TWI Industrial Member Report Summary 572/1996
G R Razmjoo
Fatigue design rules for welded steel joints are based on fatigue test results for simple plate specimens subjected to axial tension loading. The resulting stress cycle is nominally uniaxial, with a constant principal stress direction which is normally parallel or perpendicular to the weld. In real structures, the cyclic stress state is often multiaxial. In addition, the loading might be non-proportional (out-of-phase), ie the principal stress direction varies between extremes of the loading cycle.
Fatigue data for the case of multiaxial loading are very scarce, and guidance in the existing codes is limited, because of lack of data the current recommendations have not been fully validated.
For multiaxial loading, fatigue performance of unwelded materials can be correlated with that under uniaxial loading using an equivalent stress. The most commonly used stresses are the maximum principal and von Mises stresses.
For consideration of welded joints, the maximum principal stress has been adopted in current fatigue design rules (eg Ref.1 ). The von Mises equivalent stress has been suggested for welded joints as well.