Study on mechanical behavior and energy evolution of coal under different gas pressures under impact loading

To reveal the mechanical response and energy evolution mechanism of coal under the coupling action of impact load and gas pressure, dynamic impact tests under four gas pressure levels (0.4, 1.2, 2.0, and 2.8 MPa) were carried out on coal samples from a rockburst mine in Ordos using a Split Hopkinson Pressure Bar (SHPB) system and a gas pressure control system. The variation laws of coal mechanical parameters, failure characteristics, and energy evolution mechanism were systematically studied. The results show that the dynamic mechanical properties of coal deteriorate significantly with the increase of gas pressure. The peak strength gradually decreases from 41.7 MPa at 0.4 MPa to 38.6 MPa at 2.8 MPa, and the dynamic elastic modulus decreases by 25% simultaneously. The absolute value of the slope of the post-peak softening segment of the stress-strain curve increases, indicating weakened brittleness and enhanced plastic deformation. The failure mode shows an obvious gradient change. Under low gas pressure, local spalling failure dominates, with coarse fragments being predominant. Under high gas pressure, it transforms into a composite failure mode with coexisting transverse spalling and longitudinal splitting, characterized by significantly reduced fragment sizes and intensified overall fragmentation. The total input energy first increases and then stabilizes with the increase of gas pressure, reaching a peak of 382 J at 2.0 MPa. The dissipated energy for failure increases continuously, and at 2.8 MPa, it increases by 2.3 times compared with that at 0.4 MPa. The elastic strain energy shows a trend of first increasing and then decreasing, with a significant decline in storage capacity under high gas pressure. Under high gas pressure, the energy absorption rate of coal accelerates in the early stage, and the time to reach the equilibrium state is 40 μs earlier than that of the benchmark group. Gas pressure reduces the effective stress of coal through the pore pressure effect, weakens the strength of the coal skeleton, and at the same time, the elastic potential energy of gas and impact energy are released synergistically, accelerating the propagation and coalescence of fractures. The research results provide experimental support for the establishment of energy criteria and prevention and control design of rockburst disasters in high-gas mines.