## 6 Experiments of UGW Testing  
UGWs can be excited and received based on the principles including but not limited to piezoelectricity, magnetostriction, electromagnetism and laser. Magnetostrictive UGW testing has ease of implementation, low cost, excellent sensitivity and durability, and has been widely used in the nondestructive testing of pipelines [27, 28]. The magnetostrictive effect is a coupling phenomenon involving the magnetization and the size/shape change of ferromagnetic material; an UGW could be excited by the positive magnetostrictive effect, which is reflected or refracted at pipe discontinuities or defects, and received by the inverse magnetostrictive effect, which can be analyzed to determine whether a defect exists [29, 30]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**FBR-RC-SSP-PT**} & \multicolumn{2}{c}{**FBR-RC-SSP-PTM**} & \multicolumn{2}{c}{**FBR-RC-SSP-SPT**} \\ \cline{2-7}  & **SNRG (dB)** & **DCRG (dB)** & **SNRG (dB)** & **DCRG (dB)** & **SNRG (dB)** & **DCRG (dB)** \\ \hline
1–60\({}^{\circ}\) & 15.07 & 14.15 & 15.08 & 14.14 & 7.50 & 7.37 \\
1–120\({}^{\circ}\) & 12.73 & 11.95 & 14.73 & 14.15 & 6.22 & 5.24 \\
1–180\({}^{\circ}\) & 12.70 & 12.05 & 14.84 & 13.85 & 6.67 & 6.48 \\
1–240\({}^{\circ}\) & 12.66 & 13.85 & 13.07 & 12.84 & 4.82 & 3.88 \\
1–300\({}^{\circ}\) & 13.33 & 13.57 & 14.84 & 14.09 & 5.57 & 5.19 \\
2–300\({}^{\circ}\) & 13.78 & 13.96 & 13.07 & 12.18 & 6.03 & 5.16 \\
3-300\({}^{\circ}\) & 12.12 & 12.96 & 13.94 & 12.13 & 6.04 & 5.13 \\ Mean & 13.19 & 13.21 & 14.22 & 13.34 & 6.12 & 5.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SNRGs and DCRGs of simulated UGW signals processed by FBR-RC-SSP. Figure 14: Simulated UGW signals before and after applying FBR-RC-SSP (1 \(\leq\) defect number \(\leq\) 3, 60\({}^{\circ}\)\(\leq\alpha\leq 300^{\circ}\)). To perform magnetostrictive UGW testing in this study, we developed a platform called GWNDT-III, which can generate excitation signals modulated by a power amplifier module and collect received signals modulated by a signal processing module, as illustrated in Figure 15. In order to excite as pure an L(0,2) UGW at 110 kHz as possible, the length of excitation coil \(l_{c}\) should be calculated by \(l_{c}\) = 0.5\(V_{p}\)/\(f_{c}\)[31] (\(V_{p}\) is the phase velocity, \(f_{c}\) is the central frequency). The utilized pipe was the same as the one used in simulation. The static bias and dynamic alternating magnetic fields were provided by a magnetostrictive patch (FeCo alloy) and an excitation coil, where the parameters of the excitation and receiving coils were the same and the magnetostrictive patch was bonded with the pipe using epoxy resin. Furthermore, the magnetostrictive patch was magnetized using a magnetizing coil through which direct current was passed, and the excitation coil was supplied with 20 V alternating voltage and placed at one end of the pipe in order to ensure that the UGW propagates in one direction; the distance between the excitation and receiving coils was 40 mm. The experimental UGW signals with different defect circumferential angles and numbers were obtained by GWNDT-III, as shown in Figure 16. It can be seen that the direct echo is wider and the signal resolution is lower compared with those of simulated UGW signals. The degree of distinction from coherent noise is not obvious for the low amplitude of the defect echo when 60\({}^{\circ}\)\(\leq\)\(\alpha\)\(\leq\) 180\({}^{\circ}\), and this phenomenon may also occur when there are two or three defects, which may lead to the wrong detection result and further reduce the signal interpretation accuracy. GS-SSP and FBR-RC-SSP were applied to the experimental UGW signals (1-2.5 ms), and the results are illustrated in Figure 17, where the dotted lines are the packet of the  
Figure 16: Experimental UGW signals with multi-defect (1\(\leq\) defect number \(\leq\)3, 60\({}^{\circ}\)\(\leq\)\(\alpha\)\(\leq\)300\({}^{\circ}\)). Figure 15: Schematic diagram of magnetostrictive UGW testing.