Experimental and numerical study of fracturing in degraded and graded crystalline rocks at the laboratory scale

The rock masses are mineral based geomaterials which have been formed by different mineralogical compounds in terms of dimensions and mechanical properties. Geomaterials are often classified as inhomogeneous anisotropic materials due to the orientation of minerals, frequently asymmetric distribution of mechanical properties, porosity, layering, and discontinuities. Differences in deformation properties of mineral crystals under applied stresses is a potential source for crack initiation/propagation in geomaterials. Meanwhile, weathering, alteration, and thermo-chemical agents can change the distribution of mechanical properties by causing unequal effects on different minerals. Redistribution of properties changes the intensity of heterogeneity and anisotropy and ultimately changes the rock fracture mechanism. Therefore, in this study, with the use of transgranular and intergranular enrichment functions and an interaction integral for inhomogeneous materials the failure mechanism of graded and degraded rocks affected by altering factors is simulated. The concept of energy release rate was used to predict the crack initiation angle and the crack trajectory within or between mineral grains. To verify and validate this approach, the results are compared with experimental test results and those reported in the literature. The results show that the crack trajectory significantly depends on the properties of minerals and their boundaries. The smaller the ratio of intergranular to transgranular critical fracture energy is, the more likely the crack to propagate between the grains. Simulation of crack propagation in altered specimens also showed that the crack tends to preferably propagate in weaker minerals. It is also found that using homogenization techniques instead of explicit modeling of minerals may produce inaccurate results. That is because it predicts pure fracture modes at angles in which the mixed tensile-shear modes should be detected.