A model for fatigue strength of welded lap joints

The most advanced structural frames of cars and motorcycles are made from welded box beams of aluminium alloys. A recent technological solution for the beams consists of press-formed metal sheets joined by welded single-lap joints. Because the frames undergo cyclic loading, the evaluation of the fatigue endurance of this kind of connections plays a crucial role in the development of the frames. The present research deals with the evaluation of the fatigue behaviour of welded lap joints, subjected to tensile and bending loads. Fractographic evidence and numerical analyses show that the fatigue strength is controlled by the presence of defects and inhomogeneities at the tip of the weld root. In particular, these defects, which appear to lie on the maximum K path, increase the stress intensity factor (SIF) at the weld root. Such an analysis, together with a statistical description of the population defects, allows us to develop a methodology to predict the fatigue strength of lap joints. A simple method for SIF computation based on finite-element (FE) analyses and structural stresses at the lap joints is then presented.