Rock is typically a heterogeneous material composed of different types of inherent microstructures. The microstructures of a rock at the grain scale are usually associated with different mineral aggregations and microdefects such as joints, voids, and cleavage planes. In the present manuscript, a discrete element grain-based model featuring three-dimensional random Voronoi tessellations is proposed to study the deformation and fracturing of brittle rocks. Different grains, representing different rock-forming minerals, and the contacts between the grains were assigned different mechanical and physical properties, accounting for the grain scale heterogeneity observed in natural granular rocks. The simulations showed that rock specimens characterized by a wide grain size distribution are weaker than those with a narrow grain size distribution, but that spherical grains do not significantly influence the strength of specimen. Moreover, the simulations show that increasing the pre-existing porosity reduces specimen strength, due to the concentration of stress around the voids, and that intra-granular cracking is the dominant contact damage mechanism during uniaxial compression.

1. Introduction

The rock is a heterogeneous polycrystalline material in natural conditions. It composes different mineral compositions and microstructures such as grain boundaries, micro-cracks, cleavages (Eberhardt, Stead, Stimpson, & Read, 1998; Martin & Chandler, 1994). Microstructure and mineral grains of rock is known to control the complex macroscopic mechanical response and fracture pattern. Grain boundaries act as the predominant source of stress concentrating flaws. Once the local stress near the flaws exceeds the local strength of rock, the initiation of microcrack starts from existing flaws. The density of cracks increases as the load increases. Propagation and coalescence of the cracks eventually cause macroscopic failure of rock. This paper aims to simulate crack damage evolution of brittle rock at mesoscale (grain scale). Mesoscale is between macroscale (phenomenological) and microscale (atomic or molecular). We used a three-dimensional discrete element grain-based model (3DEC-GBM), consisting of an assemblage of Voronoi polyhedra grains, to represent rock by a dense packing of bonded mineral grains of non-uniform size and shape. We explored the influence of grain size distribution, grain shape and porosity on the mechanical behavior and strength of granular rock.

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