The Brittle-Ductile Transition in Porous Limestone: Failure Mode, Constitutive Modeling of Inelastic Deformation and Strain Localization

Understanding of the mechanics of the brittle-ductile transition (BDT) in porous limestone is significantly more challenging than for sandstone because of the lack of consistent acoustic emission activity in limestone, meaning that one must rely on alternative techniques. In this paper, we investigate systematically the failure modes in Indiana limestone using X-ray microComputed Tomography imaging (mu CT) and Digital Volume Correlation (DVC). Our new mechanical data show that the envelope for the onset of shear-enhanced compaction can be well approximated by an elliptical cap. The DVC analysis revealed the development of shear bands through the BDT, but no evidence of compaction bands. The shear band angles were between 29 degrees and 46 degrees with respect to the maximum principal stress. Compiling these new results with published data on Purbeck and Leitha limestones, we showed that inelastic compaction in each of these dual porosity allochemical limestones was in a good agreement with the normality condition, as defined in plasticity theory. Comparison of the observed failure modes with predictions based on bifurcation analysis showed that the shear band angles are consistently smaller than the theoretical predictions.