Fatigue life assessment of pressure hull of deep-sea submergence vehicle

With the exploration of deep-sea resources, manned submersibles play an increasingly important role in underwater applications. During the multiple submergences, evaluating the fatigue strength and remaining life of the pressure hull has become crucial for the safety of deep submersible. The pressure hull contains multiple welds, and the life of welds determines the life of the submersible. This paper first analyses the weld characteristics of the titanium alloy TC4 used in the pressure spherical shell, comprehensively considers the temperature field and the residual stress field and obtains the relationship between the plate thickness and the welding heat-affected zone. The fatigue life of a 7000 m class submersible pressure hull subject to 10–70 MPa hydrostatic pressure is then evaluated with the equivalent structural stress method considering the heat-affected zone of the weld seam. The results show that the welding seam greatly influences the stress distribution of the whole spherical shell, and the stress concentration occurs near the welding seam of the hatch cover. The fatigue life decreases sharply in the load range of 10 MPa–20 MPa and inclines to converge when the pressure reaches 40 MPa. Its remaining life is 49 years based on the current submergence frequency.