Functional Loads vs. Disturbances

When we talk about load spectra we often mean load-time histories, i.e. load as a function of time. So in fact, a load spectrum is nothing else than a set of loads acting on a structure during service. For fatigue, the relevant data for fatigue are maximum and minimum stress levels (peaks and troughs). All other stress levels in between are not relevant.
For fatigue analysis, the load-time history needs to be translated to a table with stress amplitudes, mean stress levels and corresponding number of occurences. Such a translation is usually done with the rainflow counting method.

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