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

The ability of gamma -TiAl to withstand potential foreign object damage (FO D) and/or domestic object damage (DOD) is a technical risk to the implement ation of gamma -TiAl in low-pressure turbine (LPT) blade applications. The overall purpose of the present study was to determine the influence of ball istic impact damage on the high-cycle fatigue strength of gamma -TiAl-simul ated LPT blades. Impact and specimen variables included ballistic impact en ergy, 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 spec imens. Multiple linear regression was used to model the cracking and fatigu e response as a function of the impact variables. Of the impact variables s tudied, impact energy, had the largest influence on the response of gamma - TiAl to ballistic impacting. Backside crack length (BSCL) was the best pred ictor of remnant fatigue strength for low-energy impacts (<0.74 J), whereas , Hertzian crack length (HCL) (impact side damage) was the best predictor f or higher-energy impacts. The impacted gamma -TiAl samples displayed a clas sical mean stress dependence on the fatigue strength. For the fatigue desig n stresses of a sixth-stage LPT blade in a GE90 engine, a Ti-48Al-2Nb-2Cr L PT blade would survive an impact of normal service conditions.