Effects of microstructure on mixed-mode, high-cycle fatigue crack-growth thresholds in Ti-6Al-4V alloy

Effect of microstructure on mixed-mode (mode I + II), high-cycle fatigue thresholds in a Ti-6Al-4V alloy is reported over a range of crack sizes from tens of micrometers to in excess of several millimeters. Specifically, two microstructural conditions were examined-a fine-grained equiaxed bimodal 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_II/ΔK_I = 0) to nearly pure mode II (ΔK_II/ΔK_I∼7.1), at load ratios (minimum load/maximum load) between 0.1 and 0.8, with thresholds 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 microstructural dimensions-significant effects of mode-mixity and load ratio were observed for both microstructures, with the lamellar alloy generally displaying the better resistance. However, these effects were substantially reduced if allowance was made for crack-tip shielding. Additionally, when thresholds were measured in the presence of cracks comparable to microstructural dimensions, specifically short (∼200 μm) through-thickness cracks and microstructurally small (< 50 μm) surface cracks, where the influence of crack-tip shielding would be minimal, such effects were similarly markedly reduced. Moreover, small-crack ΔG_TH thresholds were some 50-90 times smaller than corresponding large crack values. Such effects are discussed in terms of the dominant role of mode I behaviour and the effects of microstructure (in relation to crack size) in promoting crack-tip shielding that arises from significant changes in the crack path in the two structures.