Deformation And Failure Analysis Of PTFE Lined Pipes

Polytetrafluoroethylene(PTFE)is a commonly used lining material for high temperature corrosive fluid transportation equipment.It has the advantages of excellent corrosion resistance,good mechanical properties,high and low temperature resistance,no water absorption and low friction coefficient.However,during the working process,the lining not only withstands the constant stress generated by the constant temperature fluid also bears the alternating stress caused by the alternating flow of cold and hot fluids,so there is a gradual accumulation of plastic strain,that is,the ratcheting effect,and the relaxation process that develops gradually over time,that is,creep and cyclic softening.It may cause the lining pipe to collapse,bulge,crack and other damage behaviors.Therefore,it is very important to study the performance characteristics of PTFE at different temperatures and the cyclic plastic constitutive model of PTFE to ensure the safety of the pipeline.In this paper,uniaxial ratcheting,creep,ratcheting creep interaction and uniaxial torsion tests were carried out on cold-pressed PTFE round bar samples at five temperatures,the results show that the temperature has a great influence on the deformation behavior of PTFE.(1)The ratcheting strain and strain rate increase with the increase of the average stress and stress amplitude at different temperatures.The effects of strain and strain rate are both greater than the amplitude stress.This phenomenon is more obvious when the temperature exceeds 100 ℃.(2)In the experiments of five temperatures and five stress levels,PTFE enters the stable creep stage around 50,000 s.The higher the temperature,the more sensitive the creep deformation to the change of stress.(3)The Arrhenius equation at different temperatures is obtained through the PTFE creep test,and the creep rate is integrated to extract the creep deformation in the ratcheting strain while considering the PTFE creep stress threshold value at different temperatures.The creep proportion increases gradually with the increase of average stress,and the response to temperature is similar to ratcheting strain.(4)Through the circumferential cyclic softening experiment of PTFE at five temperatures,it is verified that PTFE is a cyclic softening material.The previous low torsion cycle has an inhibitory effect on the softening effect of the subsequent high torsion cycle;the previous high torsion cycle will promote the softening of the low torsion cycle.Both effects become more pronounced with increasing temperature.(5)The thermal shock test device for PTFE lined pipe was built and the thermal shock test was carried out.It was found that the flange position of the lined pipe was most prone to deformation.(6)Using three constitutive models to simulate the uniaxial ratchet strain of PTFE,it is found that the OW-Ⅱ model can better predict the ratchet strain of PTFE.(7)The finite element model of the PTFE lined pipe was established,and the OW-Ⅱ model was used to analyze and calculate the ratcheting strain of the lined pipe under cyclic thermal shock.It was found that the position of the ratcheting strain was consistent with the test results.The thermal shock cycle ratcheting boundary temperature value was determined,and the development of the ratchet effect was inhibited by the reduction of the effective thermal expansion difference during cold shock.