抄録
It is important to formulate the reliable constitutive equations which are indispensable for the accurate numerical simulations of superplastic forming processing. One of the big problems for the formulation is a lack of experimental data on the superplastic behavior under the multi-axial loading conditions, especially the yield surface, the subsequent yield surface and potential function, when we follow the standard (classical) formulation procedure based on the so-called macroscopic potential theory of plasticity. In this research work, the initial yield surface of a superplastic material was first investigated experimentally by applying some combined loads of axial force and torque to thin-walled tubular specimen of 5083Al alloy which was used in engineering applications. Tests were carried out at elevated temperatures (673K, 723K, 773K, 823K). It was found out in the experiments that the initial yield surface can not be explained by the von-Mises type J2 theory. In this paper, a new yield function is proposed.
