S-N Lines for Welded Thin Joints — Suggested Slopes and FAT Values for Applying the Notch Stress Concept with Various Reference Radii

An assessment of thin welded structures with actual IIW design lines results in an overestimation of fatigue lives (strengths) at high load levels and a conservative estimation at low load levels, in many cases, independently of the applied fatigue assessment approach (nominal, structural or notch stress). This is mainly due to the slopes of the design S-N line k = 3.0 for normal and k = 5.0 for shear stresses, which are valid for thick and stiff structures. To overcome this inconsistency for welded thin and flexible structures, the slopes k = 5.0 for normal and k = 7.0 for shear stresses are suggested, keeping the already known FAT values derived for the notch stress concept variants with rref = 1.0, 0.3 or 0.05 mm, respectively. However, the slope is not only determined by plate thickness; it is the result of an interaction between thickness, local geometry (stress concentration), structural stiffness, loading mode and last but not least residual stresses. This complexity makes it difficult to identify the driving parameters and to predict the slope of the S-N line in many cases.