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The most advanced structural frames of cars and motorcycles are made ofaluminium alloys welded box beams. A recent technological solution for the beamsconsists of press-formed metal sheets joined by welded lap-joints. Because the framesundergo relevant fatigue loads, the evaluation of fatigue endurance of this kind ofconnections plays a crucial role during the development of frame. The present researchdeals with the evaluation of fatigue behaviour of welded lap joints, subjected to tensileand bending loads. Fractographic evidences and numerical analyses showed thatfatigue strength is controlled by the presence of defects ad inhomogeneities at the tip ofthe weld root. In particular these defects, which appear to lie onto the maximum Kqqpath, increase SIF at weld singularity. Such an analysis, together with the statisticaldescription of defects population, allowed to develop a methodology able to predict thefatigue strength of lap-joints. A simple method for SIF based on FEM analyses andstructural stresses at the lap-joint is then presented.