Fracture evolution from micropore changes to macro failure of coal samples with static-dynamic loads based on dislocation and energy theory

To investigate the evolution characteristics of pores and fractures in deep coal seams subjected to instantaneous disturbance, this study conducted dynamic experiments on the coupling effect of impact load and axial static load using the Hopkinson pressure bar (SHPB) system, and analyzed the dynamic mechanical properties and fracture morphology of coal samples. The pore evolution characteristics of coal samples were characterized via low-temperature nitrogen adsorption experiments, while the macroscopic fracture and microscopic damage mechanisms were revealed based on energy dissipation and dislocation theory. The results indicate that under a constant dynamic load, both the elastic modulus and dynamic strength of coal samples increase with the elevation of axial static load. The fracture degree of coal samples decreases as the static load increases, with continuous accumulation of internal elastic energy. When the static load increases from 0 MPa to 14 MPa, the specific surface area and volume of coal sample pores increase by 10.63 % and 15.56 % respectively, and the increment of mesopores is greater than that of micropores and transition pores. Based on dislocation theory, the process from micropore development induced by dislocation accumulation to macroscopic fracture is clarified, and the dissipated energy of coal samples shows an exponential growth trend with the increase of axial static load. Under the action of dynamic and static loads, both the energy utilization efficiency and pore volume of coal samples increase gradually, which to a certain extent promotes the enhancement of coal seam permeability. This study verifies the feasibility of improving the permeability of coalbed methane reservoirs under dynamic load conditions, and can provide practical guidance for deep coalbed methane extraction.