A comprehensive creep model based on the internal stress

Design specifications for new generation of power plants require materials to perform at higher temperatures, for longer time with reduced planned maintenance schedule. In order to achieve this goal, advanced material modeling is required. For what concerns creep, not only secondary creep but also primary and tertiary creep regimes become important for an accurate creep life prediction and, therefore, need to be taken into account into more sophisticated models. Among all modeling approaches to creep, mechanism based theories are the more promising since they results in more accurate prediction of the material creep response over wide range of stress and temperature. In this perspective, it has long been recognized that the mechanics and kinetics of the deformation and recovery processes occurring at the microscale are determined by, amongst other things, the local effective stress while, at macroscopic level, it may still possible to discern an effective stress, which is different from the applied stress that drives the creep strain accumulation, [1]. Recently, the authors proposed an internal stress based model for primary creep which is independent on the creep rate formulation at the steady state ensuring the continuity of the creep curve at the transition between primary and secondary creep stages, [2]. In this work, in order to account for all creep stages, this modeling framework is extended further. In particular, for tertiary creep stage, it is assumed that microstructural changes are responsible for the reduction of the internal stress and consequence increase of effective stress and, therefore, of the creep rate. The possibility to derive the internal stress decay function from tertiary creep data is discussed. The proposed model has been applied to P91 high chromium steel and preliminary results are presented.