Assessment and Improvement of Flaw-to-Surface Proximity Rules Accounting for Flaw Aspect Ratio and Component Thickness

Abstract When a subsurface flaw is detected close to the free surface of a pressure retaining component, it must be assessed whether the subsurface flaw shall be transformed into a surface flaw in accordance with flaw-to-surface proximity rules. Those recharacterization rules implicitly assume that if the interaction between the subsurface flaw and the free surface is too high, the remaining ligament is susceptible to break leading to re-characterization into a surface flaw. This re-characterization process from subsurface to surface flaw is adopted in all Fitness for Service (FFS) Codes. In Section XI of ASME Boiler & Pressure Vessel Code, the threshold of this proximity rule depends on the ratio between the subsurface flaw to the free surface distance (i.e., the remaining ligament) and the flaw depth. This limit value depends neither on the flaw aspect ratio, nor on the thickness and the curvature of the component, nor on the type of loading. After demonstrating in a previous paper that the interaction between the subsurface flaw and the free surface of the component highly depends on the flaw aspect ratio, the present paper highlights the influence of the thickness and the curvature of the component. Consequently, the current flaw-to-surface proximity rule should account for those parameters as well. Then, the paper proposes an interaction limit criterion and presents a new limit value for the flaw-to-surface proximity factor, depending on the flaw depth, the remaining ligament, the flaw aspect ratio and the flaw depth-to-thickness ratio.