| Submersible is an important equipment for ocean exploration and deep-sea scientific research,and pressure hull is one of the most important components of the submersible.Spherical pressure hull has become the first choice for deep-sea submersible due to its simple structure,equally distributed stress on its outer surface.Deep-sea spherical pressure hulls are mostly medium-thick shells.Buckling is the main failure mode,which is strongly affected by material plasticity and geometric imperfections.However,classical theory can not accurately predict the load-bearing capacity of spherical pressure hull.At the same time,there are much difference among standards for spherical pressure hull formulated by the various classification societies and there is no unified standard.Therefore,it is still of great significance to study the stability of spherical shells and seek a unified standard.The main contents and conclusions are as follows:(1)Theoretical and numerical analyses are carried out to explore the strength,stiffness and linear buckling of spherical pressure hull.The results show that the numerical solution is well consistent with the analytical solution,and the maximum stress and the maximum deformation appear in the inner surface of spherical pressure hulls.At the same time,with the increase of the thickness-to-radius ratio,the deformation of the spherical shell is gradually reduced,but the inner surface deformation of the spherical shell with a thickness-to-radius of 0.08 is up to 4.375 mm.Therefore,the stiffness evaluation of the hull should be considered by various classification societies.In addition,buckling analysis on thin-shell theory and thickshell theory shows that difference increases with the increase of thickness-to-radius ratio,which indicates that thin-shell theory is not suitable for middle thickness spherical shells.(2)Elastic-plastic buckling analyses of spherical pressure hulls with different thicknessto-radius ratio are carried out.The results show that material plasticity plays a very important role in shell buckling.The greater the thickness of spherical pressure hull,the higher the sensitivity of spherical shell to material plasticity.Meanwhile,the nonlinear buckling analysis based on the first mode imperfection shows that thin shell has a high sensitivity to imperfection.In the case of the same imperfection amplitude,the buckling pressure increases with the increase of the wall thickness of the shell.In the case of the same wall thickness,it decreases with the increase of the imperfection amplitude.In addition,the study on the influence of the constitutive model shows that the calculated difference between the ideal elastoplastic and the real elastoplastic model are very small,and they are all suitable for predicting the buckling pressure of spherical shells.(3)For the spherical shell of titanium alloy and high strength steel,influences of material plasticity and geometric imperfection on the buckling are studied,and the plastic knock-down factor and geometric imperfection knock-down factor are proposed.Based on the thick shell buckling theory and nonlinear numerical analyses,the ultimate load-bearing capacity calculation formula of spherical pressure hull with titanium alloy and high strength steel is derived.The buckling pressure of the formula is in good agreement with the numerical results,and the results of the laboratory model test.In addition,a calculation software was developed and spherical pressure hulls of the active submersible was benchmarked.The results show that the formula is fully applicable to the design of the shell of the current manned submersible.(4)Hydrostatic pressure test and numerical analyses of stainless steel spherical shell are carried out.The results show that the nonlinear numerical analysis based on the real shape and the average wall thickness agrees well with the experimental results.The results of the buckling analysis,which consider the ideal elastoplastic and the real elastoplastic material model are in good agreement with the results of the hydraulic test.It is further suggested that the two material models are applicable to the prediction of the buckling load of the spherical shell.In addition,based on the comparative analysis of real imperfection,local imperfection and modal imperfection,it is concluded that mode imperfection are far less than real imperfection.Therefore,the nonlinear analysis with first buckling mode imperfection is very conservative. |
