Some issues in creep fatigue life prediction of fossil power plant components

Creep-fatigue damage induced by thermal stresses is of major concern with respect to the integrity of many high temperature components. The concern has been exacerbated in recent years due to cyclic operation of units originally designed for base load service. Much of the past research has been aimed primarily at crack initiation phenomena and, although useful from a design point of view, it is not always relevant to plant operators who in many instances can run components containing tolerable cracks. In terms of both crack initiation and crack growth prediction, variation in material, temperature environment, stress state, etc. have made it impossible to apply a single damage rule for all cases. The need for component-specific life prediction using appropriate material property data generated under conditions relevant to the service and using the proper failure criterion, has become very apparent. In the face of this need, thermomechanical fatigue (TMF) testing, creep-fatigue crack growth testing and bench marking against field experience is essential. This paper will assess the current state of the art with respect to creep-fatigue life prediction especially with a view to provide a plant user's perspective to the research community and to present a case study an TMF life prediction of combustion turbine blades.