## 4 Experimental Verification
### Results and Analysis of Experiment  
Through simulation comparison, it can be seen that the microcrack positioning imaging figures are ideal when the excitation and reception probes are all five and symmetrically distributed, so the experiment adopts the same arrangement and compares the microcrack positioning results at different positions. It can be seen from the positioning maps of Figure 16a-c that the experimental results are consistent with the simulation and the crack is effectively positioned. It can be seen from Figure 16d that, when there is no scanning center line passing through the microcrack, the positioning is not accurate.  
The evaluation parameters of the positioning result are shown in Table 3:  
As shown in the Figure 17 that in the four positioning maps, the minimum value of \(L_{d\_max}\) is 0.003. The minimum value of _Area-B_ is 0.018, and the minimum value of _Area-A_ is 0.002. The minimum values all appear in Figure 16c. In the actual engineering operation, by moving the scanning matrix, when the focus of multiple scanning center lines passes through the microcrack, the imaging index amplitude is inevitably the largest. By comparing the imaging index, the microcrack can be effectively located.  
However, the experimental plates originally had circular perforation and starter notch in order to prove that the method is only sensitive to microcrack and the nonlinearity is not produced by the circular perforation or the starter notch. As shown in Figure 18a, we only drill a hole and starter notch on the plate, we do not process microcrack, use the circle point as the coordinate origin, arrange the excitation and reception probes parallel to the x-axis direction, set the left side of the origin as the y-axis negative axis, and then scan and calculate the damage coefficient. As shown in Figure 18b, the damage index obtained in this case is compared with Figure 5b.  
Figure 16: Positioning results in experiment: (**a**) Microcrack in the right side; (**b**) microcrack in the lower right corner; (**c**) microcrack in the middle; (**d**) no scanning center line through microcrack.  
Figure 17: Evaluation chart.  
Figure 18: \(\beta\) in the sheet material without microcrack: (**a**) The probe layout without crack; (**b**) distribution pattern of \(\beta\).  
When there is only a circular perforation in the plate, the damage index is extremely low, and the difference in the damage index detected at different positions is small. It can be considered that the circular perforation in the plate does not have nonlinear characteristics. The imaging index amplitude of the corresponding area of the circular hole in the imaging three-dimensional map is lower. When there is microcrack in the plate, the scanning path at 10 mm from the microcrack passes through the starter notch. The damage index is lower than that of the scan path when it passes through the microcrack, and the damage index is almost the same as that of the case where only the round hole exists. The starter notch also does not produce nonlinear features. Finally, it can be proved that the positioning imaging algorithm used in this paper is only sensitive to closed microcracks and can locate the damage very efficiently.