Investigation On The Influence Of Nonlinear Buckling And Imperfections On The Spherical Cap Of Deep-sea Pressure Hull

Pressure hull is an essential component of deep-sea submersibles,all pressure hull configurations are closed shells of revolution.Actually,at least one large hole enclosed with heavy flange may be opened as an access channel of carried crews or inner equipment.Such hole must be closed by a domed head to ensure the safety of crews or equipment.Externally pressurised domed caps with positive Gaussian curvature have long attracted attention owing to their uniform stress and high load-carrying capacity,which are also prone to nonlinear buckling.The buckling performance is strongly influenced by the geometrical shape,shape imperfections,and wall thickness of head.Therefore,in order to provide theoretical guidance for the design of closed head,this paper is devoted to the buckling of spherical laboratory-scale caps under various heightto-diameter ratios,shape imperfections,and wall-thickness corrosions.Firstly,the buckling of stainless-steel spherical caps under uniform external pressure was focused on.Six nominally identical laboratory-scale caps were fabricated,measured precisely,and tested to collapse.The buckling performances of such caps were studied experimentally and numerically,and the results show good agreement with each other.Furthermore,the buckling of a series of mass-equivalent caps of various heights and diameters were numerically analyzed to identify the cap with the best load-carrying capacity.The results suggest that when a spherical cap with heightto-base diameter ratio is about 0.27~0.31,especially when the ratio is about 0.29 supports the highest buckling load.Such a cap can be applied as an end-closure for cylindrical pressure hulls or as a manhole cover for manned cabins in deep-sea vehicles.Then,the buckling of spherical caps with four different geometric imperfections was examined,including local inward dimple,increased-radius,force-induced dimple,and linear buckling mode.The influence of imperfection amplitude,meridional position,and meridional extent on cap buckling was numerically explored and partially validated using experiments.The numerical and experimental data were well consistent.Results indicated that,in the case of small-sized imperfections,the linear buckling modeshaped imperfection presented a relatively conservative cap buckling prediction compared with those of the other three imperfections.This finding contradicts previous findings that the force-induced dimple imperfection is the most unstable imperfection.Finally,the buckling of spherical caps fabricated under different conditions of wallthickness reduction which can be caused by corrosion was investigated.The spherical caps were fabricated using photosensitive resin and they were subjected to uniform external pressure.In total,42 spherical caps—6 with full thickness reduction and 36 with partial thickness reduction—were fabricated through rapid prototyping;after their external surfaces and wall thickness were measured,all caps were tested to collapse to experimentally and numerically evaluate their buckling properties,namely buckling pressures and collapse modes.Moreover,the effects of site,magnitude,and range of the thickness reduction on the buckling properties were evaluated.Additionally,the results of experimental work were compared with results given by known analytical formulas.The results show that,Wagner’s and Evkin’s formulae can be used to assess spherical caps with partial thickness reduction caused by corrosion or damage.