Creep-Fatigue Interaction Models for Grade 91 Steel

摘要

Different approaches for modelling creep–fatigue (CF) interaction are used onstrain controlled creep fatigue data of 9Cr–1Mo-VNb (P91) steel and assessedwith the target of finding suitable candidates for use in design rules. Theassessed models include time, ductility, and strain energy-based creep-fatigueinteraction methods and two simplified models. For the interaction diagrambasedmodels, the challenge of acquiring representative creep damagefractions from the dynamic material response, i.e., cyclic softening withchanging relaxation behaviour is addressed. In addition, the interactiondiagram approaches are discussed in the light of known (fatigue) materialscatter and defining representative cycles for CF data. The performance of themodel are presented and also compared against the RCC–MR design codemethodology. It is shown that the fitting accuracy of the complex interactionmodels vary significantly and that modified ductility based models seem to beless susceptible to changes in supporting creep and relaxation models.Successful and also superior prediction of the CF number of cycles to failurefor Grade 91 steel was accomplished by simplified methods with much lessfitting parameters. The practicality in using interaction diagram methods fordesign purposes, where simplicity is a key issue, is questioned.