Fracture behavior of rock plate under static and dynamic combined loads

Abstract It is critical to understand the dynamic fracture and crack propagation of confined rocks in geophysics and geoengineering application, such as earthquake fault rupture and in-situ stress measurement. The objective of this paper is to characterize the dynamic fracture behavior of rock plate especially under uniaxial compression using the DIC method combined with ultra-high speed photography. Using a modified hydraulic pump, the uniaxial compression was exerted at the top and bottom ends of the rock plate. Dynamic crack propagation tests of plate specimen were conducted using split Hopkinson pressure bar (SHPB) and the fracture processes were captured by an ultra-high speed photography. The displacement and strain fields of the dynamic fracture process were calculated using DIC. By setting the virtual digital extensometer, the crack-tip position, crack propagation velocity and the dynamic fracture toughness were obtained. Results show that the fracture toughness increases from 1.39 MPa∙m 1/2 to 2.25 MPa∙m 1/2 , and the crack propagation velocity increases from 843.6 m/s to 1148.3 m/s when the incident velocity increases from 8.95m/s to 19.3m/s. The crack propagation velocity is higher than that obtained from small specimen such as NSCB specimen (Gao et al., 2015). Crack propagation velocity in rock plate is higher than that in small specimen such as notched semi-circular bending (NSCB) specimen of rock. There is one main crack path under lower gas pressure, while crack bifurcation occurs under high loading pressure. Crack propagation velocity and crack arrest length decrease with the increase of the hydraulic compressive pressure. If the plate is free of confining stress before dynamic loading, the crack propagation velocity is about 965.0 m/s. The crack propagation velocity decreases with the increase of hydraulic confining stress, and it reduces to about 452.4 m/s at hydraulic pump pressure of 30 MPa or confining stress of 10.6 MPa. Micro cracks could be observed near the crack path and the crack tip when the rock plate subjected to hydraulic compression and dynamic loading. Therefore, DIC method combined with ultra-high speed photography could be used to study the dynamic rock fracture under confining stress, which provides a new method for high speed failure investigation of underground rock that concerned in geophysics and geoengineering application.