Strain localization in fluid-saturated anisotropic elastic-plastic porous media

An elastic-plastic constitutive model for anisotropic inelastic porous media saturated by one fluid is formulated. There are two central assumptions in the proposed model: (a) the definition of the yield function in a generalized effective stress space; and (b) the plastic coupling equation between the plastic strain rates of the fluid and solid phases. These assumptions introduce two degrees of freedom, namely the symmetric second-order tensors alpha and beta. Symmetry and decoupling of the elastic-plastic tangent stiffness are implied by particular choices of alpha and beta. On the other hand, the onset of strain localization in the fluid-saturated porous medium is shown to be governed by the underlying drained solid and consequently, to be independent of alpha and beta. Finally, generalizing the rate constitutive equations to account for the presence of a second fluid, the inception of strain localization is shown to be still controlled by the underlying drained solid.