There are biological and mineral resources in the deep sea. The deep-sea submersible is the main vehicle for deep-sea scientific research and resource development, and the "ballast cabin" is its core component, which is the safety barrier for human beings to enter the deep sea. In every operation of the submersible, the ballast cabin will bear a constant change of seawater pressure (lift-load-load-unloading), and the fatigue under this load spectrum is called Dwell fatigue. The new type of high strength and toughness titanium alloy is the preferred material for ballast tanks. It is of great significance to study the fatigue characteristics of the new type of high strength and toughness titanium alloy for the design and reliability evaluation of deep-sea submersible.
It is found that there are three fatigue failure modes of ballast cabin titanium alloy, namely, fatigue failure, ductility failure and combined fatigue and ductility failure. The interaction between load retention and fatigue load accelerates the failure of samples and leads to competition among different failure modes. It is further found that the fatigue life of titanium alloy is related to the first week cumulative maximum strain, and the relationship between them is approximately linear in logarithmic coordinate. The intermittent loading time has no effect on the fatigue behavior of titanium alloy. The experimental results and theoretical analysis reveal the load-keeping fatigue mechanism of titanium alloy, that is, the plastic deformation caused by load-keeping increases the actual stress of the sample, which promotes the propagation of the crack or damage. Meanwhile, the local plastic strain caused by the fatigue load increases the accumulation of the load-keeping fatigue strain.
