Fatigue Crack Growth Calculations for Vessels Considering Subsurface to Surface Flaw Proximity Rules

Abstract

If a subsurface flaw is located near a component surface, the subsurface flaw is transformed to a surface flaw in accordance with a subsurface-to-surface flaw proximity rule. The re-characterization process from subsurface to surface flaw is adopted in all fitness-for-service (FFS) codes. However, the specific criteria of the re-characterizations are different among the FFS codes. Recently, the authors have proposed a new subsurface-to-surface flaw proximity rule based on the experiments data and the interaction of stress intensity factors. In this study, extended Finite Element fatigue crack growth calculations were carried out for thick wall component like vessels with subsurface flaws, using the proposed subsurface-to-surface flaw proximity rule and the proximity rule provided in the current ASME Code Section XI. Different, flaw aspect ratios and ligament distances from subsurface flaws to inner surface of vessel were taken into account. As the results, the current proximity rule and proposed one provide relatively similar fatigue lives, whatever the aspect ratios of the initial subsurface flaws. However, when the thickness of the component decreases this similarity between both proximity rules appears not to be valid anymore.