Understanding the contributions of normal-fatigue and static loading to the dwell fatigue in a near-alpha titanium alloy
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I. INTRODUCTION
THE alloy Ti-6242 is widely used in the higher-temperature portion of the compressor rotor of aircraft engines. In work conducted in several laboratories around the world, a significant debit in the fatigue capability (reduced lifetimes) of this class (near-) of titanium alloys has been found when comparing results obtained from continuous cycling and holding at maximum load (dwell-fatigue) conditions. Because the magnitude of the life debit decreases with increasing temperature, the phenomenon has been called “cold-dwell fatigue.” Although it has been realized that both the creep and cyclic deformation/damage processes contribute to failure under dwell-fatigue loading conditions, the relative importance of these processes for the total damage has not been understood in a fundamental sense. The objective of this study was to obtain an understanding of the relative contributions of the cyclic (repetitive) and static loading to the overall damage under dwell-fatigue loading conditions. In the open literature, there are earlier reports about the correlation between the times to failure under static-loading and dwell-fatigue test conditions for different alloys and test conditions. For IMI 829 with a colony microstructure, the results of White et al.[1] indicate that at 89 pct of the 0.2 pct proof stress, the creep specimen did not break even at 8 times the equivalent time to failure for the corresponding dwell specimen. Hack and Leverant[2] have observed similar times to failure under dwell-fatigue and static loading conditions at 95 pct of the 0.2 pct offset yield strength for IMI 685 with a colony microstructure. Evans and Gostelow[3] have also reported similar times to failure (i.e., rupture lives) for constant-load (creep) and dwell-fatigue specimens tested at 95 to 101 pct of the 0.1 pct proof stress for a colony-microstructure IMI 685 alloy. Furthermore, the results of Kassner et al.[4] indicate that for a Ti-6242 alloy (bimodal microstructure with a primary volume of 33 pct), some creep specimens have a shorter rupture life (by a factor of 4 or more) than the corresponding dwellfatigue specimens at 113 pct of the 0.2 pct yield strength. V. SINHA, Research Associate 1-Engineer, M.J. MILLS, Professor, and J.C. WILLIAMS, Dean of the College of Engineering and Honda Professor of Materials, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] Manuscript submitted January 5, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
In this article, we have attempted to evaluate the correlation between the static-load and dwell-fatigue rupture lives as a function of stress. This information is then used in an effort to help understand the data in the literature. This study was carried out on Ti-6242 alloy in the /forged condition. The mechanical tests were conducted at two different stress levels. The variation in the relative contributions of cyclic and creep processes to the overall dwell-fatigue damage as a fu
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