MIXED-MODE THRESHOLDS IN HIGH-CYCLE FATIGUE IN Ti-6Al-4V

The influence of microstructure on mixed-mode (mode I+II) high-cycle fatigue thresholds in a Ti-6Al-4V alloy is reported for crack sizes ranging from tens of micrometers to in excess of several millimeters. Two particular microstructural conditions were examined: a fine-grained equiaxed bimodal microstructure structure (grain size ∼20 μm), and a coarser lamellar structure (colony size ∼500 μm). Studies were conducted over a range of mode-mixities, from pure mode I (ΔK_IIΔK_I = 0) to nearly pure mode II (ΔK_II/ΔK_I ∼ 7.1), at load ratios (minimum load/maximum load) of 0.1, 0.5 and 0.8. The thresholds were characterized in terms of the strain-energy release rate (ΔG) incorporating both tensile and shear loading components. In the presence of through-thickness cracks, large (>4 mm) compared to the microstructural dimensions, significant effects of mode-mixity and load ratio were observed for both structures, with the lamellar alloy generally displaying better resistance. However, these effects were substantially reduced if allowance was made for crack-tip shielding. Furthermore, when thresholds were measured in the presence of cracks comparable to the microstructural dimensions, specifically through-thickness short (∼200 μm) cracks and microstructurally-small (<50 μm) surface cracks, where the influence of crack-tip shielding is minimal, such effects were similarly reduced. Indeed, small-crack ΔG_TH thresholds were some 50 to 90 times lower than the corresponding large crack values. These results are discussed in terms of the dominant role of mode I behavior and the effect of microstructure (in relation to crack size) in promoting crack-tip shielding that arises from significant changes in the crack path in the two microstructures.