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

The objective of the AFaSR-MURI High-Cycle Fatigue program is to characterize and model the limiting damage states at the onset of high-cycle fatigue to facilitate mechanistic understanding and to develop a basis for life prediction. Efforts have been focused on the influence of foreign object damage (FAD) and fretting on a Ti-6A1-4V blade alloy and on a polycrystalline Ni-base disk alloy. Notable highlights during the fourth year include the characterization and quantitative modeling of fretting and FaD and the definition of the role of mixed-mode loading on HCF thresholds in Ti-6A1-4V. Accomplishments are outlined below: Worst-case fatigue threshold stress intensities have been measured in STOA Ti-6A1-4V using large (> 5 mm) cracks under representative HCF conditions (R > 0.95, 1000 Hz). Values provide a practical, frequency-independent (20 - 20,000 Hz) lower-bound for the growth of naturally-initiated, physically-small (> 40 gm) cracks.