Predictive Testing in Elevated Temperature Fatigue and Creep: Status and Problems

Abstract

The paper selects two well recognized problem areas of high temperature material behavior, namely, creep and fatigue, and considers those factors which contribute to the prediction of the useful service life of the component. The information available is obtained almost entirely in a laboratory environment, in which simplified load conditions and specimen geometries and controlled environments are employed for material evaluation. We do not consider transferring this information to actual design, other than to identify the importance of such problems as the analytical determination of stresses and strains in the structure, the effect of highly complex loading, or the behavior in a complex environment, but we will draw on several practical problems occurring in high temperature power generating machinery as examples of the topics we wish to discuss namely, creep, fatigue, and creep-fatigue interaction. The topics of creep and fatigue are considered separately, the discussion being built around three central topics: (1) real problems in prediction, (2) complicating instabilities including the effects of environment, deformation, and microstructure; and (3) some current predictive methods. Consideration of the less well defined problem of creep-fatigue interactions is also included.