Research On Buckling Of Cold-stretched Thin-walled Cylindrical Vessels Under External Pressure

As the commom storage and transportation equipment of low-temperature liquified gas,cryogenic pressure vessels are extensively used in many branches of engineering,including petroleum,chemical industry,machinery,energy,medical treatment,aviation,aerospace and etc.Cold stretching technique can significantly reduce the wall thickness of cryogenic pressure vessels and improve the utilization of the materials,thus becoming one of the most effective methods for light-weight manufacturing.In recent years,China's cold stretching technique has reached the international advanced level with the joint efforts of universities,enterprises and research institutions,which brings great economic benefits and plays the leading role in accelerating the green development of the pressure vessel manufacturing industry.Cold-stretched pressure vessels are mainly subjected to internal pressure,but also need to bear a certain level of external pressure in the manufacturing and inspection process.Once the buckling failure of the pressure vessel occurs,it would cause heavy economic loss and severe casualty.Therefore,the design of cold-stretched vessels under external pressure need to be carried out,and meanwhile the effects of cold stretching on its stability should be fully taken into consideration,which differs from the design of the conventional pressure vessels.However,there has been seldom research on the bucking of the cold-stretched pressure vessels under external pressure,and almost all of the existing research results are derived from numerical simulation,whose rationality remains to be verified.It is essential to dive deeper into the buckling of the cold-stretched pressure vessels under external pressure,and proposing an appropriate design method specially for the cold-stretched pressure vessels,in order to promote the lightweight development of cold-stretched pressure vessels.Supported by National Key Research&Development Program of China(Number:2016YFC0801905),studies on buckling of cold-stretched thin-walled cylindrical vessels under external pressure have been carried out,and the main research of this thesis can be described as the follows:(1)The experimental scheme of cold-stretched thin-walled cylindrical vessels under external pressure was designed,covering both long cylindrical vessels and short cylindrical vessels.The out-of-roundness of vessels was measured both before and after the cold-stretching process by a portable three-dimensional laser scanner.The results indicate that the cold-stretching process has a beneficial effect on the roundness of cylindrical vessels,especially the short cylindrical vessels.(2)Gradually increasing hydraulic pressure was applied on the test vessel placed in the quick-actuating pressure vessels,and the buckling load could be obtained according to the variation of water pressure and strain.By contrasting the buckling loads of cold-streched pressure vessels and ordinary pressure vessels,it was concluded that the cold-stretching process can significantly enhance the stability of pressure vessels under external pressure when the initial out-of-roundness of the vessel is larger than 1.5%.(3)The finite element model was established on the basis of the actual shape of the pressure vessel measured by scanning,and the nonlinear buckling analysis was performed using the true stress-strain curve of the material.Comparing the numerical simulation results with the experimental results,we came to the conlusion that the buckling loads taken by two methods were in high agreement.Further numerical simulation shows that when the initial out-of-roundness is less than 2.5%,the buckling load of cold-stretched vessel can reach up to 80%of the theoretical value.The current stability safety factor of cold-stretched thin-walled cylindrical vessels is too conservative,which prevents bringing the potential the material into full play.(4)Based on the results of the experiment and simulation,the external pressure design method for cold-stretched thin-walled cylindrical vessels was put forward.It was recommended to adjust the stability safety factor from 3 to 2.In addition,the experimental data of cold-stretched pressure vessels in Chapter 2 was used to evaluate the safety margin under different safety factor and the out-of-roundness,which proving that this design method was feasible.