On micro-cracking, inelastic dilatancy, and the brittle-ductile transition in compact rocks: A micro-mechanical study

This work introduces a micro-mechanical grain-aggregate model and numerical simulation capability to study the combined effects of grain-boundary slip and separation, as well as grain-interior plasticity on the overall deformation of compact rocks. Two major conclusions can be drawn from our simulation study: (i) At sufficiently low confining pressures, the widely-observed inelastic dilatant response in compact rocks under compression is attributable to the geometrically-mismatched grain-boundary sliding and concomitant formation of triple-junction cracks which result in an increase in volume. Failure patterns change from splitting-fracture at low confining pressures, to distributed micro-cracking in macroscopic “shear”-bands as the confining pressure increases. (ii) When the confining pressure increases to an amount such that grain-boundary sliding is suppressed due to frictional effects, the inelastic dilatancy effects disappear, and isochoric grain-interior plasticity takes over to accommodate the imposed external deformation, and this is the major cause of the brittle-ductile transition in these materials.