided wave inspections. The wave velocities are affected by the mechanical properties of the plates, which are changed by the temperature. Wave velocities of collected signals may be different if the scanning is carried out under large temperature variations, affecting the quality of the scanning image. Therefore, wave velocity compensation should be considered. In field tests, the scanning is usually finished within minutes and the temperature does not change rapidly in a short time. However, the situation can be different when the scanning is carried out at a different time (in the day or at night). Thus, wave velocity estimation is necessary before scanning. In this study, the guided waves were generated directly towards one boundary of the plate. The flight time of the reflection waves from the boundary and the distance between the MST and the boundary were used to calculate the group velocity, which is 2714 m/s. The reason for the error (15.4%) of the velocity may be the same as described in Section 3.2, since the peak of the reflection wave was used to determine the flight time.  
The results of the three methods are shown in Figure 15. The amplitudes of the images are all normalized between 0 and 1. The estimated peak value of the environmental noise is less than 0.12. The defect spot of the B-scan method in Figure 15a has large trailing, as expected, which may lower the angular resolution. By the DAS method, the trailing around the defect spot is eliminated, as in Figure 15b; however, some trailing occurs in other regions of the image, which have similar amplitudes to the defect spot, becoming a severe distraction in defect recognition. In Figure 15c, the defect spot is improved in the angular direction despite some residual trailing. It can be observed that the defect spot by the DAS method is less obvious than that by the B-scan method, whereas the defect spot by the CSAFT method is much more recognizable. Quantitative comparisons of the defect spot by three methods are listed in Table 3. The position of the defect is denoted by the peak of the spot, and the angular bandwidth is defined as in Section 3.2. The reflection waves from the boundaries are taken as references to show the significance of the defect spots by the three methods. The peak value ratio between the defect and the boundaries by the proposed method is 580% larger than that by the B-scan method, whereas the result with the DAS method does not give any improvement. Apart from the largest significance of the defect, the results by the CSAFT method have the least error in the angular localization and smallest angular bandwidth of the defect compared with the other two methods.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Name** & **Actual Values** & **B-Scan** & **DAS** & **CSAFT** \\ \hline \multirow{2}{*}{Position} & Angle (\({}^{\circ}\)) & 12 & 13.5 & 12.5 & 12 \\ \cline{2-6}  & Distance (m) & 0.825 & 0.823 & 0.815 & 0.821 \\ \hline \multirow{2}{*}{Angular bandwidth (\({}^{\circ}\))} & \multirow{2}{*}{NAN \({}^{1}\)} & \multirow{2}{*}{4.5} & \multirow{2}{*}{3.8} & \multirow{2}{*}{2.0} \\ \cline{1-1} \cline{5-5}  & & & & \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Peak value ratio \\ (defect/boundaries) \\ \end{tabular} } & \multirow{2}{*}{NAN \({}^{1}\)} & \multirow{2}{*}{0.25} & \multirow{2}{*}{0.14} & \multirow{2}{*}{1.71} \\ \cline{1-1} \cline{5-5}  & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Defect estimation by the three methods.  
Figure 15: Experimental images by the three methods: (**a**) B-scan; (**b**) DAS; (**c**) CSAFT.