Numerical investigation on the dynamic strength and failure behavior of rocks under hydrostatic confinement in SHPB testing

Rocks in the deep underground engineering are commonly subjected to hydrostatic in-situ stresses and dynamic loadings simultaneously. Accurate characterizations of the strength and the failure behavior of rocks under coupled hydrostatic confinement and dynamic loading are crucial for the safety of engineering projects. Via discrete element method (DEM), this study systematically investigates the mechanical behavior of granitic specimens under different hydrostatic confinements in split Hopkinson pressure bar (SHPB) testing, regarding seven hydrostatic confinements varying from 0 to 225 MPa and six strain rates varying from 500 to 1500 s−1. Our results show that the dynamic strength of hydrostatic pressurized rock specimen increases with increasing strain rate, while the rate sensitivity of rock strength decreases as the hydrostatic confinement increases. Simulated acoustic emission distribution demonstrates that the hydrostatic confinement significantly affects the dynamic failure patterns of rocks; as the confinement increases, the failure mode changes from a mixed failure of surface splitting and inner X-type shear to a failure in single shear band. Moreover, for a given strain rate, more integrated rock fragments are generated and the fragment size distribution becomes wider with increasing hydrostatic confinement. In addition, the mass cumulative fraction versus fragment size curves of specimen under different hydrostatic confinements can be characterized by a three parameters generalized extreme value distribution function.