Mesoscopic Damage and Fracturing of Heterogeneous Brittle Rocks Based on Three-dimensional Polycrystalline Discrete Element Method

Abstract

Rock heterogeneities often control microcracking on the grain scale and therefore the mechanical behavior of the rock on the macroscale. We therefore studied the fracture patterns, mechanical behavior and strength of granular rocks using the three-dimensional numerical modeling code 3DEC. The numerical rock specimens, built using software package Neper, comprised an assemblage of Voronoi polyhedra designed to resemble grains. 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 model accurately captured the evolution of damage on the grain scale (tensile and shear microcracks) and the macroscopic mechanical behavior, strength, and failure patterns observed in laboratory experiments performed on sandstone. Based on this validation, we explored the influence of grain size and shape distribution on the mechanical behavior of granular rock. We show that rock specimens characterized by a wide grain size distribution were weaker than those with a narrow grain size distribution, but that grain sphericity distributions does not significantly influence strength. We also found that increasing the initial porosity reduced specimen strength, due to the concentration of stress around the voids, and that intergranular cracking was the dominant contact damage mechanism during uniaxial compression. Due to the ease of flexibility in varying grain scale parameters, we conclude that the model used herein is a powerful tool that can be used to study the mechanical behavior, strength, and failure patterns of rock.