# Mechanical Behaviour of Rock Samples with Burst Liability Under Different Pre-Cycling Thresholds
## 3 Results
### AE Signal Characteristics of Rock Samples with Burst Liability  
Figure 6 shows AE signal characteristics of rock samples with burst liability. Different pre-cycling thresholds had a certain effect on AE count and cumulative AE count. The AE count and cumulative AE count were different in details. However, their evolution trends were basically consistent. The AE count and cumulative AE count showed the same changing trend: slow increase \(\rightarrow\) gentle development \(\rightarrow\) sharp rise. The sub-figures in Figure 6a,c,e are the AE failure source location in each stage under different pre-cycling thresholds. Pink dots were used to represent the failure dislocation of rock samples. Figure 5: Microstructure characteristics of rock samples with burst liability. AE signals can reflect crack generation and development of rock sample failure. The evolution trend obviously corresponded to the deformation and stress changes of rock samples during the loading process [38]. Based on AE frequency differences, [PERSON] et al. [39] divided the stress state curves of rock samples into the compaction stage, elastic stage, plastic stage and post-peak stage. [PERSON] and [PERSON] [40] divided stress-strain curves into the compaction stage, elastic stage, stable crack growth stage, unstable crack growth stage and post-peak failure stage. Based on the evolution characteristics of stress-strain curves, [PERSON] et al. [41] divided the stress-time curves of rock samples into the compaction stage, elastic stage, accelerated failure stage and complete failure stage. Based on above research results, the stress-time curves of rock samples were divided into four stages. Stage I was the compaction stage. Stage II was the elastic stage. Stage III was the crack development stage. Stage IV was the failure stage (Figure 6). The mechanical properties of the stress-time curves under each pre-cycling threshold were further analysed. (1) Compaction stage (I)  
Primary pores of rock samples were compacted by load. The primary pore opening decreased with the increasing load. For rock samples, the AE count and cumulative AE count in the first cyclic loading and unloading stage gradually increased with the increasing  
Figure 6: AE signal characteristics of rock samples with burst liability: (**a**) AE count was 0–30 \(\times\) 10\({}^{4}\) and the pre-cycling threshold was 6 MPa; (**b**) AE count was 0–0.06 \(\times\) 10\({}^{4}\) and the pre-cycling threshold was 6 MPa; (**c**) AE count was 0–5 \(\times\) 10\({}^{4}\) and the pre-cycling threshold was 12 MPa; (**d**) AE count was 0–0.06 \(\times\) 10\({}^{4}\) and the pre-cycling threshold was 12 MPa; (**e**) AE count was 0–4 \(\times\) 10\({}^{4}\) and the pre-cycling threshold was 18 MPa; (**f**) AE count was 0–0.06 \(\times\) 10\({}^{4}\) and the pre-cycling threshold was 18 MPa. pre-cycling thresholds (Figure 6b,d,f). The trend was convex. Additionally, the AE count and cumulative AE count in the loading stage were significantly greater than those in the unloading stage. This indicated that crystal particles of rock samples underwent dislocation and extrusion during the loading stage. The primary pore closure effect was significant. The bonding force between crystal particles increased. (2) Elastic stage (II)  
The curve was approximately linear. The stress increased linearly with the increasing time. AE signals were generated in the elastic stage. However, AE signals were scarce (Figure 6a,c,e). It showed that AE signals could still be detected after primary pores were not completely compacted in the compaction stage. (3) Crack development stage (III)  
When the stress reached the stress level of the secondary crack development, plastic deformation occurred. Rock samples changed from the elastic state to the nonlinear plastic state. The nonlinear plastic hardening phenomenon was obvious. The AE count and cumulative AE count showed a surge trend with the increasing time (Figure 6a,c,e). It showed that the increasing rate of dissipated energy accelerated in stage III [42].