Interactions between grain size and geometrical defects in pure aluminium in the high cycle fatigue regime

In this study, the influence of geometrical defects on the High Cycle Fatigue (HCF) resistance of aluminium is investigated, with emphasis on the impact of local microstructure on fatigue crack initiation. In order to meet this objective, an experimental approach, using a commercial purity polycrystalline aluminium alloy, is proposed. First, different thermomechanical treatments are applied to the aluminium alloy to obtain two homogeneous microstructures with respective mean grain sizes of 100 and 1000 tim. Then, fatigue specimens with an artificial hemispherical surface defect of diameter 1000 tim are subjected to fully reversed stress-controlled cyclic loading conditions. In-situ observations are carried out to monitor the crack length during fatigue tests. It is noted that, for a higher grain size, the number of cycles needed for the initiation of a 100 m-long surface crack is lower. A study of the influence of the defect size relative to the grain size is also conducted. Two sizes of defects are used, and the influence of characteristic sizes seems to be explained by the role of cyclic plasticity in the crack initiation process. Copyright (C) 2017 The Authors. Published by Elsevier B.V.