Research and numerical simulation on dynamic tensile instability evolution mechanism of coal under different impact velocities

Understanding the strain-damage-energy dissipation mechanisms and evolution characteristics of deep coal and rock under impact loading is of great significance for the prevention and control of rockbursts. In this study, a Split Hopkinson Pressure Bar (SHPB) experimental system was employed to investigate the dynamic tensile instability mechanisms of coal under different impact velocities. The stress-strain-energy responses and damage-failure modes of coal specimens were analyzed. Furthermore, a coupled numerical simulation approach integrating PFC^3D and FLAC^3D was applied, combined with fractal dimension analysis and fragment size distribution methods, to quantitatively characterize the macro-meso damage-failure features of coal under dynamic loading and to elucidate the energy conversion mechanisms and dynamic instability behavior. The results indicate that: ① Impact velocity and static loading significantly influence dynamic tensile strength, strain rate, and damage degree. As impact velocity increases, fragmentation becomes more severe; under similar impact velocities, combined dynamic-static loading produces greater damage than impact loading alone. ② At the macroscopic scale, the main crack initiates in the specimen's middle region, propagates toward the loading end, and eventually penetrates the specimen. At the mesoscopic scale, cracks initiate at the loading end, gradually extend toward the middle, and coalesce. Crack propagation exhibits stage-wise behavior, with tensile cracks dominating during the plastic deformation and failure stages. ③ Higher impact velocities result in increased incident energy, dissipated energy, and dissipated energy density; under combined dynamic-static loading, both dissipated energy and dissipated energy density are higher than under single impact loading.