Experimental study on the anisotropic fracture characteristics of shale under different loading modes

To gain a deeper understanding of the anisotropic fracture characteristics of shale under various loading modes, semi-circular bend (SCB) tests were conducted on samples with varying notch angles across three principal orientations of crack. The anisotropic fracture toughness and the deflection of cracks at the bedding planes under various loading conditions were investigated. The conditions under which cracks deflect into the bedding planes were determined through a crack deflection criterion. The results showed that significant anisotropy is exhibited in the fracture toughness of Longmaxi shale. The highest fracture toughness of mode-I (K_IC, 1.253 MPa⋅m^1/2) was observed in the Divider, while the lowest (0.605 MPa⋅m^1/2) was identified in the Short-Transverse. Bedding planes are planes of weakness in shale, exhibiting reduced resistance to crack growth. Cracks oriented normal or inclined to the bedding may deflect into the bedding plane under various loading modes. The tortuous fracture path is primarily attributed to crack deflection at the bedding planes. The crack deflection is mainly determined by the incident angle, the loading conditions, and the anisotropy in K_IC. An increase in the incident angle will result in a heightened probability of cracks deflecting into the bedding planes. The deflection range increases as the K_IC of the bedding planes decreases. Cracks are more readily diverted into weaker bedding planes, and a critical K_IC of the bedding planes exists below which all cracks will be diverted.