Influence of the shape of undercut on the fatigue strength of fillet welded assemblies - application of the local approach

With a view to improving the prediction of fatigue behaviour of welded assemblies, the Institut de Soudure and CETIM have launched an exploratory phase aimed at defining a `higher quality' for fillet welds, based on their external shape, likely to enhance their endurance limit for different joint types. The fatigue strength behaviour was assessed for fillet welded assemblies exhibiting continuous and discontinuous undercuts and welded using MIG/MAG or MMA welding in the flat and vertical up positions. The statistical analysis of the undercut geometry made it possible to evidence a homogeneous population of radius sizes at the bottom of the undercut which seems to be independent of the welding process and of the welding position. The analysis of the fatigue tests based on the nominal endurance limit revealed that SMA flat position welded assemblies displayed a higher fatigue strength than the MIG/MAG welded assemblies, in the same position. Additional investigations using numerical simulation were proposed for calculating the local stress condition at the bottom of the undercut when the assemblies are subjected to the nominal endurance limit. The analysis of the values confirmed the advantage of the concept of the local mechanical approach for predicting the fatigue strength of the fillet welded assemblies. As a matter of fact, these studies showed that the local endurance limit of welded assemblies was not only independent of the weld soundness but also of the type of undercut (continuous or discontinuous) in the weld toe.