The Master S-N Curve Method: An Implementation for Fatigue Evaluation of Welded Components in the ASME B&PV Code, Section VIII, Division 2 and API 579-1/ASME FFS-1

by David A. Osage, P.E., ASME Fellow / Martin Prager, Ph.D. / January 2010


This document provides a comprehensive coverage of the master S-N curve approach to fatigue evaluation of welded components stipulated in the 2007 ASME BP&C Section VIII Div 2 Code (VIII-2). At first, the needs to treat fatigue of welded joints differently from that of metal without being welded are discussed based on a large amount of experimental evidence. Along this line, some of the most important factors governing fatigue life of welded joints are then highlighted based on recent developments over the last two decades or so. These factors are stress concentration, joint type, loading mode, etc., among other things. There is a universal consensus among researchers and industrial practitioners that an accurate and consistent determination of stress state at a location of interest is a priori towards any reliable fatigue prediction for welded components. It will be shown that the mesh-insensitive structural stress method adopted in VIII-2 is the most reliable stress determination method to date in this regard. Then, an equivalent structural stress parameter will be formulated using fracture mechanics principles by introducing a two-stage crack growth model which encompassing both short crack and long crack behaviors. The equivalent structural stress parameter serves as an effective parameter that captures the effects of stress concentration, wall thickness, and loading mode on fatigue. Its validity has been proven by collapsing over a thousand well-documented actual weldment fatigue tests including full scale component tests into a single narrow band. As a result, the method is also referred to as the Master S-N curve approach.


In addition to providing detailed technical background on the Master S-N curve method in VIII-2, detailed responses to questions raised by some members of Code Committees during the last round of balloting of VIII-2 are also documented here. Some of the on-going research and development issues for continued refinement in the Master S-N curve method are also summarized. A recent International Institute of Welding (IIW) document is also included here as Annex A. This document further discusses recent vessel test data, the effects of environment, and design margins as they relate to the Master S-N curve approach as implemented in VIII-2. This IIW document is a complement to, and an extension of, the discussion contained in Part 5 of this bulletin.


Finally, it should be noted that this document supersedes WRC Bulletin No. 474 (2002) on this same subject area, if inconsistency exists between the two documents. Since its publication, the Master S-N curve method described in the earlier WRC bulletin has gone through a significant series of refinements, thanks to both the PVRC/ASME Division 2 Rewrite Project and Battelle Structural Stress JIP.


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