Experimental and theoretical analyses are presented that assess true mode II fracture toughness, $K_{\mathrm{IIc}}$, fracture energy, ${\mathcal{G}}_{\mathrm{IIc}}$, and associated fracture process zone (FPZ) in anisotropic rocks. The term true signifies the type of mode II crack that grows in a self-planar manner as a result of shear stresses, and thus differentiates it from a generic mode II crack that often extends by forming a tension-induced kink. We give the theoretical basis for the sinusoidal variation of $K_{\mathrm{IIc}}$, and present simplified relations for the direction dependence of the ${\mathcal{G}}_{\mathrm{IIc}}$ in an anisotropic plane. Our experimental data on anisotropic Grimsel Granite validate this theoretical model, thereby providing supporting evidence that $K_{\mathrm{IIc}}$ of this rock type indeed follows a sinusoid function. This model takes only two principal values of $K_{\mathrm{IIc}}$ to characterise fracture toughness and energy in any direction within an anisotropic plane. Our analyses also show that the values of $K_{\mathrm{IIc}}$ and ${\mathcal{G}}_{\mathrm{IIc}}$ are noticeably higher than the ones of mode I, and one must distinguish between these toughness values when formulating growth criteria in rocks. Lastly, a thorough analysis of the strain localisation obtained from the digital image correlation data gives insight on the nature of the FPZ, and demonstrates that the FPZ of a true mode II crack is of a semi-elliptical shape and has a comparable size with the one of a mode I crack.

进行了实验和理论分析，以评估真实的II型断裂韧性， ${ķ}_{\mathrm{信息中心}}$，断裂能， ${\mathcal{G}}_{\mathrm{信息中心}}$，以及各向异性岩石中的相关断裂过程带（FPZ）。术语“真”表示由于剪切应力而以自平面方式生长的II型裂纹的类型，因此将其与通常通过形成张力引起的扭折而延伸的普通II型裂纹区分开来。我们给出了正弦变化的理论基础${ķ}_{\mathrm{信息中心}}$，并给出了简化的方向依赖关系 ${\mathcal{G}}_{\mathrm{信息中心}}$在各向异性的平面上。我们在各向异性Grimsel花岗岩上的实验数据验证了这一理论模型，从而提供了支持性证据，${ķ}_{\mathrm{信息中心}}$这种岩石类型确实遵循正弦函数。该模型仅采用两个主值${ķ}_{\mathrm{信息中心}}$表征各向异性平面内任何方向的断裂韧性和能量。我们的分析还表明，${ķ}_{\mathrm{信息中心}}$ 和 ${\mathcal{G}}_{\mathrm{信息中心}}$明显高于模式I，并且在制定岩石生长标准时必须区分这些韧性值。最后，对通过数字图像相关数据获得的应变局部化进行的全面分析可以深入了解FPZ的性质，并表明真实II型裂纹的FPZ具有半椭圆形，并且尺寸可与我破解的一种模式。