Treatment of the Interaction With the Free Surface of the Component for Combined Subsurface Flaws: Technical Basis for Revision of IWA-3300 and Table IWB/IWC-3510-1

When flaws are detected in pressure retaining components, assessments have to be done in order to demonstrate the fitness-for-service (FFS) of the component for continued operation. This FFS demonstration is performed in accordance with FFS Codes providing flaw assessment procedures and acceptance standards. Before performing analyses, a flaw characterization has to be carried out in order to determine unequivocally the flaw geometry. This flaw characterization is done according to rules provided in the FFS Codes and hence appears as crucial for the rest of the flaw assessment. The first step of the flaw characterization addresses the interaction of the flaw and the free surface of the component: if a subsurface flaw is located near the free surface, this step consists of characterizing the flaw as surface or subsurface according to subsurface-to-surface flaw proximity rules. The recharacterization process from subsurface to surface flaw is addressed in all fitness-for-service (FFS) Codes. The second step of the flaw characterization addresses the interaction of the flaw with adjacent flaws: if a flaw is located near another flaw, this step consists of combining the flaws between them according to flaw proximity rules. However, in some FFS Codes and in the ASME B&PV Section XI Code particularly, there is a lack on how to treat the interaction of a combined flaw and the free surface of the component. The ASME B&PV Section XI Code flaw characterization is not clear on this topic which could lead to misinterpretations and unreliable flaw assessment results. Some typical examples of unrealistic flaw assessment results due to these misinterpretations of the ASME B&PV Code Section XI flaw characterization rules are depicted in this paper. After analyzing more in-depth the origin of the inconsistencies based on 3D Extended Finite Element Method (XFEM) calculations, the paper is used as Technical Basis for the improvement of the ASME B&PV Code Section XI in order to clarify the treatment of combined flaw in the flaw characterization (IWA-3300) and in the flaw acceptability assessment as well (IWB/IWC-3510-1).

Direct-Current Electric Potential (D-C EP) Technique Validation Through an Experimental and Computational Study on Pipe With Surface Crack

The direct-current electric potential (d-c EP) technique (also known as Electrical Potential Drop, EPD) was developed by researchers in the 1960s and applied to cracked geometries. In this investigation, measurement of the d-c EP signature from a circumferential surface-crack profile in a pipe was attempted to characterize the flaw shape with higher resolution using state-of-the-art digital equipment. A part-circular profile of crack was inserted using an EDM technique in a small diameter (4-inch diameter Schedule 160) TP304 (Type304) stainless steel pipe. Experimentally, different magnitudes of electric-current were applied to obtain the d-c EP across the length of the crack (from the shallowest to the deepest point). Finite Element Analysis (FEA) was performed to calculate the variation of the d-c EP across the length of the crack. A sensitivity study was done for various distances between the d-c EP probe locations near the crack. A comparison of the d-c EP values obtained from the part-circular crack front and a semi elliptical crack FEA (more realistically seen/assumed in service crack cases and used in the ASME Section XI calculations) were made. The study also investigated the variation of the d-c EP for various crack depths through the thickness for the applied constant amplitude direct-current. The sensitivity on d-c EP probe location distance from the surface flaw and d-c EP probe location along the length of the surface flaw (from deepest or center of the surface flaw to the shallowest point or corner of the surface flaw) were investigated. The scatter in the acquired d-c EP data across the two sides of the crack was investigated and accuracy of crack depth characterization was characterized in detail. This was done to investigate the limits of d-c EP calibration curves used for crack growth predictions. The d-c EP calibration curves are useful in determining the crack growth that occurs without destructively opening the specimen and also measuring the in-situ crack depth measurements real time during a pipe or other surface flawed component/fitting experiments.

Proposed Updates to the Buried-to-Surface Flaw Recharacterization Rules in Annex E of BS 7910

BS 7910 “Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures” [1] is undergoing revision for the next edition. Updates are proposed for the Annex E flaw recharacterization guidance. The changes aim to make Annex E more applicable to real structures and to improve the synergy between the subcritical crack growth such as by fatigue and fracture of an embedded flaw. Several studies were done by WEE/37 fracture panel focusing on brittle and ductile material behaviour to support the proposed changes. These studies are summarised in the paper. The main modifications in the proposed updated Ann ex E are in: • Removal of use of Annex E for brittle fracture. • Updated guidance on recharacterized surface flaw length for ductile fracture. • Optional guidance on use of geometry-based criterion for embedded flaw break-through under ductile conditions. • Alignment of fatigue crack growth in Section 8 with the flaw break-through in Annex E.