Rate effects in the constitutive modeling of concrete and geomaterials including plasticity and damage

This dissertation presents a unified constitutive model which can fully describe the viscoelastic-plastic-damage behavior of frictional materials. The viscoelasticity of the model is described by an appropriate modification of a viscoelastic model due to Kuhn et al. It is shown how the modified Kuhn model, which has a continuous distribution of relaxation times, can be approximated by a discrete model of the internal variable type. The problem of cyclic creep is studied and solved analytically.; The viscoelastic model is combined in series with a rate-dependent plastic model, developed after a modification of a plastic model for concrete by Lubliner et al. (1989), so that the rate dependence of the strength of the material can be described for arbitrary type of loading or boundary conditions. The rate-dependent formulation allows the prediction of the effect of stress rate on the strength of the material. In the yield criterion employed the concept of cohesion is unambiguously defined. Both hardening and softening are accounted for and are described by the evolution equations of the internal variables contained in the yield criterion. In addition to the plastic damage described by the loss of cohesion the model accounts for stiffness degradation, which is taken into account through the dependence of the stiffness on a degradation internal variable.; An extensive fitting program was undertaken using experimental data obtained on concrete specimens from the Cypress structure, destroyed in the Loma Prieta earthquake of October 1989, as well as young concrete specimens. The specimens were subjected to dynamic tests in the range 0.1 Hz-20 Hz and to short-term creep tests. Unlike current models of material damping in the time domain, the proposed model predicts a nearly constant loss tangent over a wide frequency range, which is a characteristic of most engineering materials. The fitting of material parameters was performed in the sense of nonlinear least squares by means of a Levenberg-Marquardt algorithm. (Abstract shortened by UMI.)...