Macro–Micro Damage and Failure Behavior of Creep Gas-Bearing Coal Subjected to Drop Hammer Impact

The macroscopic mechanical characteristics and microscopic pore structure evolution were investigated in gas-bearing coal subjected to creep loading superimposed with drop hammer impact loads, under different gas adsorption pressures, creep stress levels, and impact load magnitudes. The results show that, as gas adsorption pressure, creep stress level, and impact load increase, the peak strength of coal specimens decreases, while the peak axial strain and radial strain increase. Low-temperature liquid nitrogen adsorption tests demonstrated that the maximum adsorption capacity, total specific surface area, total pore volume, and the proportion of specific surface area and pore volume of micropores in intermediate-sized coal specimens decrease with increase in the three variables. In contrast, the proportion of specific surface area, total proportion of pore volume, and the fractal dimension of transition pores and mesopores increase, indicating a transformation of micropores into transition pores and mesopores. Importantly, a strong linear correlation was established between the damage rate of coal specimens and the proportion of micropore specific surface area, pore volume, and pore fractal dimension. These findings lay the groundwork for developing a macro–micro combined damage and failure constitutive model for gas-bearing coal subjected to superimposed dynamic and static loads, thereby enhancing the theoretical framework of rock dynamics.