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

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 mu m) 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 alpha grain.

Automated summary

Notches and defects are important factors influencing the dwell fatigue behavior of Ti-6Al-4V ELI alloy. Dwell fatigue is insensitive to defect size and notches compared to conventional fatigue. Notched specimens exhibit higher dwell fatigue life compared to smooth specimens, with the maximum stress dwell having no influence on fatigue life and failure mechanism. The facet feature is observed in the crack initiation region of notched specimens for both conventional and dwell fatigue. The fine line markings on the facet are attributed to steps on the fracture surface of the alpha grain, as observed through electron backscatter diffraction.