Proposal of a New Subsurface-to-Surface Flaw Transformation Rule for Fatigue Crack Growth Analyses

Abstract

When a subsurface flaw is located near the component free surface, the first step consists of characterizing the flaw as surface or subsurface in compliance with subsurface-to-surface flaw proximity rules. The re-characterization process from subsurface to surface flaw is addressed in all Fitness-for-Service (FFS) Codes. However, the specific criteria for the rules on transforming subsurface flaws to surface flaws are different among the FFS Codes. This re-characterization concept is essential and important for subsurface flaws in the flaw assessment procedures. It is applied for three stages of the flaw assessment: at service inspection for flaw characterization, at subcritical crack growth calculation, such as fatigue crack growth, and at ductile/brittle fracture assessment. In this frame, fatigue crack growth experiments were recently conducted by the authors and it was highlighted that the subsurface-to-surface transformation is highly sensitive to the aspect ratio a/l of the flaw whereas the proximity factors in the rules are defined by constant values i.e., regardless of the flaw aspect ratios a/l. The authors have therefore proposed a new subsurface-to-surface flaw proximity rule based on experimental data and equivalent fatigue crack growth rates. Then, the authors demonstrated through numerous Fatigue Crack Growth (FCG) calculations that the current ASME B&PV Code Section XI surface proximity factor should be updated according to the type of component i.e., piping or vessel. The paper summarizes all the steps leading to the improvement of the ASME Code Section XI subsurface-to-surface proximity rule, from the fatigue crack growth experiments to the studies of the suitability of the current flaw-to-surface proximity factor. Furthermore, based on additional fatigue crack growth calculations and more refined investigations, the paper proposes finally a new limit value for the surface proximity factor. As a result, a proposal for modification of the ASME Code Section XI, Appendix C is provided. The paper is used for the technical basis of this proposal.