Crack growth behaviour of aluminium wrought alloys in the Very High Cycle Fatigue regime

Fatih Bülbül,Marcel Wicke,Hans-Jürgen Christ,Martina Zimmermann,Angelika Brückner-Foit,Tina Kirsten,G Hénaff

doi:10.1051/matecconf/201816520007

Abstract

Investigations have shown that in the regime of Very High Cycle Fatigue (VHCF) “natural” crack initiation often takes place underneath the material surface leading to crack propagation without contact to atmospheric components. In order to elucidate the environmental damage contribution and its effect on the VHCF long crack propagation, fatigue experiments with alternating environment (vacuum and laboratory air) were performed. An ultrasonic fatigue testing system (USFT) equipped with a small vacuum chamber was applied that enables the in-situ examination of the long fatigue crack propagation at a resonance frequency of about 20 kHz by using a long distance microscope. By means of the Focused-Ion-Beam technique, micro-notches were prepared in the USFT specimens. The tests were carried out on the aluminium alloys EN-AW 6082 and 5083 in different conditions. It has been found that the atmosphere has a significant influence on the VHCF long crack propagation which manifests itself in the crack path as well as in the crack growth rates. Because of pronounced single sliding in vacuum, shear-stress-controlled crack propagation was detected whereas in laboratory air normal-stress-controlled crack propagation occurred. Furthermore, it has been proven that the secondary precipitation state of the aluminium alloy significantly influences the VHCF long crack propagation in vacuum.

Highlights

In the regime of Very High Cycle Fatigue (VHCF), that means at very low stress amplitudes, “natural” cracks often initiate internally underneath the material surface [1, 2]
The fatigue properties of the investigated aluminium alloys deteriorate significantly due to the aforementioned mechanism and the resulting irreversible plastic deformation manifests itself in the crack growth rates
Very pronounced single sliding takes place leading to shear-stress-controlled VHCF long crack propagation in the aluminium alloy ENAW 6082 in the peak-aged condition

Summary

Introduction

In the regime of Very High Cycle Fatigue (VHCF), that means at very low stress amplitudes, “natural” cracks often initiate internally underneath the material surface [1, 2]. In the long crack regime, the fatigue cracks propagate continuously and can be described by the Paris-Erdogan law. It is a well-known fact that the humidity of ambient air deteriorates the fatigue properties of aluminium alloys compared to an inert environment [4,5,6]. The atmospheric influence on the VHCF long crack propagation in the aluminium wrought alloys ENAW 6082 and 5083 in different material conditions is investigated. The detrimental effect of water vapour in ambient air as well as the effect of alloy microstructure on the long crack propagation will be discussed in this article

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Conclusion