## 3 Results Analysis
### Classification of Crack Types  
The magnitude of the shear crack is more significant than the tensile crack, and the AE amplitude can represent the crack scale. Therefore, a preliminary classification might be based on the amplitude for the co-existence of tensile and shear cracks in the low peak frequency band. After combining all the AE signals from the six experiments, the amplitude distribution statistics were determined for the medium- and high-frequency bands. In the low-frequency band, as seen in Figure 6, according to the prior analysis, it can be shown that 55 dB is a critical threshold value for the assessment of the crack scale. In the medium- and high-frequency bands, the percentage of AE signals with an amplitude below 55 dB is around 86.5%, whereas in the low-frequency band, the percentage is about 81.1%. The signals with low amplitude account for a more significant percentage, and the high-amplitude signal in the low-frequency band accounts for a more substantial percentage than that in the medium- and high-frequency band. Depending on the crack mode, crack-extension incidence produces different AE signals. Crack-extension modes involving relative motion of the two sides of the crack produce AE waveforms with short rise times and high frequencies. In contrast, shear cracks typically produce waveforms with lower frequencies and longer rise times, in other words, higher amplitudes [26]. Therefore, those with higher amplitudes in the lower-frequency range are shear cracks, and the rest of the signal then becomes tensile cracks. We considered signals with amplitudes \(\geq\)55 dB in the low-frequency band as shear cracks and other signals as tensile cracks. This rule was used to classify the crack types in each of the six tests. Figure 5: Peak frequency distribution characteristics of AE signal in three types of tests (moving average 50). (**a**) Specimen T2; (**b**) specimen CS2; (**c**) specimen TS2. Figure 6: Amplitude distribution characteristics in medium- and high-frequency bands and low-frequency band. (**a**) Medium- and high-frequency bands; (**b**) low-frequency band. To verify the reliability of the crack classification method based on AE amplitude and peak frequency, the RA-AF value-based method recommended by the JCMS-III B5706 was further adopted for crack classification [27]. The calculation method of RA and AF values and the crack classification principle are shown in Figure 7. Different types of crack propagation produce different waveform characteristics. Cracks appear far from the tip under tension, resulting in a shorter rise time, and the AE waveform that was collected has a higher frequency. Compared to tensile cracks, shear cracks look like longer energy waves, which produce signals with longer rise times and lower frequencies. Tensile cracks therefore generally have high AF values and low RA values, while shear cracks are the opposite. Based on this method, the crack types of the above six groups of tests were divided, as shown in Figure 8. The RA and AF values were calculated using a moving average of more than 50 counts (moving average). It should be noted that the RA-AF-based method is also an empirical discrimination method. The tensile and shear crack classification results are not entirely accurate and cannot identify the tensile-shear mixed cracks. Still, the results of existing studies show that it can meet the needs of analyzing the evolution and percentage of various kinds of cracks in the process of rock failure. Table 3 displays the crack categorization findings based on the two approaches. The percentage of shear cracks obtained by the two methods is significantly different for T1 and T51, \(-\)30.5% and 9.8%, respectively. However, for the other four tests, the classification results of the two methods are consistent within 5%. This confirms the logic of the correspondence between frequency bands and crack types, as mentioned previously.