The Peak Stress Method combined with 3D finite element models to assess the fatigue strength of complex welded structures

The Peak Stress Method (PSM) is a rapid and engineering application of the notch stress intensity factor (NSIF) approach for the fatigue strength assessment of welded structures, which employs the singular linear elastic peak stresses calculated by FEM using coarse meshes. First, the PSM was calibrated to rapidly estimate the NSIFs by adopting 3D, eight-node brick elements and by using the submodeling technique. Given the increasing adoption of 3D modelling of large-scale and complex structures in industrial applications, the PSM combined with 3D FE models has recently been even more speeded up by calibrating ten-node tetra elements, which allow to directly discretize complex 3D geometries without needing submodels. In the present contribution, the PSM has been calibrated by analysing several 3D mode I, II and III notch problems adopting either four-node or ten-node tetra elements. In particular, the PSM combined with ten-node tetra elements has been re-calibrated by analysing also other notch opening angles as compared to the previous calibration, namely 120° under mode I and 90° as well as 120° under mode III loadings. Then, an applicative example has been considered, which is relevant to a large-scale and rather complex steel welded structure, having overall size on the order of meters. The mesh density requirements to apply the PSM to the considered large-scale welded structure using either four-node tetra elements or ten-node tetra elements have been compared in terms of global number of degrees of freedom, these being directly correlated to the solution time of the generated FE models.