Multiaxial fatigue life assessment of welds based on nominal and hot spot stresses:Dissertation

A little attention has been focused on multiaxial fatigue of welded joints, even though numerous industrial applications require the consideration of multiaxial effects. Therefore, the goal in present thesis was to find methods for fatigue assessment of welded joints in multiaxial loading cases. A survey of biaxial (bending or tension and torsion) constant amplitude fatigue test results of welded connections was also carried out. Re-analysis of these 233 experimental results from eight different studies was performed based on nominal and hot spot stresses. Three potential interaction equations and three damage parameters were used in the re-analysis. The interaction equations were obtained from SFS 2378, Eurocode 3 and IIW recommendations. Of the three interaction equations SFS 2378 provided the least degree of scatter when design fatigue classes were used and with mean fatigue classes the IIW most successfully correlated the predicted and experimental lives. Principal stress range, maximum shear stress range, and a modified critical plane model for welds were used as the damage parameters. The design hot spot S-N curves were FAT 84 for maximum principal stress range, FAT 109 for maximum shear stress range and FAT 97 for the modified critical plane model, when all toe failures were analysed with a slope of 3. However, observed scatter was 70-100% larger than that observed in uniaxial loaded specimens analysed using the hot spot approach