Abstract: Functional expressions of mε, mv and mF (they are respectively strain rate sensitivity index under fixed strain ε, strain rate sensitivity index under fixed forming velocity v and strain rate sensitivity index under fixed load F) in plastic and superplastic deformation from the state equation and the basidc mechanical theory are deduced. Furthermore, experimental data of typical superplastic alloy ZnAI5 are consistent with the theoretical value. Thus, the problem-why mε, mv and mF are different with ε changing, even obtain abnormal result that the value of mF is more than 1 and the value of mv is negative-is solved theoretically, which is put forward experimentally. Then mechanical essence of m value is revealed further.

Key words: Plasticity, Strain rate sensitivity index, Superplasticity