A combined fatigue-ductile damage model with optimized parameters for predicting the mechanical response of high-Cr steels at high stress levels

In this paper, a combined fatigue-ductile damage model is presented, which aims to provide accurate predictions on the mechanical response of high-Cr steels at high stress levels. In this model, the Chaboche-type constitutive laws are adopted for the viscoplastic strain and two damage variables (i.e., the fatigue damage and the ductile damage) are incorporated to describe the failure behaviors of high-Cr steels. An efficient scheme is proposed to identify the material parameters in the model, which contains three steps. First, the initial values of the elastic and viscoplastic parameters are determined through a step-by-step procedure. Second, a parameter optimization program is designed and utilized to generate the optimized elastic and viscoplastic parameters. Third, the damage associated parameters are further determined. By adopting one set of optimized parameters, the mechanical response of high-Cr steels in the cyclic loading test, the long-term constant loading test and the loading-dwell test can be predicted at a quantitative level. Some key experimental features (e.g., the minimum strain rate and average rupture time in constant loading test, the stress amplitude evolution curves in the cyclic loading test) can also be properly described. Furthermore, it is revealed that under different loading conditions, the failure behaviors of high-Cr steels are induced by different damage mechanisms. The results obtained in this work are useful for safety design and residual lifetime evaluation of high-Cr steel components in supercritical power plants.