Deformation Modeling and the Strain Transient Dip Test

Abstract

Recent efforts in material deformation modeling reveal a trend toward unifying creep and plasticity with a single rate-dependent formulation. While such models can often describe actual material deformation with some precision, most require a number of different experiments to generate model parameter information. These experiments can be complex, time-consuming, and expensive to perform. Recently, however, a new model has been proposed which is unique in that most of the requisite constants may be found by examining creep transients brought about through abrupt changes in creep stress, namely, the so-called strain transient dip test. While the strain transient dip test is simple in concept, it is extremely difficult to perform in actuality. The critical measurement in this test is the absence of a resolvable creep rate after a stress drop. As a consequence, the result is extraordinarily sensitive to strain resolution as well as machine mechanical response. This paper presents the design of a machine in which these spurious effects have been minimized and discusses the nature of the strain transient dip test using the example of aluminum. It is concluded that the strain transient dip test is not useful as the primary test for verifying any micromechanical model of deformation. Nevertheless, if a model can be developed which is verifiable by other experiments, data from a dip test machine may be used to generate model parameters.