The Overstress Dependence of Inelastic Rate of Deformation Inferred from Transient Tests

Abstract

Transient tests are used to demonstrate unusual or even paradoxical deformation behaviors in metallic alloys and polymers at ambient and elevated temperatures. Included are repeated relaxation tests and creep tests of short duration. It is shown that creep rate need not increase with a stress increase and that, at the same stress level, creep rate can be different on loading and unloading. Also, a stress magnitude increase can be found during relaxation. Once steady inelastic flow is reached, the relaxation rate is nearly independent of the stress and strain at which relaxation starts; it depends mainly on the preceding prior inelastic strain rate. These phenomena can be modeled using an overstress (effective stress) dependence of inelastic rate of deformation and a proper evolution law for a single state variable, the equilibrium stress (back stress) within the context of a “unified” state variable theory. The author and his students have developed the viscoplasticity theory based on overstress (VBO) that can model these unusual behaviors. © 1995, The Society of Materials Science, Japan. All rights reserved.