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

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.