High temperature verification tests for constitutive equations

In our present work we follow a line of development which starts with Norton's power law for stationary creep (Norton 1929). During the past decade it has become apparent by the work of many authors (cited e.g. by Walker 1981) that the introduction of an internal back stress, sometimes also called friction stress or equilibrium stress, into Norton's power law opens up the possibility for describing not only stationary creep but also transitional visco-plastic effects of various kinds. Pioneering work in the internal stress concept applied to constitutive equations is associated with K.R. Williams and B. Wilshire (Davies et al. 1973). Although the existence of the internal back stress is widely considered, the detailed properties of the new internal variable are still unknown and have to be explored in a process of trial and error using experimental data. The fact that there is a great number of authors each favouring a different approach to back stress seems to be symptomatic for the currently unclear situation. The present approach to the internal back stress is based on yet another concept (Cords et al 1987). (orig./GL)