Fatigue Assessment of Root Cracking of Fillet Welds Subject to Throat Bending using the Structural Stress Approach

Abstract

Load-carrying fillet welds frequently occur in welded structures such as ships and offshore installations, bridges, cranes etc. Typical examples are cruciform joints where fatigue cracks may initiate not only from the weld toe, but also from the non-fused root face. A fatigue assessment of the latter is possible using for example the nominal stress in the weld throat. However, load-carrying fillet welds are in certain structures much more subjected to throat bending, which increases the risk of root failure, for example at connections with one-sided fillet welds or cover plates subjected to out-of-plane loads. In the paper, an approach for fatigue assessment is proposed, which is based on the structural stress in the leg section of the fillet weld including the bending portion, allowing a relatively simple stress analysis using coarse finite element models. It is shown that element stresses in a cross section through the weld contain uncertainties due to the stress singularity at the end of the non-fused root face, so that the stress evaluation should be based on nodal forces. The structural stresses in the attached plate can also be used in case of one-sided fillet welds. A design S-N curve based on structural weld stresses in the leg section is derived from fatigue tests of different types of specimens.