Structural Optimization Of Window Seat And Creep Analysis Of PMMA Window For Deep-sea Manned Cabin

The structure of the observation window is very important for the safety of equipment and personnel on a manned deep submersible.Based on the strength theory,the titanium-alloy window seat and the polymethyl methacrylate(PMMA)glass of the observation window on deep-sea cabin were studied.Based on the pressure-resistant spherical shell of 7000m-deep-sea manned submersible,finite element models of multi-window spherical shell and single-window spherical shell were established successively,which were used to make static calculation and strength check.The stress concentration existed near the weld toe of the window seat,where the stress distribution along the circumference of the opening and along the wall thickness was discussed.It was found that local stress concentration would be aggravated by several adjacent openings.Besides,the stress on the outer surface of the weld toe near the primary observation window was found to be lower than that on the inner surface,while the stress distribution near the secondary observation window was opposite.Three different forms were used to build the models of window form.It was concluded that the circumferential distribution of stress in the simplified model was more sensitive to the multi-opening effect.Different modeling methods for window would have little effect on the overall distribution of wall thickness stress at weld toes,but an effect on the stresses of the inner and outer surfaces.On the basis of strength analysis,an optimum design of window seat size was carried out using simplified single window model for the purpose of reducing the weight of the spherical shell.The zero-order method was adopted to complete the optimization calculationWith the strength safety of the spherical shell and the window seat guaranteed,the observation glass made of PMMA became the focus of strength checking.The traditional linear elastic model has demonstrated large limitation to simulate the creep deformation of the PMMA observation window on human occupied deep-sea vehicle.A linear viscoelastic constitutive model was built based on Prony series form.The mass scaling method together with the equivalent duration method was introduced to accelerate FEM analysis.The numerical-simulation calculation showed much stability and the results of displacement fitted well with test data.It was concluded that PMMA window showed viscoelastic behaviour when loaded gradually to a high level at room temperature.So the viscoelasticity must be considered during a whole loading-holding process.Under quasistatic condition,traditional static FEM was replaced by dynamic explicit FEM which is adaptive to the time-varying property of PMMA viscoelasticity.It was also found that a fillet could help prevent local stress concentration and improve grid convergence.