Traditionally fatigue damage is associated with time dependent loading, in the form of local stress or strain histories. But there are load situations both in the lab testing as well as in the real world, where determination and definition of the loads are more efficiently performed in the frequency domain, either as deterministic sine sweeps or random stationary processes. Nevertheless the phasing resp. correlation of the load signals still have an important impact on the fatigue – imagine the loads on a truck frame where there is a definite correlation between the loads from the front and rear axels. In this paper we present the methodology to handle multiple loads and also how to correctly handle the local stresses evaluated from multiple correlated signals, if used for a fatigue analysis. We show why traditional equivalent stress approaches bases on von Mises or principal stresses are problematic and the critical plane approach should be used.
