Semi-brittle flow of rocks: Cracks, dislocations and strain hardening

 Strain hardening is a key feature observed in many rocks deformed in the so-called ``semi-brittle'' regime, where both crystal plastic and brittle deformation mechanisms operate. Experimental observations in calcite aggregate show a negative correlation between strain hardening rate and microcrack density. Strain hardening is typically caused by accumulation of unrelaxed elastic stresses, for instance due to dislocation storage or frictional sliding, but the role of tensile cracks in that process is not clear. Here, I will first summarise key experimental observations in calcite aggregates, documenting the co-evolution of microstructural features as a function of strain, and then propose a simple microphysical hardening model that couples tensile microcracking with dislocation storage. The model relies on viewing tensile cracks as free surfaces that absorb dislocations, thus reducing the dislocation storage rate and the hardening coefficient. The model captures important qualitative features observed in calcite marble deformation experiments: pressure-dependency of strength in the ductile regime, and a reduction in hardening linked to an increase in crack growth with decreasing confining pressure. Although very promising at a conceptual level, the model has limitations and needs to be tested more systematically before it can be used to make geological predictions of strength in the semi-brittle regime.