Welded joints generally have lower fatigue strength than bolted joints and the base material itself. Different weld joints can vary a lot in fatigue strength, about a factor of four (4) on the strength and a subsequently a factor sixty-four (64!) on fatigue life.
The local geometry of the weld has a large influence on fatigue life because a weld is always a stress raiser. The designer should carefully select the joint geometry and the welding parameters. The result is influenced by the individual welder or if the process is automatic the stability of the process. Inspection of weld quality to ensure stable welding result is recommended.
Picture 1: The fatigue life is very dependend on local effects. Minimize misalignments and sharp geometrical changes. Symmetrical geometry is often advantageous.
It is possible to improve the fatigue strength of a weld joint with minor changes in the weld geometry and the process quality. Doubled fatigue strength gives doubled load capacity. However, the good news is that the life can be eight (2x2x2=8) times longer! picture 2.
Picture 2: Similar weld joints? Apparently not, there is a huge difference in fatigue performance.
Improvement methods increase the fatigue strength and might also sometimes allow to better utilize a higher strength in the base material. Weld joints that are likely to have crack initiation from the weld toe are suitable for post-treatment. If the crack is initiated from the weld root then post-treatment is only a waste.
Picture 3: Crack growth from weld toe to the left and from weld root to the right.