Effects of intermittent loading time and stress ratio on dwell fatigue behavior of titanium alloy Ti-6Al-4V ELI used in deep-sea submersibles

Abstract

Different components of deep-sea submersibles, such as the pressure hull, are usually subjected to intermittent loading, dwell loading, and unloading during service. Therefore, for the design and reliability assessment of structural parts under dwell fatigue loading, understanding the effects of intermittent loading time on dwell fatigue behavior of the alloys is essential. In this study, the effects of the intermittent loading time and stress ratio on dwell fatigue behavior of the titanium alloy Ti-6Al-4V ELI were investigated. Results suggest that the dwell fatigue failure modes of Ti-6Al-4V ELI can be classified into three types, i.e., fatigue failure mode, ductile failure mode, and mixed failure mode. The intermittent loading time does not affect the dwell fatigue behavior, whereas the stress ratio significantly affects the dwell fatigue life and dwell fatigue mechanism. The dwell fatigue life increases with an increase in the stress ratio for the same maximum stress, and specimens with a negative stress ratio tend to undergo ductile failure. The mechanism of dwell fatigue of titanium alloys is attribute to an increase in the plastic strain caused by the part of the dwell loading, thereby resulting in an increase in the actual stress of the specimens during the subsequent loading cycles and aiding the growth of the formed crack or damage, along with the local plastic strain or damage induced by the part of the fatigue load promoting the cumulative plastic strain during the dwell fatigue process. The interaction between dwell loading and fatigue loading accelerates specimen failure, in contrast to the case for individual creep or fatigue loading alone. The dwell fatigue life and cumulative maximum strain during the first loading cycle could be correlated by a linear relationship on the log–log scale. This relationship can be used to evaluate the dwell fatigue life of Ti alloys with the maximum stress dwell.