Development of an Advanced Creep Model for Type 316 Stainless Steel

Methods for predicting stress relaxation behaviour rely on either the integration of a forward creep equation derived from constant load creep data or a stress relaxation equation specifically derived from data measured during dwell periods in either monotonic or cyclic loading. In general, these methods do not explicitly address the effects of prior loading history on the observed relaxation behaviour. In the current work, forward creep data have been acquired to allow identification of a stress-temperature dependent function that represents the observed correlation between measured values of primary creep strain and the initial plastic loading strain. The prediction of stress relaxation from forward creep equations has been extended to investigate the effects of plasticity on creep deformation through the inclusion of a back stress component in the creep deformation equations. The work is based on British Energy’s evolving experimental database for Type 316H stainless steel which includes data from ongoing low stress and stress change creep tests. These tests are currently being conducted to improve the understanding of plasticity-creep interaction effects. Improved methods for predicting stress relaxation in Type 316H stainless steel based on these experimental data are assessed against their ability to adequately describe the influence of prior loading history on primary creep and stress relaxation behaviour.