13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental and analytical study of the effect of variable amplitude loadings in VHCF regime T. Müller, M. Sander* Institute of Structural Mechanics, University of Rostock, 18059 Rostock, Germany * Corresponding author: manuela.sander@uni-rostock.de Abstract Components and structures (e.g. helicopter rotors, wind turbine components or wheelset axels) are commonly exposed a very high number of cycles with variable amplitudes. For the study of the influence of variable amplitude loadings in the very high cycle fatigue regime different load-time-histories up to 109 cycles are used, which have different amounts of small amplitudes beneath the fatigue strength of the investigated material. The experiments are performed with an ultrasonic fatigue testing system with frequencies up to 21 kHz. In order to avoid an excessive heat development of the specimen, pulsed loadings with adequate pause lengths are applied. Therefore, only block loadings can be realized. The used load-time-histories have been counted by the rainflow method and then divided into different number of classes providing that within every class a minimum number of cycles restricted by the experimental performance is given. The classes have been reconstructed to a load-time-history by varying the sequence of the classes. The influence of the different reconstructions as well as of the amount of the amplitudes beneath the fatigue strength is quantified by the fatigue lifetime. These experimental data are used to proof conventional analytical approaches (Palmgren-Miner´s rule) of structural durability in the VHCF regime. Keywords very high cycle fatigue, variable amplitude loading, fatigue life prediction, load interaction effect, short crack growth 1. Introduction In technical constructions of different application fields, components are very often subjected to cyclic stresses. Components like drive shafts, gear shafts, wheelset axles, helicopter rotors or highly stressed engine parts, such as blades, are often cyclically loaded with more than 107 cycles up to 109 and more cycles. The high number of load cycles can be attributed to a very long period of service up to 30 years or to high service frequencies. In present fatigue design standards the influence and effect of such high number of load cycles on the fatigue behavior are not sufficiently taken into account. As Bathias [1] already showed in 1999, the determined fatigue strength, defined by the early investigations of Wöhler, does not exist. Thus, a decrease of the S-N curve is observable in the very high cycle fatigue regime, as numerous studies (e.g. [2-5]) have shown. The decrease in fatigue strength is caused by the transition from surface to subsurface crack initiation [5-7]. Moreover, microstructural features like cavities, non-metallic inclusions, grain boundaries, porosities or oxide layers at the surface and inside a component could lead to crack initiation. Cracks, initiated at non-metallic inclusions, usually show the typical formation of a fish-eye. In the vicinity of inclusions within the fish-eye a distinctive area could be observed [8-10]. Murakami calls it an optical dark area (ODA) with a more rough fracture surface in comparison to the remaining fish-eye fracture surface. He assumed that the area occur due to hydrogen accumulation during cyclic loading. He also proposes a relationship between the size of the ODAs and non-metallic inclusions as well as the number of cycles to failure. However, the influence of variable amplitude loading (VAL) on the fatigue behavior in the VHCF regime is, despite a few studies [11-18], poorly investigated. For instance Mayer [12] showed with two-step loading tests that low load amplitudes beneath the fatigue strength contribute to fatigue
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