On the time-dependent inelastic deformation of metals

Abstract

The research by Prof. Paul Dawson has done much to further the use of constitutive models based on internal state variables toward applications of industrial relevance. Two particular models examined by Dawson are one based on the “mechanical threshold”, developed by Kocks and co‐workers and another based on the “hardness”, advanced by Hart and co‐workers The first of these is intended to describe bulk plastic flow, and is associated with the notion of percolation of dislocations. The second is a phenomenological model that includes anelasticity and gives reliable prediction of stress relaxation. In this work, we examine the time‐dependent inelastic deformation of the aluminum alloy AA 5182‐H19 — during the manufacturing process, but also under in‐service conditions. Assessment is made through evolution of both mechanical properties and geometry. A constitutive model based on two internal variables is proposed. One variable is the mechanical threshold stress, dominating for the bulk plastic flow; the other is due to the operation of dynamic pile‐ups, providing microplasticity and based on a modification to Hart’s model by H. Garmestani. These two state parameters are not directly related to distinct mechanisms of flow resistance in this solution‐hardened alloy, but rather to long‐range and short‐range dislocation motions. As a result, this model can characterize both the large scale plastic flow and subsequent transient processes having very low strain rate, such as stress relaxation and time‐dependent springback.