Experimental study of the dynamic fracture toughness of anisotropic black shale using notched semi-circular bend specimens

Abstract

This paper investigates the dynamic fracture toughness anisotropy of black shale (BS). Notched semi-circular bend (NSCB) BS specimens are prepared with different loading angles (the angle between the stratification plane and the loading direction). NSCB specimens are dynamically loaded using a modified split Hopkinson pressure bar (SHPB) system. Formulae for calculating mode-I fracture toughness of transversely isotropic material are obtained by finite element method (FEM) in combination with the J-integral method. Dynamic fracture toughness of the NSCB sample is calculated using stress waves recorded in SHPB tests and the obtained formulae. The results show that the fracture toughness of specimens for all loading angles exhibits obvious loading rate dependence, with the fracture toughness increasing with the loading rate. Similar to the static case, the specimens with higher loading angle have larger fracture toughness values as compared with ones with lower loading angles under similar loading rates. We proposed two envelope lines to enclose the fracture toughness of specimens under different loading rate. The difference between two envelope lines decreases with the increase of the loading rate. The anisotropy of black shale diminishes under extreme high loading rate. Anisotropy index, defined as the ratio of the maximum to the minimum of fracture toughness, is equal to 1.75 under static loading and 1.52 under smaller dynamic loading. These conclusions may have significant implications to the design of hydraulic fracturing in shales, e.g., fracture fluids pumping could be considered as static loading and perforation can be considered as dynamic loading. The anisotropy of shales should be considered in both these cases.