1988 年 52 巻 11 号 p. 1051-1056
Plastic instability has been reanalyzed for superplastic materials with the strain rate sensitivity m and the flow hardening parameter γ. The condition under which a neck does not grow has been obtained as I≡(γ−1)⁄m≥0. Even though I<0, the neck broadens gradually provided I≥−1. In the case of −1>I>−2, the neck becomes initially sharper but it finally becomes broader. If I≤−2, the neck becomes sharp. Although the usual superplastic deformation is thought to proceed in an unstable state because I<0, a large elongation can take place provided that I>−2 and the neck becomes broader.