## 3 Testing Results
### AE Characteristics
#### 3.2.3 AE Hit Count  
Figure 7 shows the results of the AE hit count of the No. 4 coal specimen. The AE hit count of the coal specimen presents the following evolution laws: In the linear loading phase and the initial two loading-unloading cycles, the AE hit count is very low. This shows that the coal specimen is mainly compacted, and there occur almost no new cracks. From the third loading-unloading cycle on, the AE hit count began to increase with the improvement of cyclic loading-unloading maximum stress. It shows that the cracks in the coal specimen begin to initiate, expand, and penetrate. When the coal specimen failed, the AE hit count reached the maximum. However, it should be noted that during the 1505-1506 s, the maximum value of the AE hit count is greater than that of the next cycle but less than that of the coal specimen failure. This shows that a small-scale local failure emerged in the coal specimen at that time, and the internal structure of the coal specimen quickly reached a new stable state after readjustment. In addition, before the coal specimen failure, the AE hit count is very high in the loading phase of each cycle but very small in the unloading phase of each cycle. This shows that when the loading and unloading do not cause the failure of the coal specimen, the AE activities mainly occur in the loading phase, and there exist basically no AE activities in the unloading phase.  
Figure 5: AE ring count and axial stress versus time curves for coal specimen subjected to cyclic loading and unloading. (**a**) AE ring count scale not magnified; (**b**) AE ring count scale magnified \(\sim\)5X.  
Figure 6: AE energy count and axial stress versus time curves for coal specimen subjected to cyclic loading and unloading. (**a**) AE energy count scale not magnified; (**b**) AE energy count scale magnified \(\sim\)4\(\times\).  
The above results show that the evolution law of cumulative AE hit count is the same as the cumulative AE energy count, also increasing in a stepped shape with the cycle number. That is, the cumulative AE hit count increases in the cyclic loading phase and keeps stable in the cyclic unloading phase.  
AE hit count reflects the crack propagation rate and can better indicate the active degree of the AE phenomenon. In a loading-unloading cycle, it is inaccurate to use the maximum hit count to reflect the AE activities because the maximum hit count only reflects the AE active degree and crack growth rate at a certain time point. The AE active degree of loading-unloading cycle should be characterized by the cumulative AE hit count in this cycle. Figure 8 shows the cumulative AE hit count of different cycles in loading, unloading, and the whole loading-unloading phases.  
From Figure 8, both the cumulative AE hit count, and the AE hit count in the cyclic loading phase increase gradually with the cycle number. However, the AE hit count in the cyclic unloading phase basically keeps stable except for the failure cycle.  
To investigate the loading history effect on AE activities, the cumulative AE hit count at several key stress points in different loading-unloading cycles is recorded and analyzed, as shown in Table 2. From Table 2, in the cyclic loading phases with the equivalent stress amplitude, the AE activities of the coal specimen decrease gradually with the loading-unloading cycle number.  
Figure 8: Cumulative AE hit count in different loading–unloading cycles.  
Figure 7: AE hit count and axial stress versus time curves for coal specimen subjected to cyclic load and unloading. (**a**) AE hit count scale not magnified; (**b**) AE hit count scale magnified \(\sim\)2\(\times\).