1 Short and Large Crack, Mixed-Mode Fatigue-Crack Growth Thresholds in Ti-6Al-4V 1 Mr. R. K. Nalla, Dr. J. P. Campbell and Professor R. O. Ritchie 2 Department of Materials Science and Engineering University of California, Berkeley, CA 94720-1760 ABSTRACT: There are few experimental results to date describing the crack-propagation threshold behavior of short fatigue cracks under multiaxial loading conditions. To address this need, in the present study, the variation in mixed-mode, high-cycle fatigue-crack growth thresholds with crack size and shape are reported for a Ti-6Al-4V turbine blade alloy, heat treated to two widely different microstructural conditions, namely fine-grained bimodal and coarser fully lamellar microstructures. Specifically, fatigue thresholds are examined for through-thickness large cracks (as compared to the microstructural dimensions), through- thickness short cracks (< 200 μm in length), and small, semi-elliptical surface cracks (comparable with microstructural dimensions), under the effect of combined mode I and mode II loading for load ratios (ratio of minimum to maximum load) ranging from 0.1 to 0.8. For a range of mode-mixities, large crack, mode I thresholds, ΔK I,TH , were found to be decrease substantially with increasing phase angle. However, by characterizing in terms of the range in strain energy release rate, ΔG TH , incorporating both mode I and mode II contributions, it was observed that the pure mode I threshold could be regarded as a "worst case" large crack threshold under mixed-mode loading in this alloy. An estimation of the shielding-corrected, crack-driving forces actually experienced at the crack tip was also performed. For such near-tip (shielding-corrected) thresholds, the influence of mode-mixity was dramatically reduced. Corresponding thresholds for through-thickness short cracks and small surface cracks, where the effect of such shielding is minimized, were also substantially less sensitive to mode-mixity and corresponded in magnitude to the shielding-corrected large crack thresholds. INTRODUCTION The occurrence and control of failures due to high cycle fatigue (HCF) in turbine engine components is currently one of the most critical challenges facing the U.S. military aircraft fleet. One particular challenge is the prediction of the limiting conditions for failure in the presence of ____________________ 1 Funded by the Air Force Office of Scientific Research, Grant F49620-96-1-0478, under the auspices of the Multidisciplinary University Research Initiative on High-Cycle Fatigue to the University of California 2 contributing author: Prof. R. O. Ritchie Department of Materials Science and Engineering 463 Evans Hall, #1760, University of California, Berkeley, CA 94720-1760 tel: +1 (510) 486-5798; fax: +1 (510) 486-4995; e-mail: RORitchie@lbl.gov