Fatigue assessment of weld root failure of hollow section joints by structural and notch stress approaches

The fillet-welded joints of hollow section members are usually made from the outer side only so that the weld root cannot be controlled. At partial- or non-penetrating fillet welds, non-fused root faces are present which may act as initial cracks in the case of cyclic loading. Therefore, a fatigue assessment is necessary which not only considers the possibility of cracks initiating at the weld toe, but also at the weld root. The latter can be well assessed using the crack propagation approach, however, simpler approaches are called for in practical design. In the paper, the effective notch stress approach is described which has been developed for the fatigue strength assessment of weld toes as well as weld roots. The numerical stress analysis can be performed using the submodel technique. In addition, structural stress approaches are presented which have recently been proposed for the assessment of root failures of welds being mainly subjected to throat bending. A relatively coarse finite element mesh can be used. The requirements for meshing are discussed in detail. The procedures are illustrated by the example of a fillet-welded RHS joint subjected to axial and bending loads. Fatigue tests have also been performed so that calculated fatigue lives can be compared with those observed in the tests, showing good agreement.