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-6V 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-Delta K 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. Delta K lines were drawn for each crack size and a 'threshold' Delta K 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.