Abstract
The notch (i.e., stress concentration) and defect are important factors influencing the conventional fatigue behavior of metallic materials. What is the influence of notches and defects on the dwell fatigue mechanism and fatigue life? In this paper, the effects of notches and defects on the dwell fatigue behavior of the Ti-6Al-4V ELI alloy used in deep-sea submersibles are investigated under the load control mode. It is shown that the dwell fatigue is insensitive to the defect size (190–438 μm) compared to the conventional fatigue. For notched specimens, they all present fatigue failure mode under dwell fatigue testing, and the dwell fatigue life is higher than that of the smooth specimen at the same local maximum stress. The dwell of the maximum stress has no influence on the fatigue life and failure mechanism for notched specimens. Moreover, the facet feature is observed in the crack initiation region for both the conventional and dwell fatigue of notched specimens. Electron backscatter diffraction observation indicates that the feature of the fine line markings on the facet in the image by scanning electron microscope is due to the steps on the fracture surface of the α grain.
Highlights
IntroductionThe dwell fatigue (i.e., cold creep fatigue, which refers to the dwell loading effect at temperatures lower than 200 ◦ C [1,2]) behavior of titanium alloys has drawn great attention due to some of the components in the field of aviation and deep-sea (e.g., fan discs of engines, pressure hull of deep-sea submersibles) subjected to dwell fatigue loadings during service [1,2,3,4,5,6,7]
The dwell fatigue behavior of titanium alloys has drawn great attention due to some of the components in the field of aviation and deep-sea subjected to dwell fatigue loadings during service [1,2,3,4,5,6,7]
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]
Summary
The dwell fatigue (i.e., cold creep fatigue, which refers to the dwell loading effect at temperatures lower than 200 ◦ C [1,2]) behavior of titanium alloys has drawn great attention due to some of the components in the field of aviation and deep-sea (e.g., fan discs of engines, pressure hull of deep-sea submersibles) subjected to dwell fatigue loadings during service [1,2,3,4,5,6,7]. 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]. The actual component parts usually inevitably contain defects during the manufacturing process or from impacts of debris during service. They might have a change in geometry (i.e., stress concentration) in order to meet the demands of design. These defects [15,16,17,18] and notches [19,20,21,22] play an important role in the conventional fatigue performance of metallic materials. The specimen of EA4T railway axles with artificial defects showed a lower fatigue strength than that of the smooth specimen [23]
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