Research On Fatigue Characteristic Of Autofrettaged Ultra-high Pressure LDPE Reactor

LDPE ultrahigh tubular reactor is the key component of polyethylene production system.In order to obtain a certain amount of favourable residual stress, reducing the condition ofpeak stress and the average stress value, improve the elastic load bearing capacity and fatiguelife,it needs the autofrettage process before operation. But this kind of residual stress in harshconditions will relax as a result of many factors, especially at present situation most of theLDPE tubular reactor have operated more than20years in our country, the residual stress hasgreat relaxation, seriously affect the safe operation of the equipment. Therefore, in order toensure the production safty, reduce the economic loss, it is necessary to conduct the systemresearch for the rule of residual stress relaxation and the methodlogy how to recover theprevious residual stress level.Based on the present laboratory condition,the author of this thesis takes the residualstress relaxation of the autofrettage LDPE ultrahigh tubular reactor as the target, thegeneration and distribution of autofrettage residual stress relaxation,the relaxation attenuationand redistribution as the main tasks, from two aspects of theory and test to carry out researchwork, the reaction tube was put into use in1986and removed in2008and stored up to now.This paper mainly includes:ⅰ. The mechanism of autofrettaged residual stress are studied, theoretical analysis ofresidual stress of the LDPE ultrahigh pressure tubular reactor was then conducted based onMises yielding criterion and the result are compared with the result from ANSYS simulation,which forms the guidelines for the following experimental investigations.ⅱ. By circumferential residual stress measurement of the reactor tube end surface beforefatigue test, with the residual stress distribution along the thickness direction of the reactiontube and the circumferential residual stress of the outer wall, have been compared thecalculated value and the actual residual stress attenuation of the reaction tube;ⅲ. Used the self-designed ultrahigh pressure fatigue test device, combining the dynamicstrain stress testing method, simulating real working conditions of alternating internalpressure on the reaction tube samples for fatigue test8000cycles, after exclude the error, allthe test data was processed with ORIGIN for the external surface residual stress relaxation ofthe tubular reactor.ⅳ. Re-Autofrettage process experiment were conducted for the tubular reactor, the innerpressure and outer wall stain were dynamic monitored by precision instrument. Thenelastic-plastic boundary radius was established in terms of experimentaliε to pcurve.Base on the elastoplastic deformation relationship, compile the procedures of predicting theresidual response which is able to real-time display the residual strain after plasticdeformation.