Fatigue crack growth of titanium rotor alloys in vacuum and air

An enhanced life management system for aircraft turbine engine rotors based on probabilistic damage tolerance methods is currently under development by the engine industry and the FAA, with an initial focus on fatigue cracking at hard alpha (HA) defects in titanium. Since HA defects are usually subsurface, any resulting cracks are embedded and hence isolated from the atmosphere (i.e., vacuum-like) for at least some of their life. Fatigue crack growth (GCG) tests have been conducted in vacuum at various temperatures and stress ratios for Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo+Si rotor alloys. A brief study of vacuum levels suggests that pressures of 10{sup {minus}6} Torr are adequate to capture full vacuum effects on FCG rates. Vacuum FCG results are compared with available air data. The vacuum data demonstrate temperature and stress ratio effects comparable to air data. The vacuum and air data exhibit the same growth rates in the upper Paris regime, but apparent thresholds are significantly higher in vacuum than in air.