## 3 Results
Figure 9a,b present the maximum amplitudes (absolute values) of the ultrasonic signals obtained at 30 kHz for both materials. Similarly as for the delaminated board, the results are shown for the \"healthy\" boards and the boards with defects of selected lengths of 3, 5, 10, and 15 mm. Again, the signal amplitudes measured in the polyethylene boards are much higher (about 190 mV) than in polyamide. The third type of defect was discontinuities in the central part of the tested board, which are presented in Figure 10a,b. It is visible that the measured amplitudes (absolute values)in the \"healthy areas\", where the defects are not present, have almost the same values. It demonstrates the good repeatability of the experimental procedure. It is shown that the defects affect the amplitudes of the signals only in areas close to them. The defect manifests as fluctuations in the amplitude of the Lamb wave. The fluctuations may arise from the overlapping signals reflected/refracted from the defect's edge. In both boards, the bigger crack leads to higher fluctuations and a decrease in the wave amplitude measured. The Figure 11 summarizes the results obtained for all types of cracks. The amplitude reduction of transmission of Lamb waves over notches in the function of their depths is shown (Figure 11a shows Lamb wave amplitude vs. delamination length, Figure 11b amplitude vs. crack length while Figure 11c amplit  
Figure 10: Amplitude vs. probe position for ruptured boards. (**a**) Polyethylene. (**b**) Polyamide. The red box represents the area where defects were generated. Figure 9: Amplitude vs. probe position for cracked boards. (**a**) Polyethylene. (**b**) Polyamide. The red box represents the area where defects were generated. In the polyethylene plate, for the 5 mm defect, the decrease in signal amplitude is observed at the level of 4 dB in comparison to the signal amplitude recorded for the plate with no defects. In a polyamide plate, a defect of the same length (5 mm) decreases signal amplitude by about 3 dB. Compared to the amplitudes measured for the delaminated board (Figure 10(a)), the decrease in the amplitudes for cracked boards (Figure 10(b)) is much smaller as a function of their size. It can be attributed to the fact that during the propagation of antisymmetric (flexural) mode, the motion of the particles is perpendicular to the board surface, and delamination affects the Lamb wave more. For the ruptured boards, the fluctuations of wave amplitude became visible only when the length of the defect exceeded 5 mm. In the polyethylene board, the 5 mm crack led to an amplitude decrease of approx. 3 dB, while on the polyamide board, it is 7 dB. The input data for the ML analysis of the raw data were position and amplitude, and for the output data were defect length. The preliminary results obtained on the raw data using ML-NET were:  
* for the data from Figure 7a: classification: 0.1789, prediction: 0.7501 (SdcaRegression algorithm),
* for the data from Figure 7b: classification: 0.3280, prediction: 0.6027 (FastTreeRegression algorithm),
* for the data from Figure 7a: classification: 0.3006, prediction: 0.4762 (LightGbmRegression algorithm),
* for the data from Figure 7b: classification: 0.2708, prediction: 0.5463 (FastTreeRegression algorithm),
* for the data from Figure 7a: classification: 0.4359, prediction: 0.4949 (LightGbmRegression algorithm),
* for the data from Figure 7b: classification: 0.4860, prediction: 0.5236 (FastForestRegression algorithm). As we have already written, the above data were aggregated into one dataset and cleaned of outliers and uncertain and incomplete data. The input data for the ML analysis of the aggregated experimental data were position, amplitude, defect type, and material type and the output data were the defect length. The number of samples was 1266. The results have significantly improved for both classification (Table 3) and prediction (Table 4).