Risswachstumsverhalten von Aluminiumknetlegierungen unter zyklischer Beanspruchung bei niedriger Belastungsamplitude

In the regime of Very High Cycle Fatigue (VHCF), that means at very low stress amplitudes, it is not clear whether all the conventional crack propagation phases occur. Investigations on the aluminium wrought alloys EN-AW 6082 and EN-AW 5083 at very low stress amplitudes have shown that the “natural” crack initiation was often observed to occur underneath the material surface and crack propagation takes place without any contact to atmospheric components. Thus, fatigue experiments in vacuum on externally precracked samples were
performed in order to simulate the internal crack propagation. For this purpose, the ultrasonic fatigue testing technique (complemented by a small vacuum chamber) was applied allowing the examination of the crack growth characteristics in vacuum at a resonant frequency of about 19.2 kHz and different stress ratios by using a long-distance microscope. Furthermore,
a miniature fatigue testing system which was specifically designed and made for the use in a scanning electron microscope (SEM) was used for the in-situ observation and characterization of the crack growth behaviour at high resolution under the vacuum conditions of the SEM. Fatigue experiments in different environmental conditions were performed in order to investigate the atmospheric influence on the fatigue crack propagation behaviour. The results show that there exists a significant atmospheric influence on the fatigue crack propagation behaviour which manifests itself in the crack path as well as in the
crack growth rate. Especially the result obtained on the aluminium alloy EN-AW 6082 in the peak-aged (pa) condition revealed that shear-stress controlled crack propagation takes place under VHCF loading due to pronounced single sliding in vacuum. Thus, the VHCF long crack growth in vacuum in this material condition differs significantly from the well-known conventional long crack growth in the Low Cycle or High Cycle Fatigue regime. Against this background, a two dimensional numerical short fatigue crack growth model was adapted leading to a realistic simulation of the VHCF long fatigue crack propagation in vacuum. The model can be applied to synthetic microstructures allowing a physically based assessment of the effect and relevance of microstructural parameters on the VHCF life, which is determined in the (pa) condition by microstructure-controlled long fatigue crack growth.