## 4 Discussion
### Precursor Characteristics of Unstable AE of Granite Samples  
The AE parameters (counting and cumulative energy) of granite samples showed obvious abnormal growth from sub-stability to instability stage (see Figure 4), especially the precursory phenomenon that the AE count shows intermittent spurt is the main feature. Whether it is laboratory rock mass fracture or engineering AE monitoring dynamic disaster, the AE response characteristics of this stage have significance reference for the early warning of rock mass dynamic disaster [19].  
Before the macroscopic cracking of the sample, the AE mechanism is mainly based on the micro-yield such as the closure of the original crack of the sample and the micro-slip of the sample. As the grading loading progresses, the stress at the crack tip increases and the accumulated elastic energy increases after the original crack closing. When the energy of the crack tip (power P) is sufficient to overcome the energy (resistance R) that makes the locked bodies yield, the crack rapidly expands and the sample enters the sub-stability phase. At this stage, as the yield of the internal locked bodies of the sample increases, the strain continues to increase, and the mechanism of AE produces a transition to the macro crack. At this time, the AE signal not only depends on stress loading level, but also has a positive correlation with crack growth rate. In the crack propagation process (P > R), under the action of the locked bodies, resistance R regularly reduces the growth rate of crack propagation (a), and when the acceleration a' is a negative value, the power P and the resistance R are getting closer and closer, and when the power P and the resistance R are equal in value, the crack is stopped. At this stage, the AE signal enters a quiet period. As the stress level increases, the fracture power P will again reach the critical threshold yield of the solid lock (If the power P does not reach the critical value at this time, the crack derivative will stop the AE signal and enters the intermittent period). The locked bodies again yield plastically and the crack expands again.  
When the axial load increases to the peak stress, the last locking solid that keeps the rock mass stable is about to fracture, and, at this time, the crack growth enters the instable growth. When the fracture density formed by a large number of internal cracks of the sample reaches the last solid locked fracture threshold, the sample will undergo a macroscopic fracture, instantly release a large amount of elastic strain energy, and the AE count reaches the maximum value [20].  
From the theory of micro-fracture nucleation and multi-locked bodies fracture, it can be known that the macro-crack development process of granite is a micro-fracture clustering and forming a local fracture zone, and the local fracture zone is gradually expanded and nucleated to form a larger fracture zone. When the fracture density reaches the main fracture threshold of the sample, the sample is unstable. During this process, small cracks gradually aggregate and expand into large cracks. As the crack dimension is increased, the AE signal (counting and energy) has a sensitive high value response, which is reflected by the rising trend of the low value of the quiet phase of the AE signal, and the peak intensity is increased.