On the anisotropy of shear fracture toughness in rocks

Experimental and theoretical analyses are presented that assess true mode II fracture toughness, K-IIc, fracture energy, G(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-IIc, and present simplified relations for the direction dependence of the G(IIc) in an anisotropic plane. Our experimental data on anisotropic Grimsel Granite validate this theoretical model, thereby providing supporting evidence that K-IIc of this rock type indeed follows a sinusoid function. This model takes only two principal values of K-IIc to characterise fracture toughness and energy in any direction within an anisotropic plane. Our analyses also show that the values of K-IIc and G(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.

Automated summary

Experimental and theoretical analyses were conducted to assess true mode II fracture toughness, K-IIc, fracture energy, G(IIc), and the associated fracture process zone (FPZ) in anisotropic rocks. The studies provided a theoretical basis for the sinusoidal variation of K-IIc and simplified relations for the direction dependence of G(IIc) in an anisotropic plane. Experimental data validated the theoretical model and showed that values of K-IIc and G(IIc) are significantly higher than mode I values, with insight into the nature of the FPZ of a true mode II crack.