A theory of fatigue crack initiation along PSBs based on free energy considerations has been developed. The PSB is modelled as an accumulation of vacancy dipoles. The total free energy of the system increases with fatigue cycle number due to the increase in elastic strain energy. It is shown that there exists a critical fatigue cycle number above which the initial state of dislocation accumulation along the slip band becomes energetically unstable, leading to the initiation of a microcrack within the PSB. SN curves for initiation can be calculated and the resulting crack depths determined. The theory correctly predicts several observed phenomena of fatigue crack initiation: (i) crack initiation is easier in air than in vacuum; (ii) crack initiation is easier at room temperature than at cryogenic temperatures; (iii) just-initiated cracks are shallow.
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