New creep constitutive equation for finite element modelling including transient effects

Abstract Creep experiments are time consuming and expensive; moreover, it is not possible to carry out experiments under the actual service conditions of particular materials due to the very low creep strain rate. However, the process seems to be an ideal field for computer modelling. The experimental data are obviously only available for steady-state conditions, and so the effects of varying conditions during startup or shutdown of the components can be described by modelling. Modelling creep deformation is usually based on the so-called “creep constitutive equations”, which describe the strain rate dependence on stress, temperature and time, or creep strain. Unfortunately, the equations are derived from conventional creep experiments under constant load conditions, and so the transient effects upon stress changes are ignored. Because the stress is transformed between model elements, a correct description of the creep behaviour must contain the transient effects. In this work, the conventional approach to describing primary and secondary creep stages is combined with an internal stress model of the transient creep stage to address the problem of the stress changes, as well as that of the low-stress creep regime.