Fatigue strength of fillet welds subjected to multi-axial stresses

Fatigue endurance assessments of welded details are normally carried out by calculating the relevant stress acting on the detail and identifying a relevant fatigue class (or detail category) with its associated S–N curve. The fatigue strength of most structural details incorporated in design codes has been obtained from fatigue tests conducted under uni-axial loading conditions, which normally result in a uni-axial stress state in the detail. Many of the structural details that exist in fatigue-loaded structures experience some kind of multi-axial loading condition. The subject of the fatigue strength of welded details under multi-axial loads has been the topic of numerous research projects in recent years. The vast majority of these projects were, however, devoted to cracking in the base metal (i.e. toe cracking). Very little has been done with reference to the cracking of fillet welds in combined loading situations (i.e. root cracking). This paper presents new test results from cruciform specimens, in which weld failure initiated at the root in a multi-axial stress state. The tests have been performed at two different load levels and on three different specimen configurations giving different τ/σ ratios. This permitted an examination of the effect of the shear to normal stress ratio on the fatigue strength of fillet welds. The results of these tests, together with other relevant tests reported in the literature, are then evaluated in relation to the design models proposed in three design standards: Eurocode, IIW and DNV. No obvious dependence on the τ/σ ratios could be found. The evaluated models all appear to be able to predict the fatigue life of a cruciform weld failing from the root under combined shear and normal stress