An Integrated Approach to Creep-Fatigue Life Prediction

There has been special interest recently in developing new, reliable analytical design methods for components under higher temperature conditions. However, at present, the use of material properties is still limited to arrays of single characteristics which do not interact with each other. In this work, several creep fatigue experiments on smooth specimens of IN 800 H have been carried out at 830°C. In some tests, these have also been combined with inside hysteresis loops to investigate the different effects on deformation and damage behavior which originate in a creep and fatigue environment. As a result of these tests, it has been found that the material behavior under creep-fatigue conditions can be significantly changed compared to the material behavior under simple load conditions. Therefore there is a need for life analysis methods to be expanded to include possible variations in properties; for greater accuracy, the material properties must be treated as a complex interacting system of parameters. The examination has been extended to a typical component used under high-temperature conditions. The results of the numerical analysis show that the stress-strain history in the critical area of that component is not simply strain controlled, as it is in the typical laboratory creep-fatigue interaction life test containing a tensile or compressive dwell at constant peak strain level. At high temperatures, the conditions in the component are more severe, causing the life to be reduced compared with the typical laboratory test. In this paper, these conditions are successfully simulated with the help of a generalized Neuber law: σ · ε^p = constant. Based on this ratio, the engineering method for evaluating component geometry and loading conditions and their effects on material behavior can be established.

On the basis of the results of this study, it follows that a very large amount of information on material behavior and on its component dependence is needed when the material properties are time-dependent, as in the design of hot components. For that reason, the satisfactory solution to designing components for high-temperature conditions requires an integrated approach, with full consideration of different interaction effects from the various influences in the main areas of the material deformation and damage behavior, and of the component effects. The results of this study may apply to the standard methodology of life prediction in high-temperature areas, which shall be developed in the future.