Experimental study on mechanical properties and macro–mesoscale damage evolution of coal following true triaxial dynamic impact

During deep coal mining, the coal mass inevitably suffers damage under three-dimensional stress due to dynamic loads. This damage renders the coal highly susceptible to instability and failure under static loads, thereby posing a threat to engineering safety. Therefore, it is crucial to investigate coal affected by true triaxial dynamic damage. This study aims to elucidate the mechanical properties and damage mechanisms of coal after true triaxial dynamic impact. True triaxial dynamic impact tests were conducted on coal specimens under varying deviatoric stresses and impact velocities. Nuclear magnetic resonance (NMR) tests were performed before and after impact to examine changes in pore structure, followed by uniaxial quasi-static loading tests on the impacted coal samples. The results indicate that as deviatoric stress increases, the coal's dynamic elastic modulus, dynamic peak stress, elastic modulus, and uniaxial compressive strength decrease, while dynamic peak strain, strain rate, static peak strain, and crack compaction strain increase. Higher impact velocities elevate dynamic mechanical parameters but simultaneously deteriorate static mechanical characteristics. Additionally, with increasing deviatoric stress and impact velocity, the number of internal pores rises, the proportion of micropores declines, the proportion of meso-macropores rises, the multifractal dimension of the pore structure diminishes, pore connectivity improves, permeability increases, and damage severity intensifies. This study elucidates the fundamental mechanism by which internal pore structure damage in coal leads to the degradation of macroscopic mechanical properties and proposes a quantitative characterization of this deterioration process based on porosity evolution. By incorporating this damage mechanism, a novel statistical damage constitutive model has been developed that accurately describes the mechanical behavior of coal under true triaxial dynamic impact and reloading conditions. These findings deepen the understanding of the complex damage evolution of coal under intricate stress states, offering critical insights for safety control and stability assessment in deep mining operations.