Biaxial Experiments of Superplasticity and its Application to Constitutive Modeling

In the first part of this paper, a series of systematic experiments of 5083 Al alloy are carried out at 833 K to elucidate the characteristic features of superplastic deformation under multiaxial loading conditions. Proportional deformation tests at constant strain-rates show that the initial yield stresses obey the Prager-Drucker criterion. It is also shown that under constant strain-rate tests the material hardens under tension, while it softens under compression. Significant strain-rate dependence of the flow stresses is elucidated for tension, compression and torsion loadings. In the second part of the paper, the constitutive model of superplasticity formulated in the previous paper is improved to obtain better description of the drop of the flow stress due to cavity growth under tension loading. For this purpose, the concept of effective stress and the strain equivalence principle of continuum damage mechanics are introduced into the model, and the relevant evolution equations are modified. Comparison of the results of the proposed model with those of the corresponding experiments shows that the proposed model gives better predictions than that previously proposed by the authors.