Experimental investigation on compressive dwell fatigue behavior of titanium alloy pressure hull for deep-sea manned submersibles

The spherical pressure hull of deep-sea manned submersible is subjected to the alternating load with high dwell pressure during the cyclic working state of diving, operating and rising. Therefore, the compressive dwell fatigue behavior is a key factor to be considered in the structural safety evaluation of the pressure hull for manned submersibles. In this paper, a structure-grade dwell fatigue test of the pressure hull and a variety of specimen-grade compressive dwell fatigue tests were performed for Ti–6Al–4V ELI and Ti–6Al–2Sn–2Zr–3Mo-X alloys. Results show that no obvious structural deformation occurs in the spherical hull after the dwell fatigue test and no detectable out-of-standard defect is found in the welds when the dwell pressure value is the maximum operating pressure. The cumulative compressive strains of dwell fatigue specimens show that the base metal and welding joint have undergone plastic deformation when the maximum compressive stress value is 0.9 Rp0.2. No microcracks or changes in the microstructure characteristics are observed in the specimens. The present results support the structural design and safety assessment of deep-sea manned submersibles for long-term service.