Abstract：In order to deeply understand the propagation mechanisms of shear fractures and directivity effect of bedding，direct shear tests were performed on Longmaxi shale specimens with various bedding orientations with respect to the loading direction under different normal stresses to study the fracture propagation patterns. The evolution process of nucleation，propagation and coalescence of en-echelon fractures and directivity effect of bedding were investigated. The results show that，due to the shear-induced tensile stress，en-echelon micro-cracks are generated around the shear plane，further develop，connect and finally form a macroscopic shear fracture zone with a uniform width and obvious heterogeneity. For layered shale，the shear-induced en-echelon cracks typically develop along the bedding plane. The cracking degree and orientation of the en-echelon cracks are closely related to the bedding orientation，showing significant bedding directivity effect. It is very difficult to induce en-echelon cracks in the direction oblique to the bedding plane along which the shear force is exerted. When the angle between the shear direction and the bedding plane is 30° or 60°，the en-echelon cracks are generated along the bedding plane，especially for 30°. When the shear force is exerted normal to the bedding plane，weak bedding cracking could still be observed to some extent. The shear fracture surface formed by the coalescence of the en-echelon fractures often presents a serrated shape and occurs striation and abrasion in post-peak frictional sliding stage. However，the normal stress could obviously inhibit the formation of en-echelon cracks and make the shear plane smooth. When the shear force is exerted at a certain angle to the bedding plane，complex fracture morphology is formed by the shear induced bedding-parallel en-echelon fractures. The complex fracture morphology is mostly confined to the shear fracture zone，showing obvious localized deformation.
衡 帅，李贤忠，刘 晓，张小东. 直剪条件下页岩裂缝扩展演化机制研究[J]. 岩石力学与工程学报, 2019, 38(12): 2438-2450.
HENG Shuai，LI Xianzhong，LIU Xiao，ZHANG Xiaodong. Study on the propagation mechanisms of shale fractures #br# under direct shear conditions. , 2019, 38(12): 2438-2450.
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2019, Vol. 38 
