Dynamic mechanical response and crack evolution law of raw coal loaded by dynamic-static coupling under three-dimensional constraints

Investigation of the dynamic mechanical properties of coal rock under complex stress conditions at depth. In this work, based on the improved Separate Hopkinson Pressure Bar Test System, 15 groups of coal samples were used to perform dynamic impact tests under different conditions. The changing rules of dynamic strength, crushing, fractal dimension and damage modes of coal under different stress conditions were analyzed. And 9 groups of coal specimens were selected for numerical simulation using ANSYS/LS-DYNA. The results show that: (1) The stress-strain curves of coal specimens under different strain rates, different circumferential pressures and axial pressures basically have the same trend of a bimodal phenomenon. (2) The peak dynamic stress of the coal specimens increased linearly with the increase of strain rate and confining pressure, and the ambient pressure limited the expansion of the internal cracks of the coal specimens under impact loading. And based on the experimental and simulation data, the maximum relative errors between the two are derived as (Group A: 2.9578%, Group B: 6.177%, Group C:6.382%) (3) The damage modes of the coal specimens under the three-dimensional dynamic-static combined loading were mainly "X" type and "conical" shear damage modes. The fractal dimension increases with the increase of strain rate, decreases with the increase of circumferential pressure, and first decreases and then increases with the increase of axial pressure.