Effect of Rise and Fall Time on Dwell Fatigue Behavior of a High Strength Titanium Alloy

Abstract

Frequency is an important factor influencing the fatigue behavior. Regarding to the dwell fatigue, it corresponds to the effect of rise and fall time, which is also an important issue especially for the safety evaluation of structure parts under dwell fatigue loading, such as the engines of aircrafts and the pressure hulls of deep-sea submersibles. In this paper, the effect of rise and fall time (2 s, 20 s, 110 s, and 200 s) on the dwell fatigue behavior is investigated for a high strength titanium alloy Ti-6Al-2Sn-2Zr-3Mo-X with basket-weave microstructure. It is shown that the dwell fatigue life decreases with increasing the rise and fall time, which could be correlated by a linear relation in log–log scale for both the specimen with circular cross section and the specimen with square cross section. The rise and fall time has no influence on the crack initiation mechanism by the scanning electron microscope observation. The cracks initiate from the specimen surface and all the fracture surfaces present multiple crack initiation sites. Moreover, the facet characteristic is observed at some crack initiation sites for both the conventional fatigue and dwell fatigue tests. The paper also indicates that the dwell period of the peak stress reduces the fatigue life and the dwell fatigue life seems to be longer for the specimen with circular cross section than that of the specimen with square cross section.

Effects of Notches and Defects on Dwell Fatigue Mechanism and Fatigue Life of Ti-6Al-4V ELI Alloy Used in Deep-Sea Submersibles

“…For example, many results have shown that the dwell of the maximum stress could greatly reduce the fatigue life of titanium alloys [8][9][10] and the dwell fatigue of titanium alloys often presents the feature of cleavage or quasi-cleavage facets in the crack initiation region [1,11,12]. The results for the titanium alloy Ti-6Al-2Sn-2Zr-3Mo-X indicated that the rise and fall time significantly affects the dwell fatigue life, and the dwell fatigue life with the rise and fall time could be correlated by a linear relation in log-log scale [13]. For the dwell fatigue of Ti-6Al-4V ELI alloy, it has been shown that the stress ratio has great influence on the dwell fatigue life and dwell fatigue failure mode [14].…”