A unified viscoplastic material model based on mechanical considerations

The analysis of a torsion experiment on a heat exchanger tube at 950 deg. C is in the focus or the present investigations. It has been performed employing a constitutive equation which was developed in the Institute for Reactor Materials at the Nuclear Research Centre Juelich. Shear loads with lad reversals in cycles of 100 h cause complex time dependent torsional strains as well as strain hardening and strain softening effects and back strains at the end of the load history. The theoretical analysis clarifies how these effects are related to other effects well known from uniaxial creep tests and hot tensile tests. In the present contribution, the key role of primary creep is particularly emphasised. Primary creep is not only observed at the beginning of the loading history but also after load reversals where it extends to higher strains than the initial primary creep. This enhanced primary creep is responsible for additional plastic contributions in torsional strain as well as for the observed strain hardening and strain softening effects. It is shown that back strains and primary creep strains are related to the same basic material properties. (authors)