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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 …