Effect of load ratio and maximum stress intensity on the fatigue threshold in Ti–6Al–4V

There has been a renewed interest of late in the mechanisms responsible for the influence of the load ratio, R, and the maximum stress intensity, K max, on the threshold for fatigue-crack growth, ΔK th. While mechanistic explanations in the past have largely focused on the role of crack closure, it is certainly not the only mechanism by which K max influences ΔK th. In this work, we examine the effect of a wide range of loading frequencies (ν=50–1000 Hz) and load ratios (R=0.10–0.95) on fatigue-crack propagation and threshold behavior in a Ti–6Al–4V turbine blade alloy consisting of ∼60 vol% primary-α and ∼40 vol% lamellar α+β. The data presented in this paper indicate that at K max values above 6 MPa√m (R>0.5), where macroscopic crack closure is no longer detected in this alloy, ΔK th decreases approximately linearly with increasing K max. This result is discussed in terms of possible mechanistic explanations, including sustained-load cracking, microscopic near-tip closure, and static fracture modes, based on considerations of experimental evidence from both the current study and the literature.