Dynamic mechanical response and damage evolution of coal specimens with borehole based on discrete-continuous coupling simulation

To investigate the dynamic mechanical behavior and damage evolution mechanisms of coal specimens containing borehole under impact loading, this study employed a split Hopkinson pressure bar (SHPB) testing system in conjunction with a discrete–continuous coupling approach to conduct dynamic impact experiments and numerical simulations on coal specimens with different borehole diameters. Results show that, under high strain rate loading, the peak stress of coal specimens decreases monotonically with increasing borehole diameter, while post-peak stress drop slows and post-peak strain significantly extends. Owing to the wave-impedance effect induced by the borehole, the proportion of reflected energy increases, whereas the proportions of transmitted energy and dissipated energy decrease. In terms of failure mode, cracks in intact specimens initiate at the incident end and propagate along the stress wave direction, exhibiting tensile splitting failure; in contrast, strain concentration in specimens with borehole first occurs around the hole periphery and then extends toward both ends, resulting in a transition to tensile-shear composite failure. The total number of cracks and fragments in the borehole-containing specimens decreases, the proportion of tensile cracks declines while that of shear cracks increases, and the fragment volume becomes larger. Numerical simulations successfully reproduce crack propagation paths and damage hotspot regions, showing that crack patterns evolve from a linear "I" shape to an "X"-shaped multi-path network, providing additional channels for energy dissipation and increasing crack development volume in borehole-contained specimens. Furthermore, increasing impact velocity promotes the expansion of the damaged volume, whereas increasing confining pressure significantly suppresses its development.