Research On Some Key Issues In The Design Of Ultra-High Pressure Vessels Based On Failure Modes

As the development of engineering technology and the national economy,there are greatly increasing demands for pressure vessels with pressure larger than 100MPa,known as Ultra High Pressure Vessels.There are thousands of in-service UHPVs in the engineering fields such as Low Density Polyethylene Ethylene(LDPE),Isostatic Pressing Technology,synthetic quartz,high pressure sterilization and jet cutting.Due to the harsh working conditions,serious accidents of UHPVs have been occurred frequently with economic loss and casualties.A regulation on UHPVs in China had been proposed in the early 1990s to give some general mandatory reqreuipemnts on design,manufacturing,testing and other aspects.However,a specific design standard is still not prepared.The design of UHPVs in China are according to some technical manuals or standards of other countries.In view of the above,some studies have been carried out with the following issues in the project supported by the Special funds for Quality Supervision Research in the Public Interest("Research on Key Technologies for the design Standard of Ultra-high Pressure Vessels",Grant No.201210242):(1)Systematic discussion was performed on the accuracy and design margin of the bursting pressure formulas of UHPVs.Nearly hundred of numerical calculations based on FEA have been done to compare on those formulas;Hundreds of test data selected from papers were utilized to obtain the most accurate prediction formula.Design margin was discussed based on the failure probability.Results show:the Faupel formula which has been widely accepted in the engineering field could overestimate the bursting pressure of vessels made of high strength steels.The Flow Stress formula is proposed to be used.Meanwhile,a design margin of 2.4 instead of 3.0,is suggested for UHPVs with closed-end,such as crystal kettles.(2)A constitutive model and a fracture strain model were established to be used for the plastic analysis design of UHPVs.Tensile tests were performed on domestic high strength gun steels and results show that the thoses steels perform a multistages strain hardening behavior.A new constitutive model,the parameter of which can be obtained directly through a linear fitting,is proposed to describe such behavior precisely.Tensile specimens with differect notches were designed and tested to obtain different stress states.The results show that the ductility of domestic high strength gun steels are very sensitive to stress states and the requirement of tensile elongation and tensile area reduction is not enough.A ductile damage model of domestic high strength guns steels was obtained and a strain limit criteria based on the model is proposed to protect against local failure of UHPVs considering triaxiality effect.(3)A fatigue design procedure of UHPVs considering stress state and mean stress effect was established with a new Chinese UHPVs design fatigue curve.Findley multi-fatigue damage model based on critical plan method is utilized to consider stress states effect.The best fitting fatigue curve was obtained through push-pull fatigue tests.Considering several of influence factos,a design margin of 2/15 was utilized to create the design fatigue curve.Finally,The Safety and applicability of the method with the curve were verified by hundreds of fatigue tests data of UHPVs selected from papers.Some research results mentioned above have been adopted to the TSG 21-2016"Supervision Regulation on Safety Technology for Stationary Pressure Vessel" and the national standard"Ultra High Pressure Vessels".