Effect of Primary/Artificial Interface on Dynamic Failure Behavior of Combined Coal and Rock: Monitoring by High-Speed Imaging and Scanning Electron Microscopy.

The instability of the primary coal and rock structure significantly impacts the safety of coal mining and construction. The complex coal-rock interface cannot be simplified as a smooth surface. To investigate the dynamic response of primary composite coal and rock (primary-CCR), we studied the impact of load, hydrostatic pressure, and interface type on the mechanical behavior and macro/micro failure characteristics using a separate Hopkinson bar, a high-speed camera, and a scanning electron microscope. The results indicate a linear relationship between the composite strength, impact toughness, and impact velocity of the two types of composite coal and rock. The mechanical behavior of the primary-CCR initially increases and then decreases with the rise of hydrostatic pressure (turning point: 10 MPa). There is a positive correlation between artificial combined coal-rock (artificial-CCR) and hydrostatic pressure. The dissipative energy of combined coal and rock increases linearly with impact velocity. Initially, the dissipative energy per unit fracture area increases with hydrostatic pressure, then decreases as pressure continues to rise. Additionally, an increase in impact load causes the energy dissipation inflection point to shift forward. The primary interface significantly reduces the energy threshold for instability failure, resulting in a transition from energy transfer to energy dissipation in rock components. This transition manifests as the failure of artificial combined coal and rock cracks, which develop into rock fragments at an impact velocity of 14 m/s. Furthermore, the change in section roughness of coal and rock correlates with the degree of macroscopic crack growth, following the order: coal > primary-CCR > artificial-CCR > rock.