## 3 Results and Discussion  
### Numerical Simulation Results  
To investigate the resolution capabilities of three imaging methods for multiple damages, numerical simulations were conducted on double through-hole damages distributed parallel to the array. The damage radius r = 2.5 mm and the imaging results of different damage center distances are shown in Figures 9-12, with a comparison of the results presented in Table 2.  
Figure 8: (**a**) Experimental equipment connection schematic diagram. (**b**) Experimental platform. (**c**) PZT actuator array. Figure 10: Imaging results and imaging indicator profile of numerical simulations with two damages separated by 10 mm. (**a,d**) Time-domain DAS. (**b,e**) Frequency-domain DAS. (**c,f**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). Figure 9: Imaging results and imaging indicator profile of numerical simulations with two damages separated by 20 mm. (**a,d**) Time-domain DAS. (**b,e**) Frequency-domain DAS. (**c,f**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). Figure 11: Imaging results and imaging indicator profile of numerical simulations with two damages separated by 8 mm. (**a**,**d**) Time-domain DAS. (**b**,**e**) Frequency-domain DAS. (**c**,**f**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). Figure 12: Imaging results and imaging indicator profile of numerical simulations with two damages separated by 6 mm. (**a**,**d**) Time-domain DAS. (**b**,**e**) Frequency-domain DAS. (**c**,**f**) DORT-MUSIC. The color scale represents normalized intensity, and the red circles and lines indicate damage edges. Spatial coordinates are normalized relative to the wavelength (\(\lambda\)). As the results show above, the coordinates of the local damage imaging plane and the peak intensity profile of the damage index are converted to ratios of spatial scale and the central wavelength of guided waves in the structure (\(\lambda=7.6\) mm). The red dashed line in the profile indicates the edge position of the damage. The damage diameter is 5 mm, approximately \(0.66\lambda\), and the minimum distance between damages is \(d=6\) mm, approximately \(0.79\lambda\), both of which are close to the diffraction limit. The results of various imaging indicators are shown in Table 2. Firstly, the time-domain DAS is limited by its relatively low circumferential (parallel to the array, same below) resolution, and under the different damage distance conditions mentioned above, it is unable to clearly distinguish between dual damages, only indicating the presence of damage in the structure with a single damage index peak and relatively low positional accuracy. This is mainly because the time-domain DAS cannot account for the dispersion effect of guided waves, leading to mispositioning and lower imaging resolution as it approximates the inversion of the full frequency range of guided wave signals using the phase velocity corresponding to the central frequency. As the distance between damages decreases, the API indicator of time-domain DAS imaging also gradually decreases. This is due to the overlapping of dual damages resulting in a single damage peak distribution range that declines as the damages approach, rather than an improvement in imaging resolution. Additionally, since the presence of dual damages cannot be distinguished, the peak center intensity difference index \(\tau\) cannot be used to indicate changes in resolution under these conditions. The advantage of the time-domain DAS lies in its imaging efficiency. Since it does not require resolving and calculating delay superposition for each frequency, it can achieve near real-time imaging speeds.