Application of the 'structural stress' approach to a welded joint with complex geometry

Nowadays, local approaches for determining the fatigue strength of welded joints are commonly used in engineering practices, and regulations describe their field of application and guide designers in the execution of the calculations with numerical methods that are now extensively used. The aim of this paper is to verify the application of the structural stress approach to a welded joint used in naval construction and repair of particularly complex geometries. The structural detail is a butt weld between two bulb stiffeners, strengthened by a plate (counterpiece) welded above the bevelled but unwelded bulbs. The study has been conducted by following the guidelines of the method recommended by the International Institute of Welding (IIW), and the alternative method proposed by Xiao and Yamada in 2004 has also been applied. Large scale fatigue testing of the aforementioned joints and collapse testing on models, conducted at the DINAV Naval Structure Laboratory, have allowed subsequent comparison: indeed, the numerical analyses have been conducted with no knowledge of the experimental results. The various calculations, carried out in accordance with the standards but with different interpretations, have shown a significant spread of results, highlighting that, in the case of particularly complex geometries, this type of approach is not simple to apply, and the IIW guidelines do not provide an unambiguous method for determining joint fatigue strength. However, from the methods proposed, using those which best meet the general criteria of good FEM modelling and considering other effects certainly present in heavy structures such as misalignments, estimates of joint fatigue lifespan may be obtained which are in substantial agreement with the experimental results.