Effect of Grain Scale Geometric Heterogeneity On Tensile Stress Generation In Rock Loaded In Compression

ABSTRACT:

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. 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 crackinitiation 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.



1. INTRODUCTION

Results for an investigation into the effect of grain heterogeneity on brittle rock failure [1] suggest: (1) homogeneous grain size distributions result in higher strengths and that the peak strength is highly dependent on the heterogeneity induced by grain geometry and not by material properties (i.e. modulus contrasts between mineral grains); (2) the minor principal stress is more uniformly distributed in the homogeneous model leading to higher strengths; (3) the geometric variation in grain size and shape induces a large tensile stress-field at relatively low strains when a rock is subject to compressive stresses; (4) geometric heterogeneity has a dominant effect on crack initiation, growth and interaction; and (5) grain size distribution in materials such as brittle rock may be a good index for representing the micro-heterogeneity.