Eect 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 in ̄uence of the load ratio, R, and the maximum stress intensity, Kmax, on the threshold for fatigue-crack growth, DKth. While mechanistic explanations in the past have largely focused on the role of crack closure, it is certainly not the only mechanism by which Kmax in ̄uences DKth. In this work, we examine the e€ect of a wide range of loading frequencies (m ˆ 50±1000 Hz) and load ratios (R ˆ 0:10±0:95) on fatigue-crack propagation and threshold behavior in a Ti±6Al±4V turbine blade alloy consisting of 60 vol% primary-a and 40 vol% lamellar a‡ b. The data presented in this paper indicate that at Kmax values above 6 MPa p m (R > 0:5), where macroscopic crack closure is no longer detected in this alloy, DKth decreases approximately linearly with increasing Kmax. This result is discussed in terms of possible mechanistic explanations, including sustainedload cracking, microscopic near-tip closure, and static fracture modes, based on considerations of experimental evidence from both the current study and the literature. Ó 2000 Published by Elsevier Science Ltd.