ABSTRACT
It is well established that the presence of a corrosion resistant anodised surface film on aluminium alloys can be detrimental to their fatigue performance. This is generally attributed to initiation effects associated with the presence of process cracks in the oxide layer. What is less clear however, is the effect that such a film has on the propagation behaviour of an already initiated, and growing, fatigue crack. This paper will report the findings of a fatigue life assessment study carried out on a 2000 series aluminium alloy that had been anodised using the recently developed boric-sulphuric acid anodising process. The results obtained demonstrated the decrease in fatigue life experienced by these alloys depends not only on the easier initiation of fatigue cracks but also on film-assisted fatigue crack growth brought about by the presence of the protective oxide film. The increase in the crack growth rate can be explained, using standard LEFM techniques, in terms of two concurrent mechanisms. Firstly, constraint of the substrate material related to tensile residual stress in the oxide film, and its inherent higher modulus, results in a reduction to the plasticity-induced closure experienced by the growing fatigue crack. Secondly, the highly cracked surface morphology of the oxide film in the vicinity of the crack tip alters the growth characteristics of the propagating fatigue crack by defining a crack path of least resistance. The combined influence of these two mechanisms results in a faster growing fatigue crack, despite an observed increase in crack path tortuosity. These effects are likely to be most prevalent at low values of ΔK in thin sheet materials where the influence of fatigue crack closure is greatest and may possibly occur in other protective coating systems.
