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 approximately 60 vol% primary-α and approximately 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.