A STRAIN-RATE DEPENDENT CONSTITUTIVE MODEL FOR CLAYS AT RESIDUAL STRENGTH

Plasticity theory is extended to incorporate strain rate effects on th e residual shear strength of clays. The clay is assumed to behave elas tically 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 yie lding and subsequent plastic deformation, the stress and strain states at a point will satisfy I:he rate-dependent yield function (loading f unction). When the effective plastic strain rate decreases to a thresh old strain valuer the loading surface moves, or collapses, to the stat ic yield surface. A constant volume flow rule is used to calculate pla stic deformation. The computed stress-strain relationship is formulate d in two parts, namely a rate-independent part and a rate-dependent pa rt. The rate-independent part is the same as that used in classical el astoplastic formulations, whereas the rate-dependent part is dependent on the current strain rate of the material. The use of the model is i llustrated using a numerical example simulating a two-dimensional plan e strain test.