TWI Industrial Member Report Summary 576/1996
G R Razmjoo
Fatigue design rules for welded steel joints are based on fatigue test results for plate specimens subjected to axial tension loading. The resulting stress cycle in such specimens has a constant principal stress direction. In fabricated structures the principal stress direction often varies between extremes of the loading cycle, for example, with a moving load such as bridges or overhead cranes. At all stages during the cycle, the bending stress at the bottom of the stiffener is positive but the shear stress changes sign as the crane passes over the stiffener; the principal stresses will rotate through an angle of ? as the load moves from I to II.
Fatigue data for conditions of changing principal stress direction are very scarce. Guidance in the existing codes is also limited. BS 5400: Part 10 implies that no fatigue damage is introduced as long as the magnitude of the principal stress does not change. However, some unpublished results obtained by TWI have indicated that fatigue cracking could occur under such circumstances, if the stress direction changes. In view of this, a test programme to investigate the effect of changing principal stress direction on fatigue life was undertaken, which included commissioning a test rig to produce conditions of changing principal stress direction with little change in their magnitudes.
- To develop a loading arrangement capable of producing conditions of varying principal direction.
- To study fatigue of fillet welds under the above condition.
- To review and examine current assessment methods in fatigue design codes for the above tests.