Fracture toughness of anisotropic shale is an important parameter in shale-gas exploitation technology. Currently available literature is mainly focused on Mode I fracture toughness (K_IC) of anisotropic rock under tensile loading. Although there are the cracked straight through Brazilian disk and crack ring disk used to determine Mode II fracture toughness (K_IIC) of anisotropic rock under pure shear stress, their fracture trajectories are deviated from the original crack plane and cannot be regarded as the true Mode II fracture. In this paper, shear-box test was firstly adopted to measure K_IIC of anisotropic shale. New physical factors Y_I0 and Y_II0 were proposed to describe effects of both the geometry and the material parameters on stress intensity factors (SIFs) of the original crack plane (K_I0 and K_II0) and to derive the calculation formulae of SIFs on arbitrary (K_Iθ and K_IIθ). Calculated results show that K_IIθ reaches its maximum absolute value at θ = 0–10° where K_Iθ is negative, which promotes occurrence of Mode II fracture. The predicted planes of Mode II fracture agree well with the tested fracture trajectories (amostly along the original crack plane). The K_IIC is increased with increase of β (β = 0–90°). K_IIC is 3–4 times as large as K_IC and can be regarded as the true Mode II fracture toughness of anisotropic shale. The shear-box test is an effective method for measuring K_IIC of anisotropic rock.

各向异性页岩的断裂韧度是页岩气开发技术中的一个重要参数。目前可用的文献主要集中在拉伸载荷下各向异性岩石的I 型断裂韧性 ( K _IC )。虽然有裂纹直通巴西盘和裂环盘用于测定纯剪切应力下各向异性岩石的模式II断裂韧度（K _IIC），但它们的断裂轨迹偏离了原始裂纹平面，不能视为真正的模式Ⅱ骨折。本文首次采用剪切箱试验测量各向异性页岩的K _IIC。新物理因子Y _I0和Y _II0提出了描述几何形状和材料参数对原始裂纹平面（K _I0和 K _II0）的应力强度因子（SIF）的影响，并推导出任意（K _Iθ和 K _IIθ）上应力强度因子的计算公式。计算结果表明，K _IIθ在θ  = 0-10° 处达到其最大绝对值，其中K _Iθ为负，促进了 II 型断裂的发生。模式 II 断裂的预测平面与测试的断裂轨迹（主要沿着原始裂纹平面）非常吻合。K _IIC随着β 的增加而增加( β  = 0–90°)。K _IIC是 K _{IC 的}3-4 倍，可以看作是各向异性页岩真正的 II 型断裂韧度。剪切箱试验是测量各向异性岩石K _IIC的有效方法。