Weld Procedures and Demands for Improved Fatigue Strength of Single Load Carrying Fillet Welds

For one single sided fillet weld, in e.g. box beams, it has been found that the weld penetration has a large impact on the fatigue life. An improved penetration with 1 mm in the gap between the plates could replace approximately 2 mm of the weld throat size. This means that the focus when welding these fillet welds should be to optimize the penetration, giving opportunities to reduce the weld size or to apply higher load. In a study performed of robot welded fillet welds, with throat size 5 mm, it was shown that welds with a leg deviation, were the weld has an offset against the standing plate, has the best theoretical fatigue strength. The welds with this leg deviation have a better gap penetration and thereby a more favourable flowing of the force through the weld. The contradiction with this finding is that these welds according to the definition are undersized and should not pass a weld audit. A possible solution to this problem is to change the way of define the fillet weld and for welds were the weld penetration are important it could be a solution to define the leg sizes of the weld. A fillet with throat size of 5 mm, leg size 7 mm, could be replaced with a weld with leg size 9 mm and 5 mm. This weld would have a reduced throat size of 3,5 mm but instead have the penetration increased with approximately 1,5 mm because of the change in torch position as shown in figure 1. This weld gives improved fatigue strength without any additional cost.

 

Figure 1: Principal potential for improved weld penetration, i, with a change of fillet weld requirements and welding torch position