Delayed plastic model for time-dependent behaviour of materials

A delayed plastic model, based on the theory of plasticity, is proposed to represent the time-dependent behaviour of materials. It is assumed in this model that the stress can lie outside the yield surface and the conjugate stress called static stress is defined on the yield surface. The stress-strain relation is calculated based on the plastic theory embedding the static stress. Thus, the stress-strain relation of the model practically corresponds to that of the inviscid elastoplastic model under fairly low rate deformation. The delayed plastic model is coupled with the Cam-clay model for normally consolidated clays. The performance of the model is then examined by comparing the model predictions with reported time-dependent behaviour of clays under undrained triaxial conditions. It is shown that the model is capable of predicting the effect of strain rate during undrained shear and the undrained creep behaviour including creep rupture.