Transgranular fracturing of crystalline rocks and its influence on rock strengths: insights from a grain-scale continuum–discontinuum approach

The aim of this study is to understand the effects of micro-heterogeneity, such as grain size, morphology and mineralogy, on the initiation, propagation and coalescence of microcracks in heterogeneous materials. A multiscale grain-breakable continuum–discontinuum model incorporating realistic micro-heterogeneity reproduction method is proposed to investigate the fracturing behaviours and confinement mechanism of rocks. Crack initiation and damage stresses are intrinsic properties of rocks determined by grain-scale heterogeneity. Intergranular tensile cracks are primarily initiated as a result of local stress heterogeneity along grain boundaries. The subsequent generation of transgranular shear cracks implies a rapid proliferation of grain-crossing fractures, leads to large-scale crack interaction and coalescence. The effect of confinement inhibits crack extension and increases the appearance of shear-induced grain pulverizations. Furthermore, the effects of grain size, grain morphology and mineralogy on macro mechanical properties, including crack initiation stress, crack damage stress, uniaxial compression strength and elastic modulus, are discussed. The simulated results indicate that the larger grain size contributes to stronger local stress heterogeneity, which results in a lower failure strength of rocks. The crack initiation stress is determined by local heterogeneity and is less affected by the change in average grain size. Grain morphology plays an important role in grain interlocking while a reduction in grain size variance leads to a more homogeneous stress field. The mineralogy is evaluated with the aid of a quartz-mica-feldspar diagram, and the quantitative relationships between mineralogy and the macro-scale mechanical properties of rocks are discussed.