Consideration of runouts on the evaluation of fatigue experiments

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The determination of SN-curves is a complex mathematical and stochastic problem. With steel structures and related sectors the estimation of lifetime of a fatigue loaded notch detail is done with FAT-tables, which are based on experimentally based SN-curves. These are determined by the evaluation of fatigue tests in the high cycle fatigue regime based on a linear Basquin model. The shortcoming of this model is, that it does not provide the extrapolation in the very high cycle regime as also cannot encounter runouts. This results in an unsafe estimation of the endurance limit as also in uncertainities concerning the design of constructions for high load cycles, such as wind energy converters. The model of Castillo and Fernández-Canteli is based on a Weibull-distribution and is experimentally verified within the resarch project for unwelded fatigue details. It describes the SN-curve from finite life fatigue up to the ultra high cycle fatigue. In addition, it provides the possibilty to include runouts within the evaluation of fatigue tests as also the retesting of a runout with an increased stress range. The applicability of this model on welded connections of carbon steels as also austenitic steels should be further investigated in the research project.