## 3 Experimental Results and Analysis
### Damage Evolution Analysis of Coal Sample  
AE technology is a method that can effectively monitor perform energy release during the loading of coal and rock materials, and cumulative AE ringing counts as an effective index of damage evolution during the loading of coal and rock mass, which can be used to reveal effectively damage evolution process of microcracks formation, expansion and fracture in coal-rock mass under external force [29; 30; 31]. During the loading of coal mass, AE energies are released in the form of stress wave and received by AE probes, which can accurately reflect internal morphological changes of particle motion, fracture and microcrack propagation in process [32; 33]. The experimental results of the cumulative AE ringing counts of loading coal sample are shown in Figure 5.  
It can be seen from the analysis in Figure 4 that the curves of cumulative AE ringing counts and stress have the same growth trend during the multi-stage loading process, and the curve of cumulative AE ringing counts has three different types of growth trends, i.e., stable stage, sudden change stage, gradual increasing stage, during the compression process. In order to realize the visual analysis of damage and deformation process of coal sample simultaneously, a low-speed camera was used to record the process of deformation and failure during loading. It can be seen in the fracture process of coal mass in Figure 6.  
The values of AE accumulative ringing count are extremely small at the initial stage of loading, which are basically maintaining near zero and the curve of AE accumulative ringing count shows a stage of stable change. It indicates that the internal microfracture structures of coal sample are gradually compressed and closed during the constant loading process, and producing small amounts of elastic deformations to some extent in the process. There are no new fracture structures in coal sample, its internal integrity is quite good and it is merely occurring minor amounts of slip phenomenon of  
Figure 5: The AE accumulative ringing count curve of multi-stage loading coal mass. Figure 6: Fracture process of coal mass with low-speed camera. (**a**) 200 s; (**b**) 500 s; (**c**) 1000 s(**d**) 1500 s; (**e**) 2000 s.  
the structural plane. That is why the conductive channels of coal sample are good contact with each other and the resistivity decreases slightly compared with the initial loading stage, but the resistivity fluctuates around a fixed value due to the stable and unchanged internal microfracture and porosity under constant loading. The values of AE accumulative ringing count increase suddenly during the stress increases to the second stage loading, and the curve of AE accumulative ringing count shows a stage of sudden change. It makes clear that new cracks begin to form in the coal sample and the damage and destruction begin to emerge at this stage. However, the values of AE accumulative ringing count are tending to be stable under the constant loading. It shows that the curve of AE accumulative ringing count shows a stage of stable change and internal microfracture structures of coal sample are not further expanding, not causing further aggravation of damage. The resistivity variation curve has a slight downward trend owing to damage and failure of coal samples in this stage, and internal microfracture structures of coal sample are gradually further compressed and closed during the constant loading process, creating resistivity fluctuates around a fixed value and appears small increment. The trend of AE accumulative ringing count curve is similar to the curve of the upper level during the stress increases to the third stage loading. In this stage, it is merely occurring tiny quantity of increasing in counts, causing further expansion of microfractures and aggravation of damage and failure, which creating the resistivity variation curve shows a further downward trend.