Mixed-mode, high-cycle fatigue-crack growth thresholds in Ti–6Al–4V

Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural dimensions) in a Ti–6Al–4V turbine blade alloy with a bimodal microstructure. Specifically, the effect of combined mode I and mode II loading, over a range of phase angles β= tan −1(ΔK II /ΔK I ) from 0° to 82° (ΔK II/ΔK I∼0–7), is examined for load ratios (ratio of minimum to maximum loads) ranging from R=0.1 to 0.8 at a cyclic loading frequency of 1000 Hz in ambient temperature air. Although the general trend for the mode I stress-intensity range at the threshold, ΔK I,TH, is to decrease with increasing mode mixity, ΔK II/ΔK I, and load ratio, R, if the crack-driving force is alternatively characterized in terms of the strain-energy release rate, ΔG, incorporating contributions from both the applied tensile and shear loading, the threshold fatigue-crack growth resistance increases significantly with the applied ratio of ΔK II/ΔK I. The pure mode I threshold, in terms of ΔG TH, is observed to be a lower bound (worst case) with respect to mixed-mode (I+II) behavior. These results are compared with mixed-mode fatigue thresholds for short cracks, where the precrack wake has been machined to within ∼200 μm of the precrack tip. For such short cracks, wherein the magnitude of crack-tip shielding which can develop is greatly reduced, the measured mixed-mode fatigue-crack growth thresholds are observed to be markedly lower. Moreover, the dependence of the mixed-mode fatigue-crack growth resistance on the applied phase angle is significantly reduced. Comparison of the large- and short-crack data suggests that the increase in the large-crack fatigue threshold, ΔG TH, with an increasing mode mixity (ΔK II/ΔK I) is largely due to shielding from shear-induced crack-surface contact, which reduces the local crack-driving force actually experienced at the crack tip. Quantification of such shielding is described in Part II of this paper.