Anisotropy in mode-II fracture toughness of shale

To enhance our understanding of the anisotropy in shear fracture resistance of shale, the mode-II fracture toughness of shale with different bedding orientations was measured using the double-edge notched Brazilian disc (DNBD) method. These tests were conducted based on theoretical analyses of shear crack initiation. The initiation and propagation of fractures generated in DNBD specimens were determined, and the mechanism for testing the mode-II fracture toughness of rocks using the DNBD method was investigated. The results indicated that: (1) the high compressive T-stress in DNBD specimens significantly reduces the circumferential stress at the tips of pre-cut notches, thereby inhibiting the competitive initiation of tensile cracks and facilitating self-similar shear fractures. (2) The mode-II fracture toughness of shale increases approximately linearly with the bedding angle, displaying marked anisotropy. The fracture toughness is minimal at the bedding plane, maximal in the matrix, and nearly equal in the direction normal to the bedding compared to the matrix. As an intrinsic material property, the true mode-II fracture toughness, in the absence of normal compressive stress, is considerably lower than the apparent (experimental) value. Moreover, the true mode-II fracture toughness remains relatively constant even when wing-shaped tensile cracks initiate competitively. (3) Similar fracture propagation patterns were observed in DNBD specimens with varying bedding orientations. The findings revealed that complex fracture processes, including the competitive initiation and subcritical propagation of wing-shaped and en-echelon fractures, occurred prior to the shear fracture of the rock bridge. (4) The subcritical propagation of wing-shaped tensile cracks following competitive initiation has been identified as the primary mechanism driving self-similar shear fractures along pre-cut notches in DNBD specimens. This understanding elucidates the mechanism for measuring mode-II fracture toughness in rocks using the DNBD method.