A generalized criterion of plastic instabilities and its application to creep damage and superplastic flow

A novel criterion to distinguish between stable (homogeneous) and unstable (localized) plastic flow in tensile specimens is proposed which is based on a consideration of the structural evolution on characteristic time-dependent intrinsic or extrinsic length scales. Macroscopic localization is predicted to occur if fluctuations grow with respect to these length scales. Previous instability criteria by Hart, Estrin and Kubin, and Molinari are recovered as special cases of the present generalized criterion. Moreover, it allows one to distinguish between homogeneous and heterogeneous nucleation of slip. Two applications are discussed: (i) the peculiar necking resistance during superplastic flow is shown to depend not only on a high value of the strain-rate sensitivity but also on a large amount of deformation accommodated by grain boundary sliding; (ii) the instability criterion is applied to creep damage described in terms of a model of the Kachanov-Rabotnov type. This allows for a determination of the critical damage parameter at failure.