Experimental study on the effect of loading angle on crack propagation in bedding shale

Abstract

The dynamic fracture of rock materials is a basic problem in the field of rock mechanics, while the dynamic fracture mechanism of shale is more complex due to its anisotropic characteristics. In order to study the effect of bedding angle on dynamic fracture process of shale, a split Hopkinson pressure bar (SHPB) system was used to carry out impact tests on notched semi-circular bend (NSCB) specimens of shale. Additionally, a crack propagation gauge (CPG) was set at the crack tip, and the whole fracture process of the shale NSCB specimen was studied with the help of a high-speed photography system and the digital image correlation (DIC) method. The loading rate and Mode-I dynamic fracture toughness of the shale NSCB samples were obtained by the method recommended by the International Society for Rock Mechanics (ISRM). And the crack initiation time and crack propagation speed of the shale NSCB samples can be accurately obtained by CPG. It is found from the experimental results that the Mode-I dynamic fracture toughness of the shale NSCB samples has significant anisotropy, and the loading angle has a positive correlation with the Mode-I dynamic fracture toughness. Although the crack propagation of the C-0sample is not affected by the bedding, its crack propagation needs to cut through the shale matrix, hence the C-0 and 90° shale specimens have a high Mode-I dynamic fracture toughness. When the impact velocity is low, the bending stress on the dangerous section affects the fracture direction of the shale specimen, but with less effect on the bedding. The crack propagation path finally closes to the notch direction. With the increase of impact velocity, stress concentration and micro cracks may exist along the weak plane of bedding due to its relatively low strength. With the increase of impact velocity, the cracks between the weak planes of bedding begin to extend, and the failure planes along the direction of bedding and notch occurred simultaneously.