## 2 Experiment Test
### Sample System  
The experimental system was mainly consisting of loading control system, AE testing system and resistivity testing system. The loading control system adopted the test system of _Tianchen_ electro-hydraulic servo pressure testing machine (_Tianchen_ Testing Machine Manufacturing Co., Ltd., Jinan, China), which was mainly consisting of press and automatic control program named _TensonTest_. On top of that the control program was used to modify such as stress, strain, control modes and record experimental data in real time, which could switch the observation mode at any time for efficient sampling. Thus, it could achieve the high-precision constant load testing experiment during the process of deformation and failure in coal mass. Moreover, the resistivity testing system was mainly  
Figure 1: Specimens of coal mass. (**a**) Overhead view of coal samples; (**b**) main view of coal samples.  
composing of 3532-50 _LCR_ tester (Dongfang Zhongke Integrated Technology Co., Ltd., Beijing, China) with a measuring accuracy of 0.01 \(\Omega\cdot\)m. And it was worth noting that a layer of conductive coupling agent should be coated between coal samples and copper electrodes before the experiment. In addition, in order to avoid the effect of experimental equipment on the resistivity measurement results, a piece of insulating paper was laid under the electrode, and then the electrode was fixed and connected with _LCR_ tester. In general, the electrical resistivity decreases with the increase of test frequency during the process of loading, and it is extremely significantly that the resistivity variation range changes in process under low frequency conditions and can eliminate the resistivity polarization phenomenon effectively under low frequency conditions. Therefore, a certain frequency (usually 10 kHz) was generally used to measure electrical resistivity of coal mass in low frequency range [24; 25]. To the end, the AE-measurement system adopted data acquisition system of _AMSY-6_ AE tester (_Vallen_ Systeme, German), and there were six AE data acquisition channels in system and collecting AE parameters by data processing software _Vallen_ in real time during the loading and deformation process of coal mass. Even more important, in order to analyze the damage evolution process of coal samples in real time during the loading process, and realize the visual analysis in damage and deformation process of coal samples simultaneously, a low-speed camera was used to record the process of deformation and failure during loading. The loading control system, AE testing system and resistivity testing system is shown in Figure 2.  
In this paper, the experiment of multi-stage loading was carried out on standard coal samples of Sihe Mine, which was loading at a rate of 0.5 MPa in steps with displacement loading control mode. On top of that axial stress increased gradually from 0 MPa to 11 MPa step by step until the coal samples are completely destroyed. In the meantime, the electrical resistivity was collected in real time by using the _3532-50 LCR_ tester during the loading process, and the cumulative AE ringing counts were collected and sorted out by the AE testing system. It was worth noting that the reserved position of AE probe needed to be polished smoothly and put on Vaseline evenly between AE probes and coal sample so that the probes fitted well on the coal sample. Furthermore, the upper three probes were placed separately at 120 degrees in a cross section, and the lower probes were arranged in a cross section that was misalignment with the upper portion, the schematic diagram of AE probes is shown in Figure 3.  
Figure 2: Schematic diagram of experimental system in Multi-stage Loading (1. resistivity testing system; 2. acoustic emission (AE) testing system; 3. _3525-50 LCR_ tester; 4. AE tester; 5. Copper electrodes; 6. coal samples; 7. loading control system; 8. Tianchen electro-hydraulic servo pressure testing machine).