TIME-DEPENDENT ELASTOPLASTIC CONSTITUTIVE EQUATION BASED ON THE SUBLOADING SURFACE MODEL AND ITS APPLICATION TO SOILS

KOICHI HASHIGUCHI and TAKASHI OKAYASU

ABSTRACT: Various constitutive models for the description of time-dependent deformation behavior have been proposed. However, it is first verified in this article that a pertinent model applicable to the description of deformation for a wide range of stress below and over the elastic limit, i.e. the yield stress, has not been found up to the present. It should be noted that a stress goes out over the yield surface at a high rate of deformation, only elastic deformation being induced. The subloading surface model does not premise that a stress exists on the yield surface even in the plastic loading process and thus describes the plastic deformation induced by the rate of stress within the yield surface, exhibiting the smooth elastic-plastic transition. In this article the subloading surface model is extended so as to describe the time- dependence for a wide range of deformation rates by allowing the stress to go out from the yield surface based on the physical interpretation that a plastic deformation due to the mutual slip between microstructures is suppressed for the deformation at a high rate causing the increase of viscous resistance acting between the microstructures. Further, based on this, a time-dependent elastoplastic constitutive equation of soils is formulated by incorporating the secondary-consolidation phenomenon, and its ability to predict deformation behavior of soils is verified by comparisons with test data on fundamental time-dependent behavior with various deformation rates, creep and stress relaxation under the undrained condition.

Key words: constitutive equation of soil, creep, plasticity, relaxation, (subloading surface model), time effect (IGC: D6 /E2)