## 3 Experimental Results and Analysis
### AE Activity and Threshold Stress  
In AE tests, the ringing count represents the number of signal oscillations that cross the detection threshold. It can be used for reflecting both the frequency and intensity of the AE signal. Thus, in this study, the ringing count and cumulative ringing count were selected as the evaluation indexes of the AE activity. The curves of the applied stress, AE count and cumulative AE count versus the loading time are plotted in Figure 8.  
Based on the crack development of the samples under uniaxial compression, the corresponding AE activity can be classified into five stages. The characteristics of the stages are described as follows:  
Stage I: At this stage, the native defects slowly shut under the action of loads, which will not generate a great amount of strain energy. As a consequence, the AE activity is not very active. Thus, the quantity of detected AE count is relatively small and the cumulative AE count grows nonlinearly. Figure 7: Crack types and final failure modes of different groups of samples in uniaxial compression tests: (**a**) Sample C-3; (**b**) Sample S-2; (**c**) Sample H-3; (**d**) Sample D-1 and (**e**) Sample V-3. Stage II: As the stress rises, the samples deform elastically in this stage. Moreover, some micro cracks occur round the tips or corners of the natural defects. This gives rise to the growing increase of the released strain energy. Therefore, both the AE count and the cumulative AE count increase to varying degrees. In terms of the sample D-1, the reason why the AE signals are not remarkable at this stage may be that the coupling between the sensor and the rock sample is not very good during loading. Stage III: In this stage, the AE count of the sample remains basically the same and the cumulative AE count increases linearly. This is because the elementary tensile cracks appear and propagate stably along the loading direction under increasing loads. Therefore, this stage can be named the stable crack growth stage. Figure 8: AE responses of different groups of samples in uniaxial compression tests: (**a**) Sample C-3; (**b**) Sample S-2; (**c**) Sample H-3; (**d**) Sample D-1 and (**e**) Sample V-3. Stage IV: During this stage, the slabbing crack and posterior tensile crack occur rapidly. Consequently, the AE activity is extremely active, accompanying by several significant jump of AE count. This is induced by the rapid appearance of cracks or coalescence between cracks. It is noted that when the posterior tensile crack propagates to the V-shaped notch, the stress fluctuation is corresponded. Hence this stage is unstable cracking stage. Stage V: At the last stage, the shear cracks are formed and then intersect with the V-shaped notch or the posterior tensile crack. This triggers the instability of rock samples. Accordingly, the AE count increases drastically and the axial stress decreases to zero in a short time. Clearly, the variation of AE signals was agreeable with the crack development shown in Figure 6. Besides, according to the divided stages, it was obvious that several stress thresholds were related between the adjacent stages, namely, the crack closure stress (\(\sigma_{\rm c}\)) between the stages I and II, the crack initiation stress (\(\sigma_{\rm i}\)) between the stages II and III and the crack damage stress (\(\sigma_{\rm d}\)) between the stages III and IV as well as the peak stress (\(\sigma_{\rm p}\)) between the stages IV and V. For the \(\sigma_{\rm c}\), it corresponds to the axial stress when the stress-strain curve turns from nonlinearity to linearity. In regard to the rest stress thresholds, they can be determined by combining the crack development and AE signals; that is, if the strain localization and significant AE count occur simultaneously, new crack initiates or crack coalescence occurs. Table 4 lists the threshold stress values of the above five samples subjected to uniaxial compression.