## 3 Results  
### Laser Ultrasonic Testing  
In the LUT inspection, an A-Scan signal demonstrates the voltage amplitude of received ultrasonic waves at a certain detection point as a function of time. The A-Scans are processed to generate B-Scans. In a B-Scan image, the travel time of the ultrasonic waves are plotted on one axis, and the \(X\)-coordinates of detection points along the scanning path are plotted on the other axis. The maximum amplitude values obtained from A-Scan signals of all scanning points are extracted to generate a plan view image (C-Scan) of the sample. The colors in B-Scan and C-Scan images render the amplitude of signals. The plane of C-Scan image is parallel to the scanning surface. The incident laser energy caused displacement on the top surface of the sample. The displacement wave propagated through the sample, and it was then diffracted by the defects inside the sample and reflected by the bottom surface of the sample. Finally, the displacement wave propagated back to the top surface of the sample and was detected and saved in the form of voltage by the LDV. The conversion factor from voltage to displacement was 100 mV/nm. Multiple waves, including surface waves (Rayleigh waves), bulk waves (longitudinal waves), and shear waves, can be generated by the pulsed laser. Note that Rayleigh waves contain information of surface and subsurface defects, whereas longitudinal waves contain information of internal defects. Therefore, longitudinal pulse-echo signals were used to identify the internal holes of the sample. Figure 4a-e show A-Scan signals of holes with different diameters. The positions of the five different points were located at an interesting scanning path (\(Y=5\) mm, along \(X\)-axis in Figure 3). Longitudinal waves propagated through the sample and were then reflected by the bottom surface. In the near field of incident point, the Rayleigh (R) wave traveling along the top surface was most dominant, and firstly arrived at the detection point. The Rayleigh (R) wave penetrated down the top surface of the sample at the depth of the wavelength \(\lambda\). The wavelength of the R wave can be calculated by \(\lambda=v\)/_f_, where \(v\) and \(f\) referred to the velocity and frequency of the R wave, respectively. The calculated wavelength of the R wave was 4.17 mm. The depth of the hole was 2.5 mm \(<\) 4.17 mm, so the R wave might have interactions with internal holes. Then, the longitudinal (L) wave and shear (S) wave became prevailing in the part as they moved further away from the top surface of the sample. It can be seen that the R wave first arrived at the top surface at around 1.2 us, followed by other waveforms, including L and S waves, as shown in Figure 4a. Figure 4f shows a local amplification of Rayleigh waves propagating through the different holes in the time domain from 1.10 to 1.26 us. The existence of hole defects significantly influenced the amplitude of the ultrasonic waves. The peak value of the Rayleigh wave was reduced with the increasing internal hole diameter. The detected peak voltage of the sample without defects was 0.9499 V, whereas the detected peak voltages were 0.7817, 0.6140, 0.4235, and 0.2204 V for the 0.4, 0.8, 1.0, and 2.0 mm holes, respectively. Figure 5 shows the relationship between the peak value of the Rayleigh wave and internal hole diameter. It can be seen that existences of different holes made significant differences to the amplitude of Rayleigh waves. The amplitude of the Rayleigh waves attenuated approximately linearly with the increasing diameter of the internal hole. \begin{table}
\begin{tabular}{c c} \hline
**Parameter (Unit)** & **Value** \\ \hline Tube voltage (kV) & 220 \\ Tube current (\(\mu\)A) & 100 \\ Detector pitch (mm \(\times\) mm) & 0.139 \(\times\) 0.139 \\ Detector size (1 pF VG1) & \(1\times 1\) \\ Detector binning & \(1\times 1\) \\ Tube mode & Microfocus \\ Resolution (\(\mu\)m) & 20 \\ \hline \end{tabular}
\end{table} 
Table 3: XCT parameters used for the target sample.