| As the superplastic forming process becomes more acceptable as an efficient forming process, due to many advantages over conventional forming operations, there is a vital need for simple and accurate constitutive relations that can describe the deformation process under actual forming conditions. Most of the work that has been directed towards superplastic forming deals with uniaxial loading conditions and assumes isotropic behavior leading to the use of von Mises flow rules. Therefore, the main objectives of this study are to investigate both experimentally and theoretically the mechanical behavior of superplastic materials under various loading conditions and to examine the nature of anisotropy.; In this research, a wide range of experiments were designed to test superplastic materials under multiaxial loading conditions, including computer controlled tension-torsion machine and bulge forming system. Experimental results indicate that superplastic materials (e.g. Pb-Sn superplastic eutectic alloy) exhibit a strong degree of anisotropy and transient behavior which can be associated with the development of internal (back) stress. These results include axial effects in torsion, transient and stress relaxation effects in tension, and distortional and multiaxial effects in combined tension-torsion tests.; These phenomena are modeled within a continuum theory of viscoplasticity with internal variables in which anisotropic hardening is represented by an internal stress tensor whose evolution (including plastic spin) consists of hardening, dynamic recovery, and static recovery terms. The dynamic yield surface is assumed to be isotropic and depends on the second and third invariants of the deviatoric stress tensor (J{dollar}sb2{dollar} and J{dollar}sb3{dollar}). The results of various experiments including tension, torsion, stress relaxation, jump test, and combined tension-torsion tests are used to calibrate and verify the theoretical model. It is shown that the isotropic dynamic yield surface can successfully model the uniaxial behavior of superplastic deformation, including stress relaxation, and produces axial stresses in torsion. |
