FE modeling of strain localization in soft rock

A finite-element (FE) model of localized deformation in soft rock taking a strong discontinuity approach is presented. The model is formulated within the context of rate-independent, nonassociated Drucker-Prager plasticity with nonlinear cohesion hardening/softening. Strain localization is modeled as a jump in the displacement field and simulated within the framework of the FE method using the standard Galerkin approximation. The model is used to simulate the load-displacement behavior of Gosford sandstone deforming in plane strain, focusing on the prediction of the stress levels necessary to initiate strain localization, based on the strong and weak discontinuity criteria (jumps in displacement and strain, respectively), and on the demonstration of mesh-independence of the FE solutions in the bifurcated state. For the sandstone, the onset of weak discontinuity is detected first, before the onset of strong discontinuity, suggesting a possible coupling of the two types of discontinuities in the strain-softening regime.