Methods for Determining Creep Damage and Creep-Fatigue Crack Growth Incubation in Austenitic Stainless Steel

Most design and safety assessment procedures for estimating the accumulation of creep damage under plant operating conditions adopt either stress-based time (life) fraction or strain-based ductility exhaustion criteria for making component lifetime predictions. The UK Technical Advisory Group on Structural Integrity of Nuclear Plant (TAGSI) has reviewed new stress modified ductility exhaustion methods proposed for calculating the creep contribution to damage in components that are subjected to creep-fatigue loading. The new procedures have been found to give more reliable and less pessimistic predictions of creep damage for the isothermal laboratory data analysed than current methods in the British Energy R5 procedure. TAGSI has also reviewed the σ_d method in R5 for determining incubation periods for pre-existing and postulated defects and considers that it can be used with both stress (using the life fraction rule) and strain (using ductility exhaustion) based approaches. This paper gives the background to the reviews by TAGSI of these creep damage approaches and discusses the implications for data requirements and assessment methods.