| Hydrostatic strengthening is a kind of strengthening technology which can change mechanical performance of materials by the way of ultrahigh hydrostatic pressure,and this technology has important application value to improve the mechanical properties of materials,but the ultra-high pressure vessel usually requires greater than 1000 MPa or more of bearing capacity and harsh sealing performance for ultra-high pressure.Hence,this paper studies the ultrahigh-pressure compound cylinders for hydrostatic strengthening,that is,the compound cylinder with interlayer pressure.This dissertation systematically investigates the theoretical model of bearing pressure capacity for the compound cylinder,the high oil injection and the sealing for ultra-high pressure liquid medium,researches and designs a prototype of the compound cylinder,besides conduct a study on the influence of ultra-high hydrostatic pressure on the properties of magnesium alloy.The main research works and achievements are as follows:1.Based on elastic-plastic theory of the thick-walled cylinder and the model of plane stress,the relationship between the work pressure of the inner cylinder,interlayer pressure,position and radius ratio was investigated;and the calculation and optimum models for bearing capacity of the four type compound cylinder are built and unified into the uniform model;besides,the elastic limit values for the bearing pressure capacity of the four compound are given.2.Based on the above theoretical model and simulation,resultant stresses and residual stresses distruibution of the outer and the inner cylinder-autofrettaged type compound cylinders were investigated,which revealed the law of stresses around radius and influence of linear hardening on the stresses.The analytical and simulated results showed that under the given ultra-high pressure the compounds meet the requirement for elastic deformation and there is an optimum radius ratio;besides,the high tangent modulus of the materials lead to high resultant and equivalent stress on outer cylinder.3.A oil injection method without oil stress concentration was invented,the ultra-high compound cylinder with this method was simulated and the simulated results showed the method eliminates the oil stress concentration.Besides,the oil injection method was applied to the prototype of the compound cylinder with bearing capacity of 1200 MPa,verified by experiment,which solved the problem of structural failure due to the connection between oil injection hole and high pressure oil of high pressure vessel.4.Metal sealing scheme for the ultra-high pressure was proposed,the stress calculation model was built in metal plastic sealing,and analytical formula and the curve fitting formula were deduced respectively.Meanwhile,stress calculation on sealing surface was carried out using simulation and analytical formula,and the results based on curve fitting method agrees well with the these based analytical method.Sealing experiment showed that the sealing scheme has excellent plastic sealing performance against ultra-high pressure of 1000 MPa or more.5.A prototype with the bearing capacity of 1200 MPa was researched and developed.meanwhile,the resistance strain method was applied for measuring the outside wall of the outer cylinder;besides,the finite element model was built,and the stress and strain distribution on the inner/ outer cylinder were analyzed;test and simulated results indicated that inner and outer cylinder of the prototype were in elastic deformation,which meet the design criterion for elastic failure.6.Hydrostatic pressure experiment on magnesium alloy ZK60 was performed,and the experimental results shown that the ultimate strength is not changed significantly,but the hydrostatic pressure can obviously increase the yield strength of ZK60 magnesium alloy,that is,the yield strength under hydrostatic pressure was enhanced by 89.1%,compared with the yield strength after T6 heat treatment. |
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