Fracture pattern and toughness of layered sandstone influenced by layer orientation

In order to study the anisotropic properties of layered sandstone on fracture mechanics, the three-point bending tests on the semi-circular specimens with different layer orientations and crack angles are conducted. The variations of normalized stress intensity factors, fracture toughness and fracture patterns of these specimens with layer orientation are revealed. The results illustrate that the fracture pattern of the specimens is closely related to the angle θ between the layer orientation and the loading direction, that is, tensile splitting along the layer when θ=0°, shearing along the layer when θ=30°, tensile splitting across the layer when θ=90° and composite shearing failure across and along the layer θ=45° and 60°. The fracture toughness of the specimens with different layer orientations greatly differs with the angle between the layer orientation and the loading direction; when crack angle α=0°, the specimens with θ of 90° has the maximum fracture toughness; and when those with θ of 0° have the minimum one, the ratio equals 2.36. Modes I and II normalized stress intensity factors are calculated by using the finite element code ABAQUS. It is shown that the mode II normalized stress intensity factor of the specimens varies more evidently with the layer orientation when α equals 0°, that is, mode I fracture when θ=0°and 90°, mixed mode when θ=45° and 60°, and mode II dominated fracture when θ=30°. In addition, for the specimens with α=30°~60°, the mode I and mode II normalized stress intensity factors show different variations with the layer orientation. The crack initiation angle, mixed-mode fracture toughness and fracture trajectory of the specimens are calculated by using the extended finite element method, and are in good agreement with the experimental results. The results indicate that crack initiation angles are influenced by the layer orientation and crack angle. The findings prove to be helpful for understanding the fracture characteristics of the layered rock materials and enriching the researches on fracture mechanics and numerical simulation of anisotropic rock materials.