Remaining lives of fatigue crack growths for pipes with subsurface flaws and subsurface-to-surface flaw proximity rules

Abstract

If a subsurface flaw is located near a component surface, the subsurface flaw is transformed into a surface flaw in accordance with a subsurface-to-surface flaw proximity rule. The recharacterization process from subsurface to surface flaw is adopted in all fitness-for-service (FFS) codes. However, the specific criteria of the recharacterizations are different among the FFS codes. Recently, the authors have proposed a new subsurface-to-surface flaw proximity rule based on experimental data and equivalent fatigue crack growth rate calculations. In this study, fatigue crack growth calculations were carried out for pipes with subsurface flaws, using the proximity rule provided in the current ASME (American Society of Mechanical Engineers) Section XI and JSME (The Japan Society of Mechanical Engineers) codes and the proposed subsurface-to-surface flaw proximity rule. Different pipe sizes, flaw aspect ratios, and ligament distances from subsurface flaws to inner surface of pipes were taken into account. The results indicate the current proximity rule gives less conservative fatigue lives, when the aspect ratios of the subsurface flaws are small.