Simplified approach to fatigue strength assessment of fillet-welded attachment ends

Almost all stiffeners, brackets and other attachments in ship hulls are joined by non-penetration fillet welds for fabrication reasons. However, the ends of these attachments show frequently high stress concentration due to their geometrical shape and loading. Fatigue cracks may initiate here not only from the weld toe, but also from the non-welded root gap. A simplified approach is proposed for the fatigue strength assessment with respect to root cracking, which is based on a local nominal stress in a defined area of the weld throat and on common fatigue classes for the assessment of cruciform joints. This simple assessment is possible because the complex 3D case can be related to a simple 2D case. The procedure is verified by two typical attachments with hard and soft ends, the latter showing frequently a stress flow with pronounced shear stresses. Fatigue tests and numerical analyses of local stresses and crack propagation from the root gap have been performed. Some tests showed unexpected results in comparison with the calculations. Beneficial residual stresses were identified to be the main reason for this, which could be taken into account by a bonus factor in particular cases.