Formulation of a new elastoviscoplastic model for time‐dependent behavior of clay

Abstract

© 2020 John Wiley & Sons Ltd. This paper presents a generalized, elastoviscoplastic constitutive model, MIT-SR, that is capable of describing a wide range of time-dependent characteristics observed in clays from creep to strain-rate-dependent shear behavior. The key component of the proposed model is a novel evolution equation that attributes viscoplastic deformations to a state variable, Ra, referred to as internal strain rate, which represents the perturbation of the clay particle assembly due to historical straining. This state variable is driven by external straining actions (under compression or shear), which can be intrinsically linked to the loading step in classical plasticity theory, and decays with time representing a fading memory process. The proposed framework can be used to extend existing time-independent elastoplastic models. In this case, MIT-SR is built upon a prior elastoplastic model (MIT-S1), which uses 3-D stress-space surfaces and hardening laws to represent anisotropic effective stress-strain-strength properties, and a paraelastic approach to describe nonlinear hysteretic behavior at small strains. The paper highlights the versatility of the proposed MIT-SR model in representing a wide range of time-dependent characteristics for normally consolidation behavior and undrained shear behavior. By varying a strain-rate sensitivity parameter, β, the model can capture a full spectrum from temporary material response to changing strain rate, to isotache-type behavior where the normal consolidation and critical state lines are functions of the applied steady strain rate. The paper also showcases the model prediction for undrained creep and undrained relaxation behavior, and its promising capability in describing rate-effects under cyclic direct simple shear.