High-cycle fatigue of Ti--6Al--4V

The definition of the critical levels of microstructural damage that can lead to the propagation of fatigue cracks under high-cycle fatigue loading conditions is a major concern with respect to the structural integrity of turbine-engine components in military aircraft. The extremely high cyclic frequencies characteristic of in-flight loading spectra, coupled with the presence of small cracks resulting from fretting or foreign object damage (FOD), necessitate that a defect-tolerant design approach be based on a crackpropagation threshold. The present study is focused on characterizing such nearthreshold fatigue-crack propagation behaviour in a Ti--6Al--4V blade alloy (with ~60% primary a in a matrix of lamellar a+b), at high frequencies (20--1500 Hz) and load ratios (0.1--0.95) in both ambient temperature air and vacuum environments. Results indicate that `worst-case' thresholds, measured on large cracks, may be used as a practical lower bound to describe the onset of naturally initiated small-crack growth and the initiation and early growth of small cracks emanating from sites of simulated FOD