Robust methods of life assessment during creep

Abstract

The paper presents a simplified, yet robust analysis procedure for structural problems occurring in the high temperature regime where the combined effects of permanent creep deformation and progressive material degradation must be taken into account. The method described here is based on the continuum damage concepts proposed by Kachanov and Rabotnov in the late fifties and early sixties, and on the reference stress notions that originated in the late sixties. These ingredients are combined with well-tried numerical techniques such as finite element procedures to form a basis for rational creep/damage analyses. The excessive computer capabilities required for this type of analysis, together with the scarcity and uncertainty of material data, have led to the need to de-couple the creep stress analysis from the material degradation. This approach gives the designer adequate results with accuracies commensurate with the inherent uncertainties in material data and service loadings, and within acceptable computing time limits. At the same time the robust approach gives the designer the ability to establish which effects are most important whereas the coupled analysis does not. The development of the method is described in detail in the paper and is illustrated with an example of a pressure vessel nozzle. Monte Carlo simulations methods for dealing with randomness of material and geometrical parameters are illustrated in other examples.