Flaw Design for d-c EP Monitoring of Crack Initiation and Growth During Full-Size Pipe Experiments

A design for flaw placement in a full-scale pipe test was developed to both detect crack initiation and measure crack growth rate upon internal pressurization of a pipe exposed to a sulfide stress cracking (SSC) environment. The objective of this work was to model different sizes of longitudinally oriented, inside diameter (ID) surface flaws and lay them out on the pipe in such a manner that (1) the flaws experience their target stress intensity factor (K) value at a chosen value of internal pressure, (2) the stress interaction between flaws is minimized, and (3) the flaw layout is optimized for detecting both crack initiation and growth using a direct-current (d-c) electric potential (EP) technique. The approach to the flaw design and layout used finite element analysis (FEA) modeling and consisted of optimizing K-profiles. First, the K-profiles were optimized by designing curved-bottom flaws such that the target K along the flaw front occurred along most of the flaw length. Then, stress interactions between the flaws were checked to confirm minimum interactions were achieved and that the proposed flaw layout around the pipe circumference was acceptable. In addition, the FE models were used to predict strains on the pipe outside surface. Finally, global (single large current supply), local (individual small current supplies) and hybrid (individual medium-sized current supplies at larger distance) approaches to the d-c EP measurements were evaluated to select which methodology would be most appropriate to detect both crack initiation and growth of the flaws. The results of the analysis show that optimizing all these design factors provides a solid basis for achieving experimental success.

Assessment of Flaw Interaction Under Combined Tensile and Bending Stresses: Suitability of ASME Code Case N877-1

In the case of planar flaws detected in pressure components, flaw characterization plays a major role in the flaw acceptability assessment. When the detected flaws are in close proximity, proximity rules given in the Fitness-for-Service (FFS) Codes require to combine the interacting flaws into a single flaw. ASME Code Case N877-1 provides alternative proximity rules for multiple radially oriented planar flaws. These rules are applicable for large thickness components and account for the influence of flaw aspect ratio. They cover the interaction between surface flaws, between subsurface flaws and between a surface flaw and a subsurface flaw. The calculations of flaw interaction have been performed under pure membrane stress. However, actual loading conditions induce non-uniform stresses in the component thickness direction, such as thermal bending or welding residual stresses. Non-uniform stress fields can lead to variations in the Stress Intensity Factors of closely spaced flaws, affecting their mutual interaction. The objective of this paper is to assess the suitability of ASME Code Case N877-1 with regards to the presence of a bending part in the applied stress distribution. For that purpose, various applied stress profiles and flaw configurations are covered. The effect on flaw interaction is assessed through three-dimensional XFEM analyses.

Generic Proximity Rules for Multiple Radially Oriented Planar Flaws: Technical Basis of Code Case N-877 Revision 1

In the case of planar flaws detected in pressure components, flaw characterization plays a major role in the flaw acceptability assessment. When the detected flaws are in close proximity, proximity rules given in the Fitness-for-Service (FFS) Codes require to combine the interacting flaws into a single flaw. However, the specific combination criteria of planar flaws vary across the FFS Codes. These criteria are often based on flaw depth and distance between flaws only. However, the level of interaction depends on more parameters such as the relative position of flaws, the flaw sizes and their aspect ratio. In this context, revised and improved proximity criteria have been developed to more precisely reflect the actual interaction between planar flaws. Thanks to numerous three-dimensional XFEM analyses, a wide range of configurations has been covered, including interaction between two surface flaws, interaction between two subsurface flaws and interaction between a surface flaw and a subsurface flaw. This paper explains in detail the steps followed to derive such generic proximity rules for radially oriented planar flaws.

Proposal of New Code Case for Alternative UT Flaw Evaluation and Acceptance Criteria of Subsurface Flaw Near Component Surface in Section VIII Division 2 and Division 3

The current proximity rules for subsurface flaws near the surfaces of component in Section VIII Division 2 and Division 3 are different from those of Section XI. The current acceptance criteria of Sec. XI were revised in 2008 in order to eliminate the discontinuity of acceptable and unacceptable flaws based on the research work by Dr. K. Hasegawa and Dr. K. Miyazaki el. The fracture mechanics evaluation for the current and proposed alternative acceptance criteria for subsurface flaws near a component’s surfaces based on Section XI have been performed. The stress intensity factors for transformed surface flaws of the current acceptance criteria for subsurface flaws near component’s surfaces are much larger and too conservative compared with the overall acceptance criteria. It is confirmed herein that the proposed alternative converting rules have enough fracture margin. Therefore, the safety level of this proposal is almost the same as that of the current code. The proposed alternative acceptance criteria may avoid unnecessary weld repairs, and eliminate the discontinuity of acceptable and unacceptable flaws in the area near the component’s surface.