Abstract
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.
Highlights
In previous studies the fatigue behaviour of metallic materials at very high number of load cycles (N > 107, VHCF) was thoroughly investigated
The otherwise time and cost intensive testing in the VHCF regime can be addressed by using high frequency testing systems [4], such as the ultrasonic fatigue testing system used in this research
Previous studies could show that crack initiation for aluminium alloys may occur at subsurface regions in the VHCF regime [6]
Summary
In previous studies the fatigue behaviour of metallic materials at very high number of load cycles (N > 107, VHCF) was thoroughly investigated (for example [1]). The crack growth phase at VHCF relevant low stress amplitudes is not as intensively investigated as of yet. The aim of the present work is to analyse the fatigue crack growth behaviour of two commercially available aluminium alloys (EN AW 6082 and EN AW 5083) in the near-threshold regime at constant stress intensity factors (ΔK) using an ultrasonic fatigue testing system in order to gain insight into the barrier function of microstructural inhomogeneities such as large precipitates or grain boundaries. The threshold ΔKth is determined experimentally using ASTM E 647 [5] This implies using the load-shedding method, in which the threshold is reached by continuously decreasing the applied load amplitude until the crack growth rate is less than 10-11 m/cycle. A micronotch was inserted by means of focused ion beam technology (FIB)
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