Study on failure characteristics of coal sample under different unloading stress paths

The existing research on the failure characteristics of coal samples has some problems, such as single mechanical parameter test and large limitation of stress loading direction. The numerical simulation effect has deviation in the inversion of real geological conditions, and the determination of coal and rock dynamic disaster and bursting liability is based on the macro-research of field experiments. There are few studies on the failure mechanism of coal samples under different unloading stress paths in true triaxial. In order to solve the above problems, taking the engineering geology of Hujiahe Coal Mine in Binchang, Shaanxi Province as the research background, the true triaxial test of coal samples under three different unloading stress paths is designed by using the triaxial dynamic and static load test system of high-frequency vibration acquisition and borehole imaging. And the failure characteristics, peak strength characteristics, acoustic emission response characteristics and fractal law of coal samples are studied. ① The results show that the failure modes of coal samples under three different unloading stress paths are tensile-shear composite failure, and the initiation failure of macro-cracks mostly occurs in the coal samples with relatively low strength. The axial stress of each coal sample increases continuously, and each horizontal stress provides tensile stress in the process of gradually decreasing, resulting in significantly different surface failure forms of coal samples under different unloading stress paths. ② There are obvious differences in the stress of the three different unloading stress paths at the peak failure stage, and the standard deviation reaches 4.35 MPa, accounting for 29.25% of the average peak strength. When the stress load exceeds the average peak strength of the three stress paths by 14.87 MPa, the coal samples are damaged. ③ Under the action of high static load, the pores of the coal samples are compacted after initial loading. The internal structure is relatively uniform, and no fracture expands. The damage variable value is 0 in the initial stage. In the stable development stage of damage, the internal pores of the coal samples reach the limit state and break to form micro-fracture, and the damage variable value is 0.04-0.17. During the loading process, the micro-fracture develop rapidly, expand and converge into the fracture network, and the coal sample is macroscopically damaged. The bearing capacity of the coal sample decreases rapidly, the damage variable value increases sharply first and then stabilizes, and the maximum damage variable value reaches 1.0 in the stage of accelerated damage development. The acoustic emission(AE) energy value increases suddenly when the coal sample is damaged by stress instability and tensile-shear failure. When the AE energy intersects with the damage variable curve, the coal sample begins to break, and the AE energy has a good coupling with the coal sample failure. ④ Under different unloading stress paths, the larger the fractal dimension of coal sample, the higher the degree of fragmentation.