Thresholds for high-cycle fatigue in a turbine engine Ti-6Al-4V alloy

The characterization of critical levels of microstructural damage that can lead to fatigue-crack propagation under high-cycle fatigue loading conditions is a major concern for the aircraft industry with respect to the structural integrity of turbine engine components. The extremely high cyclic frequencies characteristic of in-flight loading spectra necessitate that a damage-tolerant design approach be based on a crack-propagation threshold, ΔK_TH. The present study identifies a practical lower-bound large-crack threshold under high-cycle fatigue conditions in a Ti-6Al-4V blade alloy (with approx. 60% primary α in a matrix of lamellar α+β). Lower-bound thresholds are measured by modifying standard large-crack propagation tests to simulate small-crack behavior. These techniques include high load-ratio testing under both constant-R and constant-K_max conditions, performed at cyclic loading frequencies up to 1 kHz and R-ratios up to 0.92. The results of these tests are compared to the near-threshold behavior of naturally-initiated small cracks, and to the crack initiation and early growth behavior of small cracks emanating from sites of simulated foreign object damage.