On Brazilian disk test for mixed-mode I/II fracture toughness experiments of anisotropic rocks

This paper presents the mixed-mode I/II stress intensity factors (SIFs) of the through-thickness cracked Brazilian Disk (TCBD) specimen made of an anisotropic rock. We show that the SIFs of an anisotropic TCBD specimen not only depend on the geometry and loading, but also on two non-dimensional parameters, the anisotropy ratios of the Young's modulus and the apparent shear modulus, as well as the anisotropy orientation. For most rocks, the Saint-Venant relation gives a good approximation of the transverse shear modulus. We have therefore used this approximation to efficiently formulate the dependency of the SIFs on the material constants based on only the anisotropy ratio of the Young's modulus and the anisotropy orientation. Reference plots of the SIFs, obtained from the finite element (FE) simulations, are reported for the entire range of the mixed-mode I/II loading for various geometrical and material configurations. The results show that the SIFs of the TCBD specimen depend significantly on the anisotropy ratio of the Young's modulus, and the anisotropy orientation. Therefore, elasticity anisotropy must be considered when designing mixed-mode fracture toughness experiments using the TCBD tests.