عنوان مقاله [English]
Elasto-viscoplastic constitutive models, based on the Perzyna overstress concept, have been applied vastly to describe the time-dependent behavior of soils. The response of these models is divided into two parts: time-independent elastic and time-dependent viscoplastic. Despite their simplicity, some important behavioral aspects, such as the plastic failure state and plastic deformation in rapid loading, cannot be modeled. In fact, the consistency condition is not verified. On the other hand, the yield surface can be crossed by the stress state. In this manner, for rapid loading, the model response is elastic, and viscoplastic strain occurs during the time. However, in the case of rapid loading, a great part of the response is plastic. In this approach, an elastoplastic-viscoplastic model is proposed. The model has been developed from a basic elastoplastic model by considering an additional viscous mechanism. This model is able to explain the time-dependent behavior of soils such as creep (primary and secondary), stress relaxation and strain rate effects. In addition, existing problems in the classical elasto-viscoplastic models related to the plasticity failure and rapid loading are solved in the proposed model. The physical meanings and the identification strategy of the model parameters are clearly given. A simple example of model validation has been presented in the last part of the paper. The results of validation indicate the good capability of the proposed model.