Fatigue assessment of welded joints under slit-parallel loading based on strain energy density or notch rounding

Three types of welded joints exhibiting weld roots with slit-parallel tensile stresses combined with predominant mode 2 loading are assessed with regard to their fatigue strength based on the strain energy density (SED) concept in comparison to the fictitious notch rounding (FNR) concept: the butt weld joint with permanent backing plate, the fillet-welded single-sided attachment joint and the fillet-welded double-sided lap joint. This mainly numerical analysis extends a more fundamental previous investigation of the authors. The common feature of these joints is the markedly one-sided angular distribution of the SED or notch stress at the slit tip of the weld root. The main geometrical influencing parameters, plate thickness ratio and joint face width ratio, are varied systematically for a main plate thickness of 10 mm. The effect of slit closure is considered where the effect may occur. Fatigue-effective stress concentration factors are determined. Corresponding endurable structural membrane stresses are given based on Lazzarin’s uniform image–N curve on the one hand (SED approach) and based on the endurable notch stress for the reference radius ρr = 1 mm in the IIW recommendations on the other hand. Finally a welded sandwich panel joint is considered, for which some fatigue test data are available. Sufficiently accurate results can already be achieved by the SED approach using an extremely coarse FE mesh.