TWI Industrial Member Report Summary 574/1996
S J Maddox
One issue which was debated by the committees producing the current British and European fatigue design rules for welded aluminium alloys was the significance of the constant amplitude fatigue limit for conditions of variable amplitude loading. In particular, it was recognised that account must be taken of the fact that although stresses below the limit are non-damaging under constant amplitude loading, they gradually become damaging as a fatigue crack develops under variable amplitude loading. Faced with the same problem in relation to steel, fracture mechanics was used to deduce an effective S-N curve extrapolated beyond the constant amplitude fatigue limit which, when used in conjunction with Miner's rule, was equivalent to a full fracture mechanics fatigue life prediction, involving the threshold stress intensity factor. The outcome was a curve, of the form S nN = constant, which was extrapolated beyond the constant amplitude fatigue limit at the shallower slope of n+2.
The same approach may be suitable for welded aluminium alloys, but calculations performed for steel included the simplifying assumption that the simple 'Paris' fatigue crack growth law was applicable down to the threshold stress intensity factor.