An investigation of yield potentials in superplastic deformation

Recent results (Khraisheh et al., 1995 and 1997) have indicated that superplastic materials exhibit a strong degree of anisotropy and that the plastic flow cannot be described by the isotropic von Mises flow rules. In this study, the yield potential for the model Pb-Sn superplastic alloy is constructed experimentally for different effective strain rates using combined tension/torsion tests. A generalized anisotropic “dynamic” yield function is also proposed to represent the experimentally constructed yield potentials. The anisotropic function is not only capable of describing the initial anisotropic state of the yield potential, it can also describe its evolution through the evolution of unit vectors defining the direction of anisotropy. The anisotropic yield function includes a set of material constants which determine the degree of deviation of the yield potential from the isotropic von Mises yield surface. It is shown that the anisotropic yield function successfully represents the experimental yield potentials, especially in the superplastic region.