Strain localization conditions in porous rock using a two-yield surface constitutive model

Abstract

Abstract This work examines theoretical conditions for localized deformation in porous rock, with emphasis on two recently identified deformation structures: compaction bands and dilation bands. Field and laboratory observations report that compaction/dilation bands consist of pure compressional/dilational deformation, which form perpendicular to maximum/minimum compression. A bifurcation approach is employed, with a two-yield surface constitutive model, to develop localization conditions under axisymmetric stress states for different stress paths. The first yield surface corresponds to a dilatant, frictional-damage mechanism (brittle regime), while the yield surface cap corresponds to a compactant mechanism (ductile regime). In the transitional regime, where both mechanisms are active, this model successfully predicts compaction bands and shear bands observed in axisymmetric compression tests. Due to discontinuities in the predicted band angle for probable material parameter values, this model may explain the lack of low angle compacting shear band observations in experiments. The two-yield surface model may also be applicable for a non-traditional axisymmetric extension stress path: increasing confining pressure with constant axial compression. Conditions for dilation band formation for this stress path are significantly less restrictive than corresponding compaction band conditions, suggesting that dilation bands could be a common deformation mode for high porosity sandstone.