# An Omnidirectional Near-Field Comprehensive Damage Detection Method for Composite Structures
## 4 Experimental Results Analysis  
The positions of the piezoelectric elements and single damage point P are shown in Figure 9. Using 401 glue paste, a hollow hexagon screw with a diameter of 13 mm was used to simulate structural damage whose polar coordinates were \(P\) (90 mm, 109\({}^{\circ}\)) and whose Cartesian coordinates were \(P\) (\(-\)29.3 mm, 85.1 mm). Thus, according to (2), \(P\) was located in the near-field region. Taking array I with the excitation at the piezoelectric element 0 and piezoelectric sensor 6 as an example, the influence of a tank structure on the Lamb wave signal propagation was analyzed. In the experiment, an excitation signal with a central frequency of 50 kHz was used, and the sampling frequency was 4 MHz. Additionally, 2000 points before the data sampling point were used in the calculation. The excitation signal of the piezoelectric element 0 is shown in Figure 10a, and the reflection signal of the Lamb wave received by the piezoelectric plate 6 is shown in Figure 10b. As shown in Figure 10b, the received signal contained a lot of overlapping reflected signals from the Lamb wave signal, which resulted from the reflection from the bottom boundary. At last, there were mixed, reflected Lamb wave signals, which resulted from the reflection from the left and right boundaries above the array edge. In Figure 10b, it can be seen that as the propagation distance increased, the attenuation of the Lamb wave increased relatively fast. Therefore, in order to identify the damage and make it easier to determine, we used the damage scattering signal; in other words, we used the difference between the sensor signal collected in the health state and the sensor signal collected in the damage state to analyze the structural damage. Figure 8: The tank speed curve. Figure 9: The locations of the piezoelectric elements and single damages. As shown in Figure 11, in array I, the piezoelectric element 0 generated the excitation signal, and the other sixth piezoelectric elements received the signal. A received sensing signal containing no damage was considered as a health signal, as shown in Figure 10(a). A received sensing signal containing any kind of damage was considered as a damage signal, as shown in Figure 10(b). The difference between the health and damage signals denoted the damage scattering signal, as shown in Figure 10(c). In Figure 10(c), the front signal denotes the crosstalk signal, having the same position as the excitation signal. The damage scattering signal originated from the Lamb wave signal generated by the excitation signal in the plate and reflected by the damage. The final damage scattering signal in Figure 10(c) was produced by the Lamb wave signal generated by the excitation signal in the plate, then reflected by the river boundary, and lastly reflected by the damage. Data acquisition was only required to make all the array elements generate the excitation signal while the remaining elements were doing the sensing, without distinguishing the angle scanning. As mentioned previously, array I was along the \(x\)-axis direction, and array II was along the \(y\)-axis direction. During data acquisition, the health and damage signals were collected for each excitation-sensing combination made of seven piezoelectric elements, where one of the seven piezoelectric elements generated the excitation signal and the other six piezoelectric elements sensed and received the scattered signals generated in the structure. The damage scattering signal of each piezoelectric sensor was obtained by subtracting the corresponding health signal and damage signal. The near-field phased-array sampling damage  
Figure 11: The sensor and damage scattering signals. Figure 10: The excitation signal and the sensor signal. imaging monitoring method presented in Figure 3 was used to determine the time delay of the damage scattering signals.