Abstract: Due to the importance of establishing the quantitative mechanical analysis theory of superplastic deformation, the mechanical meanings of material parameter k and strain-rate sensitivity index m in Backofen’s constitutive equation are discussed. The canonical measurement formulae and methods of m value under constant strain ε, constant velocity υ and constant load F3 typical deformation routes are given Moreover, the precision of traditional measurement method is compared with that so computer simulation under each typical deformation route. Under the above deformation routes, the strain-rate sensitivity indexes mε, mυ and mF of typical superplastic alloy Zn – 5% A1 for room temperature (18℃) and superplastic state (340℃) are obtained by the experiments. The results indicate that there exist large deviation among mε, mυ and mF , they have different variational rule with strain rate ε respectively. These prove that the strain – rate sensitivity index depends tightly on the deformation route from experiment, even on the condition that those stress-state are similar. So new problem emerges, it requires giving the explanation in theory why there exist deviation and different variational rule with strain rate respectively, and further disclose the mechanical essence of mε, mυ and mF.

Key words: Strain – rate sensitivity index, Superplasticity, Tension deformation