The peak stress method applied to the fatigue assessment of tube-tube steel joints with weld ends under multiaxial loadings

Abstract

The Peak Stress Method (PSM) is an approximate, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints, which is based on the singular linear elastic peak stresses calculated from FE analyses performed by using coarse mesh patterns. By adopting the averaged strain energy density (SED) as a fatigue strength criterion, a design stress (the equivalent peak stress) can be defined; in conjunction with a reference design curve previously defined, the fatigue strength assessment of welded joints subjected to multiaxial fatigue loadings can be performed. In the present contribution, the PSM has been applied to the fatigue assessment of tube-tube steel joints with weld ends, which have been fatigue tested in a previous contribution under combined loadings: namely pure axial, pure torsion and in-phase as well as out-of-phase axial-torsion loadings, all of which with two load ratios, i.e. R = 0 and R = -1. The experimental fatigue results have been re-converted in terms of equivalent peak stress by adopting a 3D FE model including an idealised weld end geometry. The equivalent peak stress has proved to assess the fatigue crack initiation location in agreement with experimental observations, moreover a quite good agreement has been obtained between the experimental results and the PSM-based design scatter band.