High-Cycle Fatigue and Time-Dependent Failure in Metallic Alloys for Propulsion Systems

This program is focused on the definition, microstructural characterization and mechanism-based modeling of the limiting states of damage associated with the onset of high-cycle fatigue failure in titanium and nickel-base alloys for propulsion systems. Both experimental and theoretical studies are aimed at three areas, namely HCF/LCF interactions, the role of notches and foreign object damage, and fretting fatigue. The approach is to combine new experimental techniques for characterizing microstructural damage, with detailed micro-mechanical modeling to facilitate the prediction of the effects of such damage. During the third program year, notable highlights include the characterization and quantitative modeling of fretting and FOD, and the definition of practical "lower-bound" HCF threshold stress intensities including the role of mixed-mode loading in Ti-6Al-4V. The ultimate aim of this work is to provide a mechanistic basis for the evolution of HCF damage, enabling comprehensive life-prediction schemes to be formulated for fatigue-critical components in turbine engines. Prepared in cooperation with the Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA; the Div., of Applied Sciences, Harvard University, Cambridge, MA and the Dept. of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI.