On the factors affecting the propagation of long fatigue cracks in thin-sheet wrought aluminium alloys

The paper deals with the propagation of long fatigue cracks in commercial heat-treatable thin-sheet wrought aluminium alloys with focus on the mid-region of the cyclic stress intensity factor. The experimental observations support the distinction of two groups of alloy conditions according to the degree of coherency of the strength-controlling precipitates. A model explaining the observations for the first group, which is characterized by predominantly incoherent precipitates, is proposed. These precipitates cannot be cut by dislocations and both a homogeneous slip distribution and a continuum-mechanics dominated crack path are favoured. The reasons why the alloys with a high degree of coherency of the strength-controlling precipitates deviate from the fatigue crack growth behaviour common of the representatives of the first group are analysed.