On the failure characteristics of superplastic sheet materials subjected to gas pressure forming

The forming pressure profile, thickness distribution of the bulged sheet, and the failure characteristics of superplastic materials subjected to gas pressure forming were examined. Different failure modes were observed for different forming rates. Slow forming failure can be related to the nucleation and growth of voids. When forming under constant effective strain rate, it was observed that failure occurs at a time near the peak value of the forming pressure profile constructed based on the assumptions of balanced biaxial stretching and isotropy. A simple instability analysis predicted that failure occurs at a much lower thickness at the pole than the actual measured value.