Many investigations show that components become damaged even though they were designed beneath their fatigue strength. In particular high-frequently stressed parts, for example rotors, turbines, bearings and engines reach lifetimes up to 10^7 cycles and above. Within this region referred to as very high cycle fatigue (VHCF) regime, different damage mechanism occur. Especially in reference to variable amplitude loading insufficient knowledge exist.
Ultrasonic fatigue experiments under variable amplitude loading are performed at a load ratio of R = 0 at the high-strength steel 34CrNiMo6. Within the present study the experimental results using the wind turbine based load spectrum WISPER are compared to the SN-test data acquired by Sander et al. in .
For further studies finite element models are created to work out the stress distribution surrounding interior inclusions and cavities, respectively. Geometrical and mechanical properties as well as positions of imperfections are modified in order to compare the stress intensity factors of a circumferential crack initiating at the imperfection with analytical solutions. Moreover, the determined geometry factor solutions are used to predict residual lifetimes from analytical crack propagation simulations in NASGRO in comparison to the results of the experiments with constant amplitudes as well as with variable amplitudes in terms of the WISPER load spectrum. In addition residual lifetime predictions with the 3D fracture analysis software FRANC3D are intended. Furthermore, elastic-plastic simulations are performed to evaluate the crack closure behaviour due to constant and variable amplitude loading.
 Sander, M., Müller, T., and Lebahn, J. 2014. Influence of mean stress and variable amplitude loading on the fatigue behaviour of a high-strength steel in VHCF regime. 9th Fatigue Damage of Structural Materials Conference. International Journal of Fatigue 62, 10–20.