## 4 Results and Discussion
### Comparison of FPAPr and Conventional PA Probe  
The FPAPr and conventional PA probe flaw detection sensitivities were compared to verify the performance of the FPAPr, and the results were expressed in terms of their amplitudes. In Figure 20, the angles for each inspection position are specified, as specimens with various curvatures had different contact surfaces depending on the inspection position. All experiments were performed based on a bidirectional inspection. However, data acquisition from the conventional PA probe inspection was problematic, owing to unstable  
Figure 19: S-scan image results of flaws detected by the FPAPr on the pipe specimens: (**a**) flange–elbow weld zone surface notch flaw image; (**b**) tapered elbow weld interior LoF flaw image; (**c**) upper-tapered weld upper notch flaw image.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Flaw Type** & **Length (mm)** & **Width (mm)** & **Height (mm)** \\ \hline Notch & 6, 10–30 & 1–2 & 0.6–2 \\ LoP & 10–20, 5 & 2 & 2–3 \\ LoF & 6–10, 12 & 2 & 2–2.5 \\ SDH & 25 & - & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 5: The dimension of the defects presents inside each specimen.  
curvature surface contact. Therefore, the data were obtained from the one side that could be inspected. Additionally, a scanner that suited each purpose was used in all the experiments.  
Figure 21 shows the amplitude comparison between a total of 17 flaws on the flange-elbow-elbow specimen. A relatively small 4 MHz probe and the FPAPr were used for the flange-elbow-elbow specimen with a large curvature and a small outer diameter (OD). The bidirectional inspection was performed in the experiment, but the general probe was only able to inspect one side due to the curvature at the bending near the flange-elbow (0\({}^{\circ}\)) and the overall area of the elbow-elbow. Therefore, low sensitivity at the flaw and flaw detection failure was observed in Figure 21a (7) and Figure 21b (4) and (8), respectively.  
Figure 22 shows the amplitude comparison between a total of 12 flaws on the flange-reducer-elbow specimen. Similar to the flange-elbow-elbow inspection, a 4 MHz probe and the FPAPr were used. Near the 0\({}^{\circ}\) of the flange-reducer and the 180\({}^{\circ}\) and 270\({}^{\circ}\) of the reducer-elbow, only one side could be inspected with the general PA probe due to the specimen curvature. Experimental results were similar for the three types of probes. Despite inspecting only one side of the specimen, the PA probe showed results similar to those of the FPAPr.  
Figure 20: Angles for each inspection position on the experimental welding test pieces: (**a**) 100 mm curvature pipe with elbow–flange construction geometry; (**b**) 75 mm, 100 mm curvature pipe with elbow–flange-reducer construction geometry.  
Figure 21: Flange–elbow–elbow specimen inspection results: (**a**) flange to elbow; (**b**) elbow to elbow.  
Figure 23 shows the results for the tapered elbow and branch specimens. A 32-channel 5 MHz probe with a long near field was added to the PA probe. The relatively large 5 MHz probe showed low amplitudes due to the specimen curvatures, as shown in Figure 23a. In flaw (7), flaw detection failed in both directions due to poor contact. Contrarily, for the branch specimen in Figure 23b,c, the FPAPr showed results similar to those of the PA probe because there was devoid of surface curvatures. Therefore, this implied that the FPAPr was applicable in general specimen environments.  
Figure 23: Tapered elbow and branch specimen inspection results: (**a**) tapered elbow, (**b**) branch pipe, (**c**) branch pipe.  
Figure 22: Flange–reducer–elbow specimen inspection results: (**a**) flange to reducer; (**b**) reducer to elbow.