## 5 Experiment  
To verify the accuracy of the finite element simulation, and the correctness and applicability of the conclusions in this study, a piezoelectric ultrasonic guided wave damage identification test system was established for a laminated pipeline structure, based on six working conditions in the finite element simulation. The damage index values obtained from the test system were then compared with those obtained from the finite element simulation. The test system was based on four laminated pipes, and six sets of damage identification tests were conducted. For the reader's convenience, the working conditions established in the test system have the same numbers as those in the finite element simulation. The information regarding the damage location and other details are shown in Table 10. \begin{table}
\begin{tabular}{c c c c} \hline
**Content of Influence** & \(PD/mm\) & **Performance** & **Reasons** \\ \hline \(I\) value & \(750\leq PD\leq 1800\) & \(I_{\text{double defect 0}^{\circ}}>I_{\text{double defect 90}^{\circ}}\) & Changing the circumferential position caused a phase difference, which led to a decrease in the overall waveform amplitude. \\ \hline \(I\) value & \(750\leq PD\leq 1350\) & \(I_{\text{double defect 90}^{\circ}}\) showed a & When \(I\leq 1.36\lambda\), the phase difference caused the reflection echoes to cancel out or reinforce each other. \\ \(1.7\lambda\leq PD\leq 3\lambda\) & \(I_{\text{double defect 90}^{\circ}}\) showed a & In addition to an increase in \(I\), the phase difference also played a role. \\ \hline \(I350\leq PD\leq 1500\) & \(I_{\text{double defect 0}^{\circ}}\) and \(I_{\text{double defect 90}^{\circ}}\) & The damage echo of the damage gradually separated, and the pulse width of the waveform increased. \\ \hline \(I\) & \(I_{\text{double defect 0}^{\circ}}\) showed a slow & At this point \(I>2.04\lambda\), the reflection echoes were completely separated, and the amplitude of the waveform decreased. \\ \(1500\leq PD\leq 1800\) & \(I_{\text{double defect 90}^{\circ}}\) showed a rapid & The phase difference caused the overlap of the waveform between the reflection echoes of the defect and the boundary reflection echoes. \\ \hline \end{tabular}
\end{table} 
Table 9: Comparison of damage index values based on different circumferential positions. The geometric and material parameters of the laminated pipe structure used in the test system were the same as those in the finite element simulation, as shown in Table 1. The radial cross-section of the laminated pipe structure can be seen in Figure 13.  
Due to the fact that the laminated pipe structure was covered with an outer layer, piezoelectric sensors were placed at the end of the structural layer, to ensure the integrity of the outer layer. In this test, 12 mm \(\times\) 6 mm \(\times\) 1 mm PZT-4 was used as the actuator and sensor, and the number and arrangement of the sensors were the same as those in the simulation. First, two intact pipeline structures were selected for single-defect tests, such as case 5 and case 8. Based on the single-defect pipeline structures, double-defect tests were conducted at the same circumferential position, such as case 13 and case 16. Then, two intact pipeline structures were selected for double-defect tests at different circumferential positions, such as case 21 and case 24. The circumferential dimensions of the damage studied in the experimental system were consistent with those in the finite element simulation, and the damage schematics are shown in Figure 3. A L (0,6) mode, with a central frequency of 70 kHz and a 5-cycle single-tone sine wave superimposed signal, was selected as the excitation guided wave. The experimental system setup is shown in Figure 14.