Critical View on the Creep Modelling Procedures

Process of creep deformation is rather complex, consisting of many interconnected subprocesses, mainly: (i) the creep strain itself, based on dislocation mobility as well as grain boundary activity, (ii) development of dislocation substructure (work strengthening and dynamic recovery), (iii) development of phase structure (phase transformations, precipitation, particle coarsening, etc.), and (iv) nucleation and development of voids and microcracks, i.e. creep damage. The creep experiments are time consuming and expensive, moreover, it is not possible to make experiments under the service conditions of particular materials due to very slow creep strain, the process seems to be ideal field for computer modelling. The experimental data are obviously available for the steady conditions only, so the effects of varying conditions during startup or shutdown of the components can be described by modelling. The model of creep deformation is obviously based on the so-called "creep constitutive equation", which should describe the strain rate dependence on stress, temperature and some other variables. Nevertheless, the comprehensive physical description of all the above mentioned processes is still missing. This paper should illustrate the shortcomings of most "creep constitutive equations", confronting them to some experimental results on common structural materials under non-steady loading conditions.