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Deriving a continuous fatigue life curve from LCF to VHCF

: Wagener, R.; Melz, T.


Warrendale, Pa.: SAE, 2017, 8 S.
SAE Technical Paper, 2017-01-0330
World Congress Experience (WCX) <2017, Detroit/Mich.>
Fraunhofer LBF ()

Fatigue testing is known to be time consuming and expensive. Therefore, it should be the main target of fatigue research to accelerate the derivation of fatigue properties. Depending on the required properties, strain- or load-controlled fatigue tests have to be performed. Carrying out load-controlled fatigue tests is necessary to derive the influence of mean stresses and notches on the fatigue strength and fatigue life of different materials and joining technologies. In the case of material samples, increasing test frequencies could be a proper way to accelerate the fatigue testing, as long as the increased test frequencies have no influence on the resulting fatigue life. In the case of strain-controlled fatigue tests, it is not possible to increase the test frequencies in order to accelerate the fatigue tests. Therefore, the Incremental Step Test, which allows the derivation of the cyclic stress-strain curve with only one test, was introduced. Due to the cyclic material behaviour of aluminium alloys, which has caused an on-going discussion, regarding the course of the strain life curve, over several years, it is, in contrast to sheet steel alloys, not possible to use the Incremental Step Test and approaches based on its results to estimate the strain life curve. In order to derive a fatigue life curve from the Low Cycle Fatigue (LCF) regime, across the High Cycle Fatigue (HCF) regime and up to the Very High Cycle Fatigue (VHCF) regime, a combined method using strain- and load- controlled fatigue tests will be discussed. This continuous fatigue life curve for aluminium wrought alloys, based on the evolution of the elastic-plastic material behaviour as well as on the results of high frequency testing up to the VHCF regime, will be presented.