Investigation On Buckling Characteristics And Restoration Of Hemispherical Shells With Large Imperfections Under External Pressure

Pressure shell is an important part of deep-sea diving equipment,which is used to ensure the safety of scientists and equipment.The pressure cabin usually consists of two hemispheres connected by welding or bolts to form an intact spherical shell.Adequate buckling strength of the hemispherical shell is an important guarantee for the safety of the pressure cabin.During deep-sea exploration,due to collisions or impact,geometric imperfections are generated on the hemispherical shell,which makes the pressure bearing capacity of the pressure cabin drop sharply.The buckling load of a hemispherical shell with imperfection is related to the geometrical shape,amplitude and location of the imperfection.After the imperfection occurs,in order to ensure the safe of the pressure cabin,it is necessary to restore the ultimate bearing capacity of the dented hemispherical shell.Therefore,this thesis conducts an in-depth study on the buckling characteristics of the hemispherical shell with dented imperfection and the restoration of the ultimate bearing capacity of the dented hemispherical shells.The specific contents are as follows:First,the buckling characteristics of hemisphere with different shapes of dented are studied,and the influence of the amplitude of the shape imperfection on the buckling load of the hemispherical head is analyzed.Through stamping and argon arc welding,20 nominally identical stainless steel hemispherical heads were manufactured,and the wall thickness measurement and 3D scanning test of the above-mentioned 20 hemispherical shells were carried out.Conical indenter,spherical indenter and cylindrical indenter are used to produce dented imperfection of different shapes on the hemispherical shell.Test their buckling load through experiments.Comparing with the numerical analysis results,obtain the critical buckling load of the hemispherical shell with dented imperfection and the corresponding post-buckling failure form and the stress distribution of the hemispherical shell during critical buckling,and compare the numerical simulation results with the experimental results to confirm the numerical analysis correctness.In addition,numerical analysis of geometic imperfection with different imperfection amplitudes,including dented,linear buckling mode-shaped imperfections and force-induced dimple imperfections,is carried out to obtain the influence of different imperfection amplitudes on the buckling load of the hemispherical shell.The results obtained from the above analysis have many potential applications in the field of deep-sea equipment.Then,the recovery of the ultimate bearing capacity of the hemispherical shell with shape imperfect is studied.Six hemispherical shells were fabricated,including two hemispherical shells without shape imperfect,two hemispherical shells with dented and two hemispherical shells with welded reinforced spherical heads.Geometrical measurements and hydrostatic pressure tests were carried out,respectively.The experimental and numerical simulation results are compared and analyzed.The relationship between the thickness to diameter ratio of the reinforced spherical head and the buckling load of the reinforced hemispherical shell is numerically simulated and analyzed.The results show that the ultimate bearing capacity of the hemispherical shell with dented can be recovered nearly100% under the condition of a certain thickness to diameter ratio of the reinforced spherical head.The numerical simulation shows that the ultimate bearing capacity of the reinforced hemispherical shell increases with the increase of the thickness to diameter ratio of the spherical head,and the post-buckling mode of the reinforced hemispherical shell shifts from the bottom of the spherical head to the joint between the spherical head and the hemispherical shell,and finally returns to the post-buckling mode with the perfect hemispherical shell.When the thickness to diameter ratio of the spherical head is 0.057,the ultimate bearing capacity of the reinforced hemisphere and the post-buckling mode are the same as that of the perfect hemisphere.Finally,the buckling characteristics of the bolted manned cabin under the condition of dented imperfections are studied in this chapter.The material is martensite nickel steel.The influences of different geometric imperfections and different imperfect amplitudes on the hemispheric manned module were obtained by numerical analysis.Furthermore,the hemispherical manned capsule with geometric imperfection was strengthened,and the relationship between the thickness to diameter ratio of the reinforced spherical head and the hemispherical carrying capacity of the manned capsule with geometric imperfection was studied.Results show that under the condition of small imperfections,indentation flaws and modal of hemisphere manned space limit bearing capacity is almost the same,the influence of the modal imperfect hemisphere manned space limit bearing capacity under the condition of smaller,under the condition of big imperfections,the dented hemisphere manned space is bigger,the influence of using dented imperfection as a hemisphere manned space prediction of the ultimate bearing capacity in a more conservative and safe.The use of spherical head has a perfect strengthening effect on the hemispherical manned module under the condition of dented imperfections.