A strain rate dependent constitutive model for clays at residual strength

Plasticity theory is extended to incorporate strain rate effects on the residual shear strength of clays. The clay is assumed to behave elastically before yielding and then in a perfectly plastic manner with no volume change during yielding. The Mohr-Coulomb failure criterion is used in the rate-dependent model in which the strain rate affects the mobilized effective friction angle of the material. During initial yielding and subsequent plastic deformation, the stress and strain states at a point will satisfy I:he rate-dependent yield function (loading function). When the effective plastic strain rate decreases to a threshold strain valuer the loading surface moves, or collapses, to the static yield surface. A constant volume flow rule is used to calculate plastic deformation. The computed stress-strain relationship is formulated in two parts, namely a rate-independent part and a rate-dependent part. The rate-independent part is the same as that used in classical elastoplastic formulations, whereas the rate-dependent part is dependent on the current strain rate of the material. The use of the model is illustrated using a numerical example simulating a two-dimensional plane strain test.