Crack growth behaviour of aluminium wrought alloys in the very high cycle fatigue regimeWednesday (08.11.2017) 16:40 - 17:00 Part of:
The stage of crack growth gains significance when the components considered contain initial micro-cracks, e.g. due to manufacturing processes. In the case of low strain amplitudes, the crack growth behavior in particular of long cracks has not been explored sufficiently so far. In the case that components were free of defects, crack initiation was often detected in the interior underneath the material surface in the regime of Very High Cycle Fatigue (VHCF). Consequently, crack growth takes place under vacuum conditions different to preexisting surface cracks. In order to separate environmental effects on the long fatigue crack propagation, fatigue experiments in air and in vacuum are necessary to be performed. For this purpose, an ultrasonic fatigue testing system equipped with a small vacuum chamber and a miniature fatigue testing system, which was specifically designed for the use in a scanning electron microscope (SEM), were used for the characterization of the long VHCF crack growth behavior. The results show that the VHCF long crack growth in vacuum is strongly affected by the microstructure of the alloy studied and consequently very different to the conventional long crack growth in the Low Cycle or High Cycle Fatigue regime. Under the applied conditions (low values of the stress amplitude), stage I crack growth prevails. In contrast, microstructural long crack propagation was only observed in the threshold regime in air, and it turned out that primary precipitates impede the crack propagation. At constant stress amplitude and after reaching a certain crack length, only stage II crack growth takes place in air. Moreover, crack propagation in air was detected at reduced stress intensity factors as compared to vacuum. Electron Backscatter Diffraction analyses (kernel average misorientation method) were systematically applied to give information about the plastically deformed area ahead of the crack tip for fatigue crack propagation in air and vacuum. Local dislocation arrangements as well as the density of dislocation were analyzed by means of transmission electron microscopy (TEM). For this purpose, TEM lamellae are prepared very accurately from the plastic zone by applying the focus ion beam technique.
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