## 2 FPAPr Design and Fabrication
### FPAPr Fabrication  
The FPAPr was fabricated using a composite material consisting of lead zirconate titanate (PZT) and epoxy resin, as illustrated in Figure 9, which was combined with a polyamide film that included a connector for transferring data. The piezoelectric element was fabricated in a neoprene rubber layer housing. Flexibility and bending functions for closely adhering to the curvature-shaped weld zone enabled accurate inspection data acquisition and scanning. The elements were designed to place their matched layer and damper at the end of one side to maintain sufficient accessibility at the weld zone [46; 47; 48; 49; 50]. Figure 10 shows the shape of the fabricated FPAPr. The two types of probes were fabricated as a one-dimensional (1D) linear structure P/E type and a dual linear structure T/R type, as shown in Figure 10a,b, respectively. Figure 8: Simulation results from various curvature specimens; (**a**) upper notch of the flange-elbow specimen; (**b**) lack of fusion of the tapered elbow specimen; (**c**) upper notch of the upper-tapered specimen. Figure 7: Scan plan drawing for simulation; (**a**) index offset 8 mm, sweep angle 35–75\({}^{\circ}\) assigned for the flange–elbow structure; (**b**) index offset 20 mm, sweep angle 35–70\({}^{\circ}\) assigned for tapered elbow structure; (**c**) index offset 62 mm, sweep angle 35–70\({}^{\circ}\) assigned for tapered upper structure. The FPAPr was designed as a broadband system by widening the frequency bandwidth to a center frequency of 5 MHz to increase the flaw detection resolution above a certain sensitivity. Then, the frequency of the designed FPAPr was analyzed. The center frequency and the frequency bandwidth of each element were analyzed, and the result was presented as an average value for all channels. Figure 11 shows the frequency spectrum and the radio frequency (RF) signal received at the element for analyzing the FPAPr frequency. The JPR-50p from Japan Probe Co., Ltd. was used to analyze the frequency spectrum, wherein the device settings were 0.2 MHz and 100 MHz for the high-pass filter and sampling frequency, respectively [51]. Additionally, the signal was acquired by perpendicularly propagating an ultrasonic beam from a 25 mm thick carbon steel. Figure 11b,d present the 16 th channel frequency analysis results from the P/E type and T/R type, respectively, where the center frequencies were 5.4 MHz and 4.6 MHz, respectively [52; 53]. The bandwidths of the frequencies were greater than or equal to 110% and exhibited broadband features. Figure 11: _Cont._  
Figure 10: Images of phased array probe excluding solid wedge; (a) a photo of an FPAPr manufactured in P/E type; (b) a photo of an FPAPr manufactured in T/R type. Figure 9: Images of phased array probe with a flexible structure geometry: The cross-section of T/R probe. The center frequencies measured in all channels were plotted on a graph in Figure 12. The results show similar center frequencies for all channels. The P/E type average center frequency was about 5.4 MHz, and the T/R type average center frequency was about 4.6 MHz. This satisfies the ISO 18563-2 standard of permission of \(\pm\)10%. The difference between the average center frequency and each element frequency was a maximum of 6.5% and minimum \(-\)7.0%, which satisfies the \(\pm\)10% standard of permission of the same standard. The fabricated FPAPr had probes with 32 arrays where each channel was required to show a constant amplitude. This was because flaw detection and quantitative size evaluation are only possible when the amplification rate is constant with the time axis linearity and amplitude linearity for each channel. Figure 13 shows the measured amplitude of each channel. The sensitivity deviation of the P/E and T/R type FPAPr were found to be 2.8 dB and 1.5 dB, respectively. They satisfied the ISO 18563-2 standard of permission of \(\pm\)3 dB. Table 3 summarizes the aforementioned results and the FPAPr specifications.