## 5 Experimental Research on the Positioning of Armored Composite Materials
### Experimental Research on Single Damage Imaging and Localization  
In order to conduct experimental research on damage imaging and localization in armored composite material structures, as shown in Figure 6, a sandwich armored composite material was used as the experimental object. Four piezoelectric sensors are arranged on the structure, with the origin set at sensor 1. The diameter of the PZT sensor is 8 mm. The resonant frequency of PZT is 2 kHz. The coordinates of sensor 2 are (160 mm, 0 mm); sensor 3 is located at (160 mm, 160 mm); and sensor 4 is located at (0 mm, 160 mm). All sensors are connected into the monitoring PC interface, which has one excitation PCI interface card, one eight-channel piezoelectric signal condition PCI interface card, and one processing card. The excitation PCI interface card is in charge of the excitation signal to omit the excitation sensor, and the piezoelectric signal condition PCI card is in charge of receiving the response signal from responding sensors. The excitation signal was configured as a narrowband modulated five-cycle sinusoidal signal with a center frequency \(\omega_{c}\) of 50 kHz. The sampling system was configured with a sampling frequency of 5 MHz and a total sampling length of 10,000 points, which included a pre-sampling length of 2000 points. The triggering voltage was set to 300 mV. Damage simulation was conducted using weights and absorbing rubber. The damage locations were designated at the central position of the armored composite material structure and the four corner regions, with the specific coordinates provided in Table 6. \begin{table}
\begin{tabular}{l c} \hline \hline
**Serial Number** & **Coordinates/mm** \\ \hline Simulated damage 1 & (70, 120) \\ Simulated damage 2 & (120, 80) \\ Simulated damage 3 & (130, 130) \\ Simulated damage 4 & (50, 40) \\ Simulated damage 5 & (80, 80) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Coordinates of simulated damage locations. Figure 6: Schematic diagram of the experimental system for damage imaging and localization and the arrangement of piezoelectric sensors. The approximate operational procedure for the damage imaging and localization experiment is illustrated in Figure 7. Initially, the structure is in a healthy status. We set three operation procedures for the experiment. First, the first sensor is set as excitation, and we can obtain response signals from the second, third, and fourth receptors. Then, the second sensor is set as excitation, and we can obtain a response signal from the third and fourth receptors. Finally, the third sensor is set as excitation, and we can obtain a response signal from the fourth receptor. In each setting, the excitation sensor emits the excitation signal, and receptor sensors receive the response signals, which comprises a total of six baseline signals. In contrast, as the structure is in a damaged status, the same channel parameters were maintained, and the signals were repeatedly collected as damage signals in damage status. This process was repeated to acquire a total of 20 sets of signals. In a healthy or damaged status, the response signals from each excitation sensor channel are processed using the Shannon continuous complex wavelet transform to extract the narrowband signals with a center frequency. The damage-scattered signals for each excitation sensor channel were determined. The previously obtained propagation velocity (group velocity \(c_{g}\)) of the narrowband signal with \(\omega_{c}=50\) kHz inside the structure was employed, as calculated in the previous experiment. The distances from the hypothetical damage location \((x,y)\) within the structure to the piezoelectric sensors in each excitation sensor channel were computed. Subsequently, the damage-scattered signals were phase-combined, and the amplitude at that location was utilized as the pixel value for the image. The imaging process was conducted for all unit points within the entire region, resulting in the generation of the damage imaging.