Analysis of dynamic behavior and damage characteristics of gas-bearing coal-rock combinations under impact loading

To investigate the dynamic behaviors and damage characteristics of gas-containing coal-rock combinations under impact loading, a triaxial dynamic compression experiment was designed and conducted using a gas-containing coal-rock impact damage-seepage experimental system and an industrial micro-CT scanning system. The 3D crack structures of coal-rock combinations before and after dynamic loading were reconstructed through experiments. The dynamic stress-strain curves, mechanical parameters, energy dissipation laws, crack propagation patterns and impact damage characteristics of coal-rock combinations under different CO₂ gas pressures were comparatively analyzed. The results indicate that the stress-strain curves of the coal-rock combinations under impact loading can be divided into the initial deformation stage, coal-controlled elastic stage, transition stage, rock-controlled elastic stage, strain hardening stage and macroscopic failure stage. With increasing CO₂ gas pressure, the dynamic compressive strength of coal-rock combinations gradually decreases, while the dynamic peak strain and strain rate exhibit an exponential decline. The first elastic modulus of coal-rock combinations initially increases and then decreases with increasing CO₂ gas pressure, while the second elastic modulus fluctuates within a certain range. Under impact loading, the incident energy of coal-rock combinations is greater than the absorbed energy, which in turn exceeds the reflected and transmitted energies. With increasing CO₂ gas pressure, all four energy components show a decreasing trend, while their relative proportions remain essentially unchanged. The increase in CO₂ gas pressure enhances the degradation effect on the coal, leading to an increase in the internal crack volume and crack surface area of coal-rock combinations. With the increase in CO₂ gas pressure, the shear and tensile crack areas on the axial slices of coal-rock combinations increases. The primary damage mode of coal-rock combinations is compression-shear damage, which predominantly occurs in the coal component, while the damage mode of the rock component is often influenced by the coal component.