An Additive Fatigue Damage Accumulation Modeling Procedure Without Needing Cycle Counting

Cycle counting is often a significant part of the current time-domain fatigue analysis processes for structures subjected to uniaxial and multiaxial variable amplitude loadings. However, cycle counting approaches including rainflow cycle counting are essentially empirical and only serve as a convenient choice for variable amplitude uniaxial loading. By its nature, cycle counting itself has no substantial physical basis that hinders its extension to complex multiaxial fatigue analysis, in both time and frequency domains. In this paper, a load-path based additive fatigue damage accumulation modeling procedure that can essentially eliminate cycle counting is developed based on strain energy exhaustion during cyclic loading, and only a fatigue S-N curve obtained under constant amplitude loading is needed to establish all required fatigue damage parameters. The fatigue damage in the new theory is perceived as the product result of the incremental loading path length in stress space and the current state of stress, which is distinctly different from the range and mean measures derived from the concepts of peak and valley in cycle counting methods. The new additive fatigue damage method has several advantages over cycle counting related fatigue analysis methods: (1) it embeds both stress range and mean stress within a single unified additive damage parameter, and (2) it unifies uniaxial and multiaxial fatigue with the same theoretical framework.