Anisotropy in shear-sliding fracture behavior of layered shale under different normal stress conditions

Abstract

Shear-sliding mode (mode II) fracture of rocks is a vital failure form in deep underground engineering. To gain deep insight into the anisotropic shear fracture behaviors of a typical shale under high normal stress conditions, a series of direct shear tests were conducted on double-notched specimens in three typical bedding orientations (i.e., the arrester, divider, short-transverse orientations) and under five normal stresses. The mode II fracture toughness (K_IIc) is found to exhibit a significant 3D anisotropy. The maximum K_IIc is obtained in the divider orientation, followed by those in the arrester and short-transverse orientations. In contrast, the 3D anisotropy in the critical mode II energy release rate (G_IIc) is not as significant as that in K_IIc, and G_IIc in the arrester orientation is quite close to that in the divider orientation. The anisotropy in the prepeak input energy accumulated during shearing is found to be exactly consistent with that in G_IIc, which has not been noted before. Furthermore, the anisotropies in the mode II fracture resistances will, unexpectedly, not be weakened by the high normal stress. Owing to the layered structures, tensile cracks are involved during the mode II fracture process, resulting in the formation of rough fracture surfaces.

Keywords

shale / mode II fracture / fracture toughness / critical energy release rate / surface morphology

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