Stress concentrations and the Mott mechanism in fatigue failure: Jackson, P.J. Mater. Sci. Eng. A 1 Nov. 1989 A119, (1–2), 1–6

Abstract

The fatigue crack growth characteristics of the near-alpha IMI 834 Ti alloy (nominal composition Ti-5.8Al-4.0Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C) have been studied for material heat treated to yield microstructures consisting of 5, 15 and 25% primary alpha phase in a transformed beta matrix. Although crack initiation was observed to be specific to the alpha phase the global crack propagation characteristics of the material were found to be relatively independent of the alpha-phase volume fraction and were governed primarily by the crack length, the level of cyclic damage introduced prior to crack initiation and by the magnitude of the applied stress. A significant short-crack effect was identified for the material with short self-initiated cracks continuing to grow at ΔK-values significantly below the long-crack threshold. The large R-ratio effects observed for long through cracks were not transferred to the short-crack regime, and short cracks propagated at both high (R=0.7) and low (R=0.1) stress ratios, exhibiting significantly higher growth rates than their long-crack counterparts at R=0.7. However, crack growth was not continuous and, when crack arrest points were included in the calculation of the crack growth rates, the average crack propagation rates obtained for short cracks below the short/long-crack transition at both R=0.1 and R=0.7 became coincident with the crack growth rates observed for long cracks grown at R=0.7.