2016:27 Validation of fatigue fracture mechanics approaches


In a previous study (SSM research report 2015:38) fatigue experiments were performed on welded austenitic stainless steel piping components. The fatigue experiments offer an opportunity to evaluate fatigue flaw tolerance assessments used in industry, which are based on the fracture mechanical approach implemented in ProSACC.


The present study aims to validate the used fatigue flaw tolerance approaches by comparing experimental results obtained for the total fatigue life of the considered piping component and the computed fatigue life estimate. Safe and reliable long term operation (LTO) of the plant has to be demonstrated when NPPs approach the end of their design service life time, and this process includes amongst others the evaluation of fatigue resistance of components.


The study indicates that an ASME inspired flaw tolerance approach causes extensive conservatism, implying that the propagation fatigue life at most represents 10% of the total fatigue life. A best-estimate flaw tolerance approach on the other hand presents a significant reduction of conservatism, which indicates that fatigue initiation represents a negligible contribution to the total fatigue life. The estimated 90% prediction limits of the best-estimate approach show good agreement with the experimental results. Overall conservatism of the fatigue flaw tolerance approach is preserved by assuming a relatively large initial flaw size and neglecting effects from inelastic material behaviour, sequence effects for variable amplitude loads and crack closure effects. The results support the use of flaw tolerance approaches for demonstrating reliability of a component.