## 7 Conclusions  
This paper investigated the second defect in laminated pipeline structures, and determined the relative axial position between the first and second defects, based on the damage index value. The relationship between the relative axial position of the damage and the damage index value was obtained through finite element simulation, and the accuracy of this relationship was verified experimentally. The specific relationship can be summarized as follows:  
1. When comparing the damage index values (\(I\)) of double defects at the same and different circumferential positions with those of single defects in pipeline structures, it was found that within the range of \(1.7\lambda\leq PD\leq 3.4\lambda\), the order of damage index values was \(I_{\text{double defect 90}^{\circ}}<I_{\text{signal defect}}<I_{\text{double defect 0}^{\circ}}\). Based on this relationship between the damage index values of double and single defects in pipeline structures within the range of \(1.7\lambda\leq PD\leq 3.4\lambda\), simulations were conducted to obtain the damage index value for a single defect. The wavelength \(\lambda\) of the guided wave was calculated using the relevant parameters, and the simulated damage index value was compared with the measured value obtained from actual engineering applications, to locate the relative axial position of the defects. These findings highlighted the importance of accurately detecting and repairing defects in pipeline structures, especially double defects, to ensure their safe and reliable operation. 2. When comparing the damage index values (\(I\)) of double defects at the same and different circumferential positions with those of single defects in the pipeline structures, it was found that when \(3.7\lambda\leq PD\leq 4\lambda\), the order of damage index values was \(I_{\text{double defect 90}^{\circ}}<0.3<I_{\text{double defect 0}^{\circ}}<I_{\text{signal defect}}\). Within the range of \(3.7\lambda\leq PD\leq 4\lambda\), the reflected waves from the two defects began to separate. At this point, the relative axial position of the defects could be located through two main methods. Firstly, as the reflected waves from the defects began to separate, the relative axial position of the defects could be calculated using the pulse-echo method. Secondly, based on the relationship between the damage index values of double and single defects within this range, simulations could be conducted to obtain the damage index value for a single defect. The wavelength \(\lambda\) of the guided wave was calculated using the relevant parameters, and the simulated damage index value was compared with the measured value obtained from actual engineering applications, to locate the relative axial position of the defects. 3. In order to investigate the impact of changes in the relative circumferential positions between double defects on the damage index (\(I\)) in pipeline structures, we compared the values of \(I_{\text{double defect 90}^{\circ}}\) and \(I_{\text{double defect 90}^{\circ}}\). Our study revealed that within the range of \(1.7\lambda\leq PD\leq 4\lambda\), changes in the relative circumferential positions of the