# Damage Detection Method Based on Continuous Wavelet Transformation of Lamb Wave Signals
## 6 Experimental Evaluation  
The accuracy of the proposed method was further tested through experimental investigation performed on an \(800\times 800\times 1.5\) mm\({}^{3}\) aluminum plate with PZT wafers arranged in the form of two square cells of \(300\times 300\) mm\({}^{2}\) and \(240\times 240\) mm\({}^{2}\), as shown in Figure 26a. The minimum size of the damage that can be estimated using the Lamb wave should be equal to or greater than half of its wavelength. For 383 kHz frequency excitation and 5.31 \(\times\) 103 m/s group velocity, the wavelength was about 14 mm, therefore, a damage was formed as a punch hole of 7 mm diameter located at \((-90,30)\) mm. The size of each PZT wafer used in experiments was 5.4 mm diameter, and they were made up of PSN-33 with a density of \(7.70\times 10^{3}\) kg/m\({}^{3}\). The PZT wafers had a resonance frequency of 383 kHz, and they could excite/sense the Lamb wave signals in radially outward directions. The real-time laboratory setup of the experiment is shown in Figure 26b. The signal generator (RIGOL, DG1022) was programmed to generate the excitation signal shown in Figure 3a. The signal was amplified to 4.2 V using a power amplifier (KROHN-HITE, 7602M). The dynamic response signals were recorded at each receiving transducer using a four-channel oscilloscope (LeCroy, LC574 AL). The response signals were captured over a sampling time of 200 \(\upmu\)s at the sampling rate of 1 GHz. In order to reduce the noise, each signal was acquired for an average of 500 times. When A1 excited the Lamb wave signal, the damaged signal recorded at B1 is presented in Figure 27a, which contains three fluctuations: excitation, damage-scattered, and edge-scattered. Similarly, when A2 was excited, the recorded signal at B2 is presented in Figure 27b, which contains two fluctuations: one is excitation fluctuation and another is damage-scattered overlapped with edge-scattered. The envelopes of the coefficient of CWT for signals at A1B1 and A2B2 are presented in Figure 27c,d, respectively. The time compensation was already made in the experimental signals during the data recording as the oscilloscope was set to trigger or start recording when peak of the voltage signal appears. The damage possibilities for sensing paths A1B1 and A2B2 are presented in Figure 28a,b, respectively, along with actual size and location of damage D3. The damage-scattered fluctuation in Figure 28b is overlapped with edge-scattered fluctuation, therefore it does not represent the correct possibilities of damage existence. In the current experimental evaluation, the baseline signals from the undamaged plate were not utilized, therefore the damage possibilities from sensing pair A2B2 were not used in the aggregate damage imaging. Similarly, the damage possibilities for all useful sensing paths in the pulse-echo configuration were evaluated and fused to get the aggregate damage image, presented in Figure 29.  
Figure 25: Baseline-free aggregate damage image for the multiple damage scenario in pulse-echo configuration with SNR 30 dB: (**a**) without time compensation; (**b**) with time compensation. Figure 28: Possibilities of damage existence for sensing path: (**a**) A1B1; (**b**) A2B2. Figure 26: Experimental evaluation: (**a**) schematic diagram; (**b**) laboratory setup. Figure 27: (**a**) Damaged signal at A1B1; (**b**) damaged signal at A2B2; (**c**) coefficient of CWT at A1B1; (**d**) coefficient of CWT at A2B2. The results validate that the proposed method can locate the damage with relatively good accuracy also through experimental data. The method proposes the arrangement of PZT transducers as a network of square detection cells on a large structure. Since, the wave velocity and location of the plate edges are already known, the total time for wave propagation can be calculated accordingly to avoid the edge scatterings for all the central cells.