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, K_I(0) and K_II(0), in order to calculate the SIFs on arbitrary crack plane, K_I(θ) and K_II(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 K_I(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.

各向异性页岩水力压裂的断裂机理在页岩气开采技术中具有非常重要的意义。经典的断裂准则可以更好地预测在任意载荷条件下（纯拉伸，纯剪切和混合模式）的拉伸（I型）断裂，但很难预测剪切（II型）断裂。本文基于应力强度因子（SIFs）与任意平面的断裂韧度之比，建立了各向异性岩石的混合模态断裂准则（修正的K比准则），以预测裂纹萌生角度和断裂模式。 。提出了新的物理影响因素来描述各向异性岩石的几何形状和材料参数对原始裂缝平面的SIF （K _I（0）和K）的影响_II（0），以便计算任意裂纹平面上的SIF，即K _I（ θ）和K _II（ θ）。预测结果表明，在施加于原始平面上的纯拉力，纯剪力和拉剪荷载下，I型断裂可能发生在沉积平面或K _I（ θ） _max平面上。I型或II型断裂可在压缩剪切载荷下发生。不同ψ值的各向异性页岩标本的半圆弯曲试验结果 （裂纹倾斜角）和β（沉积平面角）与预测结果非常吻合，可以验证修改后的K比标准的有效性。