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Propagation of microcracks at stress amplitudes below the conventional fatigue limit in Ti–6Al–4V

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Fatigue crack growth below the conventional fatigue limit was examined in Ti–6Al–4V in two different microstructural conditions, bi-modal and fully lamellar. Tests conducted at R = −1, at room temperature and in air showed that there is a stress dependency in the da/dN–ΔK behaviour in both microstructures. The increasing crack front roughness associated with increasing crack size results in a decrease in the crack growth rate relative to the crack growth rate in a single grain. The da/dN vs. ΔK lines were drawn for each crack size and a ‘threshold’ΔK was determined using the intercept of the lines with da/dN = 10−10 m cycle−1. These values were used to construct modified Takahashi–Kitagawa diagrams to predict microcrack growth below the fatigue limit for each microstructure. A comparison of the two microstructures indicated differences in behaviour in microcrack and macrocrack growth that were explained by differences in crack front roughness at a given crack size.

Keywords: high cycle fatigue; microcracks; titanium alloys

Document Type: Research Article


Affiliations: Technical University Hamburg–Harburg, Eißendorferstr. 42, 21073 Hamburg, Germany

Publication date: 1999-08-01

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