On yielding, failure, and softening response of rock

Mechanical descriptors of inelastic behavior of rock are reviewed and approaches to measure material and system response are highlighted. A non-traditional testing method that involves biaxial deformation developed through a Vardoulakis- Goldscheider plane strain apparatus is used to assess dilatancy and friction, as well as dilatancy of the shear band after localization. The Paul-Mohr-Coulomb failure criterion, which includes the intermediate stress, is applied to interpret the experiments at peak stress. Post-peak response, often referred to as softening, is explained as a size effect within the context of class I-class II stability.