Understanding the anisotropic fracture mechanism of shale is of significance to assess the development of a stimulated reservoir volume. In this study, three-point bending tests were carried out using notched deep beam specimens of shale with 7 different bedding dip angles to investigate the anisotropic fracture behavior. The anisotropic characteristics of the fracture toughness, energy release rate, inelastic zone and crack propagation path were comprehensively analyzed and highlighted. The results showed that the fracture toughness decreases with increasing bedding dip angle. When the bedding dip angle is 0° and 90°, the fracture toughness reaches the maximum and minimum values, respectively, with a ratio of 1.60. The variation tendency of the energy release rate with the bedding inclination angle is consistent with that of the fracture toughness, but the ratio of the maximum value to the minimum value of the energy release rate is about 3.24. The size of the inelastic zone first decreases and then increases at different bedding inclination angles from 0° to 90° and reaches the maximum size of 1.51 mm when the bedding plane inclination angle is 45°. The crack propagates along the bedding plane rather than straight along the initial crack direction when the bedding dip angle is higher than 45°. As the bedding plane dip angle increases from 0° to 45°, the crack surface roughness of the tested shale first increases, then decreases and finally reaches the maximum. The discussion indicated that the presence of the Mode II fracture has a considerable influence on the crack surface roughness and crack propagation path, although it has little effect on the failure load. Compared with the maximum energy release rate (MERR) criterion, the maximum tangential stress (MTS) criterion seems more accurate in predicting the anisotropic Mode I fracture toughness. The applicability of these two classical crack initiation criteria of fracture mechanics has been analyzed based on our experimental results, which demonstrates the validity of the MTS criterion to assess the anisotropic fracture toughness of shales.

了解页岩的各向异性压裂机理对于评价增产储层体积的发育具有重要意义。在这项研究中，使用具有 7 个不同层理倾角的页岩缺口深梁试样进行三点弯曲试验，以研究各向异性断裂行为。综合分析并突出了断裂韧性、能量释放率、非弹性区和裂纹扩展路径的各向异性特征。结果表明，断裂韧度随着层理倾角的增加而降低。当层理倾角为0°和90°时，断裂韧性分别达到最大值和最小值，比值为1.60。能量释放率随层理倾角的变化趋势与断裂韧度的变化趋势一致，但能量释放率的最大值与最小值之比约为3.24。非弹性区的尺寸在0°～90°的不同层理倾角下先减小后增大，当层理面倾角为45°时达到最大1.51 mm。当层理倾角大于45°时，裂纹沿层理平面扩展，而不是沿初始裂纹方向直线扩展。随着层理面倾角从0°增加到45°，被测页岩裂缝表面粗糙度先增大后减小，最后达到最大值。讨论表明，模式 II 断裂的存在对裂纹表面粗糙度和裂纹扩展路径有相当大的影响，尽管它对破坏载荷的影响很小。与最大能量释放率 (MERR) 准则相比，最大切向应力 (MTS) 准则在预测各向异性 I 型断裂韧性方面似乎更准确。基于我们的实验结果分析了这两个断裂力学经典裂纹萌生准则的适用性，这证明了 MTS 准则在评估页岩各向异性断裂韧性方面的有效性。最大切向应力 (MTS) 准则在预测各向异性 I 型断裂韧性方面似乎更准确。基于我们的实验结果分析了这两个断裂力学经典裂纹萌生准则的适用性，这证明了 MTS 准则在评估页岩各向异性断裂韧性方面的有效性。最大切向应力 (MTS) 准则在预测各向异性 I 型断裂韧性方面似乎更准确。基于我们的实验结果分析了这两个断裂力学经典裂纹萌生准则的适用性，这证明了 MTS 准则在评估页岩各向异性断裂韧性方面的有效性。