Creep ductility which is assumed to be constant at a given temperature in many creep constitutive models, actually varies with temperature, stress level and creep strain rate, etc. In this paper, the relationship between creep ductility and stress levels of ferritic steels has been briefly discussed from the perspective of failure mechanisms. It can be generally divided into three regimes, including the upper shelf, lower shelf and the transition regime. The four-parameter logistic model has been adopted to quantitatively describe the stress-dependent creep ductility. Furthermore, a modified mechanism-based continuum damage mechanics (CDM) model for ferrtic steels has been proposed using the stress-dependent creep ductility model. Uniaxial creep tests of 2.25Cr1Mo0.25V steel at three stress levels have been carried out and the experimental data points realistically reflecting the creep behavior have been carefully selected to fit the improved CDM model using genetic algorithm (GA). It is shown that the improved model has the capability to characterize the whole creep process of ferritic steels and the stress-dependent creep ductility over a wide range of applied stress.

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