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 conditio ns is a major concern for the aircraft industry with respect to the structu ral integrity of turbine engine components. The extremely high cyclic frequ encies characteristic of in-flight loading spectra necessitate that a damag e-tolerant design approach be based on a crack-propagation threshold, Delta K-TH. The present study identifies a practical lower-bound large-crack thr eshold under high-cycle fatigue conditions in a Ti-6Al-4V blade alloy (with similar to 60% primary alpha in a matrix of lamellar alpha+beta) Lower-bou nd thresholds are measured by modifying standard large-crack propagation te sts to simulate small-crack behavior. These techniques include high load-ra tio 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 resu lts of these tests are compared to the near-threshold behavior of narurally -initiated small cracks, and to the crack initiation and early growth behav ior of small cracks emanating from sites of simulated foreign object damage . (C) 1999 Elsevier Science Ltd. All rights reserved.