The anisotropic fracture mechanical behavior of layered rocks is an important issue in hydraulic fracturing and other subsurface engineering applications. However, current laboratory tests and numerical simulation studies only focus on the effects of isotropy or 2D anisotropy on the fracture mechanical behavior of layered rocks, and the effect of 3D anisotropy on fracture mechanical behavior was seldom investigated. Therefore, in order to reveal the effect of 3D anisotropy on the fracture mechanical behavior of layered shale under mode-I loading, the semi-circular bending tests were carried out on layered shale with considering the effect of 3D anisotropy in combination with acoustic emission (AE) and digital image correlation (DIC). The results indicated that the apparent fracture toughness (AFT) and failure modes of layered Longmaxi shales were strongly dependent on both the orientation angle (α) and loading angle (β). The anisotropy ratios along the loading and orientation directions were 2.03 and 2.84, respectively, indicating that the effect of orientation angle was greater than that of loading angle. According to the results of the DIC failure modes and AE parameters, when orientation angle α = 0°, the failure modes were the tensile failure along the bedding plane at β = 0–15°, the tensile-shear mixed failure at β = 30–75°, and finally the tensile failure across the bedding plane at β = 90°. When orientation angle α > 0°, the failure modes were mainly tensile-shear mixed failure caused by 3D bedding structure and showed a characteristic of progressive failure. The 3D anisotropy had less influence on the fracture process zone (FPZ), the FPZ’s shape remained still semi-elliptic, and the length and width of FPZ obeyed the normal distribution with the length–width ratio of 2–3. Meanwhile, an approximate linear correlation between the AFT and the apparent tensile strength was observed under the 3D anisotropy condition.
Highlights: The SCB tests were conducted to investigate the 3D anisotropy of apparent fracture toughness of layered shale.Both AE and DIC technologies were combined to monitor the 3D anisotropic fracture behaviors of shale.The 3D anisotropic characteristics of apparent fracture toughness, failure process and mode, acoustic emission and fracture process zone were analyzed.The relationship between apparent fracture toughness and apparent tensile strength was discussed.The apparent fracture toughness, failure mode, and the fracture angle of layered shales were discussed comprehensively under 3D space.