Dynamic response and fracture mechanisms of cubic jointed rock under true triaxial static-dynamic loading

Rock masses typically exist in a three-dimensional stress state and are often subjected to a combination of in-situ stresses, static loads, and dynamic engineering loads. To investigate their mechanical behavior under dynamic conditions, a series of impact tests were conducted on both intact and jointed rock specimens. The triaxial Hopkinson pressure bar (THPB) apparatus was employed to simultaneously apply initial static and dynamic loads. By capturing stress wave signals, the axial stress variations in the specimens during impact were measured, allowing the dynamic stress-strain curves to be determined. The effects of initial static and dynamic loading on the stress-strain response, compressive strength, and failure modes were analyzed. The role of the initial intermediate principal stress was also examined. Initial intermediate principal stress σ 2 0 alters lateral confinement and fracture patterns, which are not captured in conventional SHPB tests. Subsequently, 3DEC numerical simulations were performed to examine the evolution of stress and displacement fields during dynamic failure. Results from both experiments and simulations indicate that the dynamic stress-strain response can be categorized into two types corresponding to distinct failure mechanisms. For intact specimens, as the loading rate increases, the strength increases from 235.2 MPa to 255.1 MPa, an increase of 8.5%; for jointed specimens, it increases from 111.8 MPa to 131.6 MPa, an increase of 17.7%, showing a stronger rate sensitivity in jointed rocks due to stress wave reflection and transmission at joint surfaces that amplify the strain rate effect. Moreover, dynamic compressive strength decreases with increasing initial axial stress but rises with higher lateral confinement. The findings also reveal a positive correlation between dynamic strength and the initial intermediate principal stress.