Anisotropic characteristics of crack initiation and crack damage thresholds for shale

The inherent anisotropy is the most important structural feature of shale. Most research on the mechanical behavior of shale has focused on its anisotropic strength, deformation and failure patterns. Few studies have examined the anisotropic micro-failure mechanism and failure process of shale. To investigate the anisotropic crack initiation mechanism and evolution of shale during the progressive rock failure process, two stress thresholds (i.e., crack initiation stress (sigma(ci)) and crack damage stress (sigma(cd))) and two strain thresholds (i.e., crack initiation strain and crack damage strain) of shale under various bedding layer orientations and confining pressures are identified on the basis of the crack volumetric strain. The anisotropic characteristics of the four thresholds are described. The crack initiation stress can be seen as independent of the bedding layer orientation, but the other thresholds are strongly dependent on the bedding layer inclined angle. The physical mechanism of this phenomenon is analyzed in detail. The anisotropic features of the mutual relationships among the peak stress sigma(c), sigma(ci) and sigma(cd) are presented. The ratios sigma(ci)/sigma(c) and sigma(ci)/sigma(cd) at confining pressures greater than 30 MPa and the ratio sigma(cd)/sigma(c) increase first and then decrease with the increasing inclination angle. A high confining pressure constraints shale from entering the unstable crack propagation stage but has little effect on preventing crack propagation once it has entered this stage. The anisotropic characteristics of these thresholds can provide meaningful information for understanding the anisotropic deformation and failure mechanism of shale.