Abstract: In the present paper, a thermo-viscoplastic damage potential and Hill’s normal anisolropy (isotropy in plane).yield criterion to be quadratic in the stress components are combined to describe the interaction process of damage and instability during superplastic deformation, based on the kinetic equation for damoge during superplastic deformation and the deformation features of superplastic materials. The superplastic deformation process is devided into four Slages, namely, Stable, quasistable, Strain path drift to plane Strain and piane strain stages, and a damageinstability model during superplastic deformation of sheet metals is developed through numerical method. On the basis of the above results and taking the occurrence of localized inslability (dε₂=0) or the cavity volume fraction reaching the critical value(f_c) as a fracture criterion. the forming limit during superplastic deformation of sheet metals is predicted.

Key words: superplasticity, damage-intability, forming limit