To investigate the fatigue crack growth under VHCF relevant stress amplitudes high frequency testing equipment such as ultrasonic fatigue testing becomes essential. It allows the examination of long fatigue crack growth at a testing frequency of about 20 kHz. However, a quantitative description of the damage accumulation and crack growth data at such high frequencies is a true challenge. In order to gain insights into the barrier function of microstructural inhomogeneities such as large precipitates or grain boundaries, a direct correlation between local microstructure fatigue crack growth behavior has to be analyzed.
Previous investigations of the aluminum alloy EN-AW 6082 under tension-compression cycling (R = -1) have shown that especially the ferritic precipitates are influencing the crack growth rate despite the fact that the crack is in the long crack growth regime. Grain boundaries seem to have only a slight influence.
In the present study fatigue crack growth tests were performed at a stress ratio of R = 0.1. The aluminum alloy was examined in two different aging conditions (peak-aged and overaged). The cracks were initiated by means of a focused ion beam notch and a long distance microscope was used for in-situ observation of the crack growth. First of all the threshold was determined by means of the load shedding method. Then, crack growth was investigated at constant stress intensity factors close to the threshold. During the in-situ investigation a change in crack growth velocity can be detected. It is assumed that the barrier function of primary precipitates is again the major reason for crack growth retardation. The microstructural influence becomes more important with decreasing ΔK values, close to ΔKth the primary precipitates can stop the crack growth entirely. Meanwhile the average crack growth rate decreases simultaneously with decreasing ΔK. But in comparison to the experiments under fully reversed loading the influence of the microstructure for the overaged condition seems already reduced when the applied stress intensity factor is 40 % above the threshold. For R = -1 an influence could still be seen at this ΔK range.