The Design Master S-N Curve in ASME Div 2 Rewrite and its Validations

Abstract

A master S-N curve approach has been adopted in the new 2007 ASME Div 2 as an alternative method to the existing ASME fatigue evaluation procedure for fatigue assessment of welded components through a rewrite effort over the last few years. The master S-N curve has been developed based on two key technical advances in fatigue research over the last decade: (1) a robust structural stress method that offers good mesh-insensitivity in stress concentration calculation; (2) an equivalent structural stress based single master S-N curve that represents a large amount of fatigue test data from various joint types, loading modes, plate thicknesses, etc. Based on the statistical characterization of over 1000 weldment fatigue tests, a design master S-N curve has been proposed to provide both technically sound and reasonably conservative fatigue evaluation for pressure vessel and piping components. In this paper, two new series of recent fatigue tests on Div 2 vessels and related weld details will be used to further validate the design master S-N curve proposed. The emphasis will be placed upon its relative safety margins in comparison with those inherent in the traditional ASME fatigue procedure in lights of these recent tests.