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 crack‐propagation threshold. The present study is focused on characterizing such near‐threshold fatigue‐crack propagation behaviour in a Ti–6Al–4V blade alloy (with ~60% primary α in a matrix of lamellar α + β), 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.

The phenomenon of crack closure, which involves the premature closing of fatigue cracks during the unloading portion of a fatigue cycle resulting in the development of crack‐tip shielding due to crack wedging, has become widely accepted as a critical mechanism influencing many aspects of the behaviour of fatigue cracks in metallic materials; these include effects of load ratio, variable‐amplitude loading, crack size, microstructure, environment and the magnitude of the fatigue threshold. Recently, however, the significance of crack closure has been questioned and alternative suggestions made for many of these phenomena, e.g. the effect of the load ratio (i.e. the ratio R of the minimum to maximum loads) on threshold behaviour. In the light of this, the present paper provides evidence to rebut the assertion that crack closure is an insignificant process. Particular attention is given to the effect of crack closure on the threshold level as a function of load ratio.