In shear, the fatigue load giving an endurance of 106 cycles may be up to 20 or 25% of the static shear failure load. However, spot welds are much more sensitive to peel loading and a load in the region of only 5% of the static peel failure load may give an endurance of 106 cycles.
The characteristic failure of a spot weld in fatigue is for a crack to grow from the interface, through the thickness of the material, sometimes partly round the outside of the nugget. On breaking through the sheet surface close to the edge of the electrode indentation, the crack normally propagates tangentially to the weld in an arc into the parent material.
In a complex sheet structure, the mode of stress and the share of the load between spots is not well defined.
Fatigue data for spot welds are normally expressed as applied loads rather than a stress. Efforts have been made to express the data as a local stress range taking into account the local axial and bending stress components of a lap joint in shear. Maddox  compared a range of results for sheet steel in the range 0.8 to 2mm thickness using a local stress range per spot:
Local stress range
Where P is the applied load, C is the weld circumference, W is the sample width per spot and T = the material thickness.
The result of this comparison is shown below and the data suggest that spot welds may be considered as meeting the fatigue rating: IIW class 125.
Improvement in fatigue performance may be achieved by reducing the load (particularly any peel component) on individual spot welds by changing the number or distribution of the welds. Increasing material thickness can improve fatigue life, but there is normally only a small benefit to be achieved by increasing weld size.
The fatigue performance of spot welds is not affected by limited internal porosity or cracking in the weld nugget as the majority of the test load acts on the edge of the weld, and the stress in the weld itself is low. Indeed, a hole, up to 1/3 of the diameter of the weld, may be drilled through the centre of the weld without substantial effect on the fatigue performance.
One technique which has been used in a few special cases to improve fatigue life significantly, is to cold compress the weld area after welding. This reduces the tensile residual stresses of the crevice at the edge of the weld. The tool design and procedure need to be proved experimentally to ensure the redistribution of residual stresses has the desired effect.
- Maddox S J: 'International conference on the performance of dynamically loaded structures'. IIW 50th Annual Assembly Conference 1997.
Resistance welding of sheet metals - a guide to best practice