Physically short crack propagation in metals during high cycle fatigue

In metals, during high cycle fatigue on plain specimens, almost the entire fatigue life is spent as short crack initiation and propagation. The fatigue short crack life can be schematically divided into two subsequent phases: microstructurally short crack and physically short crack. Recently, Chapetti proposed a physically short crack threshold and propagation driving force model [Chapetti MD. Fatigue propagation threshold of short cracks under constant amplitude loading. Int J Fatigue 2003;25(12):1319-1326]. In his model the physically short crack behavior is obtained from the long crack propagation, just introducing the reduced threshold due to unsaturated closure. In the present paper the physically short crack propagation is similarly modeled by means of a driving force equation, but independent from the long crack propagation. In this way, a better description of the short crack behavior is provided, however short crack propagation data is required. Physically short crack propagation model parameters were obtained, by fitting experimental data drawn from the literature, for two aluminum alloys and a titanium alloy at two different heat treatment conditions and load ratios. By calculating the physically short crack plus long crack propagation, and assuming microstructurally short crack as part of the initiation stage, a purer information about crack initiation can be drawn from the S-N curves, and it is shown in the paper for the investigated materials. A precise crack initiation size and the number of cycles just for initiation are then provided. This information is useful to accurately predict fatigue life for blunt notched and for thick components, where the propagation is much higher than in the small plain specimen. A validation of the model was obtained by predicting the fatigue life of a notched specimen. An accurate prediction was obtained both when the initiation was much smaller than propagation and when almost the entire fatigue life was initiation.