S-N Curves for Welded, Non-welded or Improved Welded Details of Marine Structures

Fatigue cracking is surely the more common failure mode of marine structures. The return experience shows that in the majority of the cases, cracks start at welded joints and so the verification methods developed since the 70s deal with welded joints. More recent cases on FPSOs have shown that cracks can also start from non welded areas and the development of the ULCS (Ultra Large Container Ships) points out the necessity of methods for the hatch corners which are non welded areas. To provide a solution for the design of these ships Bureau Veritas developed a local stress S-N curve formulation including as-welded, non welded and improved welded details. The formulation is based on the accumulated knowledge in fatigue verification approaches since the 70s, the marine return experience and the analysis of available published data. The paper presents the development of the formulation starting by the definition of a S-N curve for as-welded joints covering the low and high cycle domains, then the extension of to the stress release welded joints with different R levels and finally the generalisation to non welded and improved welded details. The non-welded detail S-N curves include the effects of the yield strength on the low cycles domain and of the mean stress level on the slope of the high cycles domain. The defined S-N curves are calibrated versus existing methods for welded joints and mechanical component fatigue verification and also versus test data. Finally, an illustration of a practical application on two ship deck details is given showing acceptable results.