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Response of metallic structural materials to loading sequences

: Buxbaum, O.

Beukers, A.; Jong, T. de; Sinke, J.; Vlot, A.; Vogelsang, L.B.:
Fatigue of Aircraft. Proceedings of the Specialists. Conference dedicated to the 65th birthday of J. Schijve
Delft: Delftse Universitaire Pers, 1992
pp.3-18 : Abb.,Tab.,Lit.
Fatigue of Aircraft Materials <1992>
Conference Paper
Fraunhofer LBF ()
fatigue strength test; Lastfolge Schwingfestigkeit; load sequence; material fatigue; mean stress; Mittelspannung; rechnerische Lebensdauerabschätzung; theoretical fatigue life estimation; Werkstoffermüdung

The differences between block program and randomly combined half cycle test results cannot be described by a general relation, because they depend amongst others on type of material, state of stress and strain in the critical cross section, and type of loading. The effect of sequence on fatigue life will be much more severe if mean stress variations occur. The lesson learned from this was, as far as testing is concerned, that so-called flight-by-flight sequences of loads will be the only answer to the problem of ground-air-ground cycles superimposed on a continuous random stress time history due to flight loads. To take into account sequence effects in fatigue life calculations, two general approaches can be observed: 1. Integral models, like relative Miner rule. 2. Models taking into account the damage produced by individual stress fluctuations. Since the sequence of an individual stress fluctuation cannot be determined for a random process, only a description by means of statistical parameters seems to be successful which will describe sequence effects in an integral form. Therefore, a criterion for sequence effects in a random stress-time history was established, the so-called intermixture which can be calculated from the Markov matrix. It is shown, that the relative life reduction observed for test results is well represented by a function of the intermixture.