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In the present study two different aluminium alloys, the precipitation hardening alloy EN AW 6082 (peak-aged and overaged) and the work-hardening alloy EN AW 5083 (soft annealed) were examined. Fatigue cracks were initiated by means of a focused ion beam notch and a longdistance microscope was used for in-situ observation of the crack growth behaviour. The crack growth was investigated at constant stress intensity factors near the threshold regime. During the insitu investigation a change in crack growth velocity was detected. It could be observed that the barrier function of grain boundaries and primary precipitations are the major reason for crack growth retardation despite the fact that the crack is in the long crack growth range. The microstructural influence becomes more important with decreasing ΔK values, meanwhile the average crack growth rate decreases simultaneously. Experimental results have shown that the Febased precipitates are influencing the crack growth rate for both aluminium alloys. Meanwhile, grain boundaries are causing a deceleration of the crack growth rate primarily in case of the work hardened aluminium alloy. This is assumed to be the reason for the smaller average crack growth rate in EN AW 5083 compared to that observed for the precipitation hardening alloy while applying comparable ΔK values.

Influence of microstructural discontinuities on the behaviour of long cracks in the VHCF regime for the aluminium alloys EN AW 6082 and EN AW 5083