Transient Stress Relaxation Test to Identify Material Constants in Dislocation Density Model

Abstract

The constitutive behavior of metallic materials in terms of dislocation kinetics can be successfully described using a dislocation density-based model. Although the kinetics of thermally activated plastic deformation is well described by such models, the number of material constants associated with the model leads to non-unique solution set. In the present work, transient stress relaxation test is used to identify rate-dependent material parameters. The stress relaxation test, when used in conjunction with stress-strain curve can reduce the uncertainty associated with parameter identification. The proposed methodology is demonstrated using aluminum alloys subjected to severe plastic deformation processes such as cryorolling and constrained groove pressing. Kocks-Mecking-Estrin (KME) dislocation density model is implemented as a user subroutine in commercial finite element analysis software. The parameter identification procedure is validated by comparing the experimental results of monotonic tensile and limiting dome height tests. Using dislocation density model, it is shown that unlike the general understanding, the limiting strain is not related to the strain hardening exponent. The limiting strain correlates only with the extent of dynamic recovery, a component of strain hardening.