P91-type steel is widely used for the high-temperature pipe work components in advanced power plants. The creep behavior of the P91-type steel has been studied by many researchers during the past years. Since it is well known that the creep behavior of P91-type steel cannot be satisfactorily described by a simple, Arrhenius-type, power-law constitutive model. While Norton-Bailey creep is a deviatoric temperature-dependent creep model, furbished with a time-hardening creep model, which is the most common model for modeling primary and secondary creep together, and Kachanov-Rabotnov creep damage theory described with Norton creep model can be used to model tertiary creep. Both of them based on Norton creep constitutive equation. In this paper, based on the Norton-Bailey creep law and Kachanov-Rabotnov creep damage theory, a new combined constitutive model has been developed, in which the creep and damage function are both considered as nonlinear variables. The damage parameters in the model have clear physical meaning and can be determined from the benchmark experiment. The results indicated that this combined damage model was applicable to describe the full damage evolution for P91-type steel.

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