Characteristics of Crack Deflection and Mixed-Mode I-II Fracture Controlled by Bedding in Oil Shale Under Three-Point Bending

Oil shale often exhibits well-developed internal bedding planes, microcracks and organic-rich weak interfaces, while a mixed failure mode of tensile fracture and shear slip along weak bedding planes can be observed under bending loads. In this study, three-point bending tests were performed on the oil shale, and with the combination of acoustic emission (AE) monitoring to analyze the crack propagation paths, the crack path selection and mixed-mode I-II fracture behavior controlled by bedding were symmetrically investigated. The experimental results demonstrate that crack propagation does not always steadily proceed along the initial direction of pre-existing crack, and instead, the occurrence of pronounced deflection can be observed near the weak bedding planes, indicating a trend of transition from tensile crack to shear slip along bedding, while the obvious mixed-mode I-II fracture characteristics are presented. Meanwhile, this process is also accompanied by the enhanced AE activity and the occurrence of a localized high-energy event. Furthermore, based on theoretical fracture mechanics analysis, it is interpreted that the localized driving force conditions at the crack tip can be altered by the mechanical differences between the bedding weak planes and the matrix, which provides a theoretical explanation for why the crack deflection along the structural weak planes is promoted. These research findings correlate the crack propagation path evolution, AE response and mixed-mode fracture characteristics, which can provide the experimental evidence for understanding the controlling role of crack path selection in brittle shale under bending conditions.