## 4 Data Acquisition  
### Experimental Setup  
To validate the proposed method, some testing experiments were conducted. Two flat plate welded joints (600 mm\(\times\) 400 mm\(\times\) 90 mm) were prepared by joining flat plates made of 20# carbon steel using argon arc welding in combination with arc welding. Five artificial cracks were embedded in the middle of one (called Joint1) and at the root of the other (called Joint2), respectively, which were evenly spaced in the weld. Table 2 gives details of the defects. In the middle of Joint1, five defects of the same size were set. To increase the difficulty for the proposed method, five defects of varying size were set at the root of Joint2, as is shown in Figure 6. As these defects are far from the probe, the diffracted signal will become weak, which may make it more difficult for the measurement method to identify defects. Meanwhile, its measurement performance can be checked under varying conditions.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Welded Joint Number** & **Defect Number** & **Length** & **Height** & **Depth** \\ \hline \multirow{6}{*}{Joint1} & D1 & 25 & 6 & 50 \\  & D2 & 25 & 6 & 50 \\  & D3 & 25 & 6 & 50 \\  & D4 & 25 & 6 & 50 \\  & D5 & 25 & 6 & 50 \\ \hline \multirow{6}{*}{Joint2} & D1 & 15 & 2 & 84 \\  & D2 & 15 & 3 & 84 \\ \cline{1-1}  & D3 & 20 & 4 & 84 \\ \cline{1-1}  & D4 & 25 & 5 & 84 \\ \cline{1-1}  & D5 & 25 & 6 & 84 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Detailed parameters for defects embedded in welded joints.  
Figure 5: The intelligent denoising framework.  
The ultrasonic diffraction equipment in the experiments contains an ultrasonic transmitter-receiver module, a computer system controlling the ultrasonic module through the matched software, two probes (transmitter and receiver) in pitch-catch configuration, and a scanner device for fixing the probes. The computer sends corresponding instructions to the ultrasonic module to generate ultrasonic pulses in the transmitter and receive the response pulses in the receiver. The received pulses are digitized and stored on the computer. The scanner device is used to carry the two probes to scan the weld and record the signal at each position. Here, the step of the scanner is set to 1 mm, meaning that one signal is recorded every 1 mm. The used ultrasonic probes have a beam angle of 60\({}^{\circ}\) and central frequency of 5 MHz, and the sampling frequency is 100 MHz. As the defects exist in the lower part of the weld, the distance between two probes is set to 208 mm according to Equation (1), i.e., PCS = 252 mm. Figure 7 shows the experimental setup.  
Figure 8 shows an example of ultrasonic diffraction images obtained from the two welded joints. The image of Joint1 obviously demonstrates the five cracks in its middle while careful observation needs to be taken to see the cracks in the image of Joint2. The diffracted waves from the cracks at the root of Joint2 travel for so long that they suffer from large attenuation. Therefore, weak defect signals appear in its image, which increases the difficulty of defect evaluation, especially by manual means.  
Figure 6: Drawings of joints and simulation defects.  
Figure 8: Ultrasonic diffraction images obtained from the two welded joints: (**a**) image of Joint1; (**b**) image of Joint2.  
Figure 7: The ultrasonic diffraction equipment in experiments.