The peak stress method combined with 3D finite element models for fatigue assessment of toe and root cracking in steel welded joints subjected to axial or bending loading

In fatigue design of welded joints, the local approach based on the notch stress intensity factor (NSIF) assumes that the weld toe profile is a sharp V-notch having a tip radius equal to zero, while the root side is a pre-crack in the structure. The ratio between the mode I NSIF and the linear elastic opening peak stress evaluated at the point of singularity (i.e. the V-notch tip) by the finite element (FE) method depends only on the type and size of the elements adopted in the discretization. The ratio between the mode II SIF and the linear elastic sliding peak stress evaluated at the crack tip has the same property. This useful property of the opening and sliding peak stresses evaluated by the FE method gives rise to the so-called peak stress method (PSM). In the present paper, it is adopted as an engineering, FE oriented application of the NSIF approach to fatigue design of welded joints. Originally applied to 2D FE models, the peak stress method is combined here with 3D numerical models to assess the fatigue strength of steel welded joints having complex geometry and characterised by toe as well as root cracking. A properly defined design stress, that is, the equivalent peak stress, is shown to correlate with good approximation all analysed experimental data.