A new mixed-mode fracture criterion of anisotropic rock

Abstract

Fracture mechanism of hydraulic fracturing for anisotropic shale is of very importance in shale gas mining technology. The classical fracture criteria can better predict tensile (Mode I) fracture under arbitrary loading condition (pure tensile, pure shear and mixed-mode), but have difficultly in predicting shear (Mode II) fracture. In this paper, a new mixed-mode fracture criterion (modified K-ratio criterion) of anisotropic rock is established based on the ratio of stress intense factors (SIFs) and fracture toughness of arbitrary plane to predict both the crack initiation angle and fracture mode. New physical influencing factors are proposed to describe effects of both the geometry and the material parameters of anisotropic rock on the SIFs of the original crack plane, KI(0) and KII(0), in order to calculate the SIFs on arbitrary crack plane, KI(θ) and KII(θ). Prediction results show that under the pure tension, pure shear and tension-shear loads applied onto the original plane, Mode I fracture can occur on the sedimentary plane or on the plane of KI(θ)max. Mode I or Mode II fracture can occur under compression-shear load. The semi-circular bend (SCB) test results of anisotropic shale specimens with different ψ (crack inclined angle) and β (sedimentary plane angle) are in good agreement with the predicted results and can verify the validity of the modified K-ratio criterion.