Effect of grain scale geometric heterogeneity on tensile stress generation in rock loaded in compression

Brittle failure of rock is dominated by tensile mechanisms even in an overall compressive stress field. Heterogeneities play a key role in the development of the localized tensile conditions. However, details on how heterogeneities affect brittle rock failure processes are still open for debate. Through the use of regular honeycomb grain arrangements progressing to highly irregular Voronoi arrangements, the impact of grain geometric heterogeneity through finite element tools and discrete element methods was assessed and it is shown that non-uniformity of grain size distribution is not a critical parameter to evaluate crack initiation, peak strength, or micromechanical behaviour. The results demonstrate that grain boundary orientation and grain system arrangements control tensile stress generation inside a brittle rock specimen under compression and thus impact the crack-initiation stress level. This suggests that crack interaction and peak strength is then affected by the kinematic and allowable degrees of freedom in the grain assembly of the damaged rock. At this stage, grain deformability and more importantly grain breakage is needed to increase the degrees of freedom required for the linkage and formation of a macroscopic rupture. Based on this it is suggested that if it is possible to characterize grain boundary orientations or arrangements, various micro-mechanical behaviour could potentially be forecast.