Characterization of the Long Crack Propagation Behaviour in a Hardenable Aluminium Alloy in Very High Cycle Fatigue Regime

摘要

In the regime of Very High Cycle Fatigue (VHCF), it appears questionable whether all the well-known stages of fatigue crack propagation occur. Deviations must be expected through the very small dimension of the cyclic plastic zone ahead of the crack tip and, hence, a strong interaction with microstructural features seems very likely. Investigations have shown that "natural" crack initiation often takes place underneath the material surface leading to crack propagation without contact to atmospheric components. In order to reproduce the crack growth behaviour, an ultrasonic fatigue testing system (USFT) equipped with a small vacuum chamber was used for fatigue experiments in vacuum. The tests were carried out on the aluminium alloy EN-AW 6082 in the peak-aged (pa) condition. Micro-notches were prepared in the USFT specimens by means of the Focused-Ion-Beam technique. Systematic measurements in laboratory air in the region of the threshold value of long fatigue crack growth revealed that primary precipitates significantly influence the crack growth behaviour. Depending on the spatial distribution of the primary precipitates, strong crack retardation and localized crack arrest take place even far above the threshold value. In vacuum only shear-stress-controlled VHCF long crack propagation were detected in EN-AW 6082 (pa) due to very pronounced single dislocation slip associated with the secondary precipitates of the aluminium alloy.