Deformation, fracture, and energy evolution characteristics of coal-rock under dynamic–static combined loading

Deep coal-rock formations are subjected to complex stress environments characterized by high static stresses and dynamic disturbances. To study the damage, fracture, and energy evolution characteristics of coal-rock under dynamic–static combined loading, a new multiscale constitutive model for coal-rock under dynamic–static combined loading is proposed based on micromechanics, and it is implemented into the LS-DYNA solver. A numerical model of coal-rock Split Hopkinson Pressure Bar under dynamic–static combined loading is established using LS-DYNA, and research on the mechanical and energy evolution characteristics of coal-rock under one-dimensional and three-dimensional dynamic–static combined loading is conducted. The results show that under one-dimensional dynamic–static combined loading, with the increase of precompression, the dynamic peak stress linearly decreases while the combined peak stress linearly increases, and the dissipated energy of the specimen shows a decreasing trend. The fracture patterns of the coal-rock specimen include internal shear fracture and external tensile fracture, and eventually, these two modes of fracture intersect to form macroscopic mesh cracks. As the axial pressure increases, the degree of specimen fragmentation gradually increases. Under three-dimensional dynamic–static combined loading, with the increase of preconfining pressure, the stress–strain curve of the specimen will transition from “stress drop” to “stress rebound” after the peak. The peak stress increases with the increase of confining pressure, and the energy dissipation density of the specimen increases first and then decreases with the increase of confining pressure. With the increase of confining pressure, the hoop deformation of the specimen plays a constraining role, and the degree of specimen fracture gradually weakens, and the time of fracture occurrence gradually delays. The research results contribute to revealing the mechanical and energy mechanisms of rockburst disasters in deep coal mines.