Elastic-Plastic Coupling and Localization of Porous Sandstone Under Axisymmetric Compression

Multiple tests using a weak porous sandstone deformed under axisymmetric compression in dilational, compactional, and transitional regimes are analyzed to investigate elastic-plastic coupling (i.e. the influence of plastic deformation on elastic properties) and localization behaviors. Acoustic emissions track the onset and progression of high- and low-angle shear bands and low-angle compactional features. Multiple unloading loops show consistent degradation of elastic constants pre-failure and this degradation is parameterized as a function of work-conjugate plastic strains and increasing functions of stress magnitude. Interestingly, shear moduli (assuming elastic isotropy) in all tests which experience localization are observed to degrade to approximately the same magnitude just prior to onset of localization. Results were applied to both localization (bifurcation) theory and to an elastic-plastic model, and to test assumptions of isotropic hardening. The relevance of elastic-plastic coupling is seen when partitioning the measured shear and volume strains into elastic and plastic portions. Significant contributions to each are shown as “coupling” strains associated with the degradation of elastic moduli. This has an observable effect on magnitudes of bifurcation parameters and thus on the comparison of theoretical predictions with experimental results. A better comparison of shear band angles between theory and experiment is seen when including the elastic-plastic coupling. Low-angle compaction bands observed experimentally in the transitional regime are only predicted when including the coupling. A 3-invariant, mixed-hardening, continuous yield surface, elasto-plasticity model is applied to calculate stress-strain paths leading up to localization. Model features important to incorporate to best describe porous sandstone constitutive behavior include non-associativity, nonlinear elasticity, elastic-plastic coupling, and kinematic hardening. The authors gratefully acknowledge the U.S. Department of Energy Basic Energy Sciences Program and the National Science Foundation Tectonics Program (award EAR-0711346) for funding. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under contract DE-ACOC4-94AL85000.