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, DeltaK(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 DeltaK(th) In this work, we examine the effect of a wide range of loading frequencies (v = 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 similar to 60 vol% primary-alpha and similar to 40 vol% lamellar alpha + beta. The data presented in this paper indicate that at K-max values above 6 MPa rootm (R > 0.5), where macroscopic crack closure is no longer detected in this alloy, DeltaK(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. (C) 2000 Published by Elsevier Science Ltd.