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Abstract-Fatigue crack growth and threshold behaviour have been examined in three commercial aluminium alloys in both air and vacuum environments. It was observed that, in air, the threshold stress intensity range AK,,, varied linearly with the R ratio. In contrast AK, in vacuum was found to be independent of R. Over the whole growth rate range examined fatigue crack growth in vacuum was AK controlled and failure occurred by a dimple and ductile striation mechanism. This also applied to failure in the intermediate growth rate ranges in air. However, at slow growth rates in air, fatigue crack growth was structure sensitive and crystallographic facets were formed during the crack propagation process.

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