CONSTITUTIVE MODELING OF SUPERPLASTIC DEFORMATION .1. THEORY AND EXPERIMENTS

In this work, we investigate the mechanical behavior of superplastic m aterials under various loading conditions and examine the nature of an isotropy. This and other phenomena are modeled within a continuum theo ry of viscoplasticity which includes an overstress function, 'yield su rface' and internal variables. Anisotropic hardening is represented by an internal stress tensor whose evolution (including plastic spin) co nsists of hardening, dynamic recovery, and static recovery terms. The 'dynamic' yield surface (within a viscoplasticity formulation) is assu med to be isotropic and depends on both J(2) and J(3). Various experim ents on a Pb-Sn superplastic alloy including tension, jump test, stres s relaxation, fixed-end torsion, and combined tension-torsion tests we re performed and the results are used to calibrate and verify the theo retical model. It is shown that the isotropic dynamic surface can succ essfully model the uniaxial behavior of superplastic deformation, incl uding stress relaxation, and produces axial stresses in fixed-end tors ion. (C) 1997 Elsevier Science Ltd.