The effect of ballistic impacts on the high-cycle fatigue properties of Ti-48Al-2Nb-2Cr (atomic percent)

Abstract

The ability of γ-TiAl to withstand potential foreign object damage (FOD) and/or domestic object damage (DOD) is a technical risk to the implementation of γ-TiAl in low-pressure turbine (LPT) blade applications. The overall purpose of the present study was to determine the influence of ballistic impact damage on the high-cycle fatigue strength of γ-TiAl-simulated LPT blades. Impact and specimen variables included ballistic impact energy, projectile hardness, impact temperature, impact location, and leading-edge thickness. The level of damage induced by the ballistic impacting was studied and quantified on both the impact (front) and backside of the specimens. Multiple linear regression was used to model the cracking and fatigue response as a function of the impact variables. Of the impact variables studied, impact energy, had the largest influence on the response of γ-TiAl to ballistic impacting. Backside crack length (BSCL) was the best predictor of remnant fatigue strength for low-energy impacts (<0.74 J), whereas Hertzian crack length (HCL) (impact side damage) was the best predictor for higher-energy impacts. The impacted γ-TiAl samples displayed a classical mean stress dependence on the fatigue strength. For the fatigue design stresses of a sixth-stage LPT blade in a GE90 engine, a Ti-48Al-2Nb-2Cr LPT blade would survive an impact of normal service conditions.