Experimental study of the static mechanical response of impact-damaged coal

The intense dynamic loading induced by mechanical coal cutting and blasting pre-damage the bearing capacity of surrounding rock. To examine the influence of impact pressure on the evolution of damage to the coal and the effects on its static mechanical response, coal specimens were subjected to controlled multiple isobaric impact via a Split Hopkinson Pressure Bar (SHPB) system, generating specimens with progressive degrees of damage. Micro-focus CT scanning was used to characterize the morphology of fracture distribution, followed by uniaxial compression tests on damaged coal using an MTS-816 testing machine to investigate static mechanical properties. Key findings reveal: 1) The wave velocity progressively declined followed by accelerated reduction with increasing number of impacts, while the degree of damage exhibited initial gradual growth preceding rapid escalation; 2) The fracture porosity, fractal dimension, and fracture volume increased rapidly then stabilized, whereas the three-dimensional (3D) connectivity rose continuously. The volume of connected fractures ascended with the decelerating rate of growth, and the connected fracture ratio initially dropped then rose. Layer-wise porosity profiles indicated larger damage at specimen ends versus central regions; 3) The peak stress decreases rapidly − steadily − rapidly with the increase of impact times, while the elastic modulus shows a trend of gradually decreasing decline. Before the circumferential strain reaches the peak stress, the stress rises rapidly. The particle size distribution of the specimen after failure accumulates from more than 12.5 mm to less than 1 mm with the increase of the number of impacts; 4) Compare the correlation curves of the microstructural parameters and the macro-mechanics parameter, and compare the magnitudes of the correlation coefficients. By comprehensively comparing the relationship between the microstructural parameters and the peak stress and elastic modulus, it was found that the correlation coefficient between the fracture area and the peak stress and elastic modulus of the specimen was the highest, which were 0.976 and 0.990 respectively. These results provide theoretical and engineering foundations for mitigating instability hazards of coal mines.