Development of titanium alloy dwell fatigue for pressure chamber of deep sea submersible

There are biological and mineral resources in the deep sea. Deep sea submersible is the main carrier of deep-sea scientific research and resource development. "Pressure chamber" is a core component, which is the safety barrier for human to enter the deep sea. In every operation of the submersible, the pressure chamber is subjected to a constant sea water pressure (load lifting, load holding and unloading). The fatigue under this load spectrum is called dwell fatigue. A new type of high strength and toughness titanium alloy is the preferred material for pressure cabin. It is very important for the design and reliability evaluation of deep-sea submersible to study its fatigue characteristics.

It is found that there are three fatigue failure modes of titanium alloy in pressure cabin, namely fatigue failure, ductility failure and mixed fatigue and ductility failure. The interaction of holding load and fatigue load accelerates the failure of specimens and leads to competition among different failure modes. It is further found that the fatigue life is related to the first cycle cumulative maximum strain, which is approximately linear in the double logarithm coordinate. Intermittent loading time has no effect on the fatigue behavior of titanium alloy. The experimental results and theoretical analysis reveal the fatigue mechanism of titanium alloy, that is, the plastic deformation caused by the holding load increases the actual stress of the sample, which promotes the propagation of cracks or damage. At the same time, the local plastic strain caused by the fatigue load increases the accumulation of plastic strain.