Numerical and experimental investigation into dynamic fracture and energy dissipation of red sandstone under multi-axial confining pressure

Abstract

Investigating the dynamic behavior of rock under confining pressure is instrumental in enhancing the understanding of rock fracture characteristics and energy efficiency of deep rock blasting. Related investigations have been conducted extensively by using modified Split Hopkinson Pressure Bar (SHPB) apparatus. However, in these experimental tests, the real-time observation of the dynamic fracture process of rock remains a significant challenge due to confinement. Numerical simulation offers a viable solution to overcome this obstacle. In this study, a full-scale numerical model of a true triaxial SHPB test system is first developed by using the hybrid discrete element method and finite difference method (DEM-FDM). Then the numerical triaxial SHPB is experimentally validated through assessments of stress wave propagation, dynamic stress equilibrium and dynamic stress–strain behavior in rock. By employing the numerical triaxial SHPB tests in combination with the experimental tests, the dynamic fracture behavior and energy dissipation of red sandstone under biaxial and triaxial confining pressure are investigated. Based on these investigations, the energy required for rock fragmentation by blasting under in-situ stress is discussed. The numerical and experimental results show that the strain rate effect on the dynamic compressive strength of red sandstone is related to the state of confining pressure, exhibiting a decrease with increasing axial pressure and an increase with increasing lateral pressure. Both tensile and shear microcracks are generated within the rock specimen under the coupled uniaxial impact loading and multi-axial confining pressure. As the axial pressure increases, there is an augmentation in the total number of microcracks, accompanied by an escalated proportion of tensile cracks. Consequently, the dynamic fracture behavior of rock is predominated by tensile failure. Conversely, the total number of microcracks decreases and shear failure becomes the primary fracture pattern with an increase in the lateral pressure. The dynamic specific fracture energy required for creating a new fracture surface of unit area correspondingly decreases with increasing axial pressure while increases with increasing lateral pressure. Compared to biaxial or triaxial anisotropic pressure states with the same average pressure, the energy required for rock fracture is highest under the triaxial isotropic pressure condition. Regarding deep rock blasting under in-situ stress, the fracture zone decreases and the energy consumption in rock fragmentation increases with the increase of the stress level, indicating that rock blasting under high in-situ stress becomes more challenging.