| Polytetrafluoroethylene(PTFE)was widely used in various industries because of its insulation,stable chemical structure,small friction coefficient and high temperature resistance,especially as gasket for flange joint.At present,most scholars have studied the properties of PTFE gasket at room temperature,but very few literatures researched on compression fatigue and sealing property of PTFE at high temperature,which is the main work of this paper.The main work and conclusions were as follows:(1)The high temperature gasket compression creep fatigue test device was designed.The effect of holding time on the deformation of PTFE gasket under creep fatigue loading was investigated experimentally.Results showed that the compressive mechanical properties of PTFE gaskets were typical temperature-dependent.With the increase of temperature,the compression modulus of PTFE gasket decreased rapidly.At the same peak stress 6.8MPa,temperature 150?,stress rate 0.2MPa/s,the cumulative deformation of the holding time 1min and 0.5min was slightly larger than that of the static creep,but it was significantly larger than the static creep strain at200?.At room temperature,the accumulated creep-ratcheting strain corresponding to400 cycles for peak holding lmin was nearly 1.06 times that for peak holding 0.5min and 1.3 times that without peak holding.While,at 100?,150? and 200? they were almost 1.1 times and 1.6 times,1.1 times and 1.7 times,1.05 times and 1.9times,respectively.This mean although the accumulated creep-ratcheting strains for peak holding 0.5min and lmin were very close to each other,short peak holding time still had significant effect on the accumulated deformation comparing that without peak holding.(2)The deformation behavior of PTFE gasket under fatigue loading was studied experimentally.Although obvious difference of ratcheting rates took place during the initial 50 cycles,the ratcheting rates under various temperatures and stress rate became almost the same constant and less than 0.01% during the subsequent cycles.This indicated that the ratcheting behavior usually took place during the initial 50 or more cycles,but always turned to shakedown finally in the subsequent cycles undervarious temperatures and stress rates.Moreover,the load rate independent compression ratcheting took place when the stress rate was less than 0.1MPa/s under various temperatures..The evolution of strain range with number of cycles could be divided into two stages.In the primary stage(before the initial 50 cycles),the strain range decreased with number of cycles rapidly,while it almost kept constant during the secondary stage(after 50 cycles).This indicated that the cyclic hardening behavior took place mainly during the initial 50 cycles,while the cyclic stability occur in the subsequent cycles.In the first 20 cycles,the ratcheting rate and strain range decreased rapidly,then the ratcheting rate and strain range were almost unchanged in the stable stage.It was noted that the stability of the ratchet effect and the strain range were much smaller.When the stress rate was equal to or less than 0.1MPa/s,the change of compressive stress-strain curve was very small,which shows the PTFE gasket had no correlation in the condition.(3)The gasket sealing performance testing system under the condition of high temperature fatigue was designed.The relaxation rate and leakage rate of PTFE gasket under static and dynamic conditions were tested.In the static relaxation experiment and the small deformation amplitude relaxation test,the PTFE gasket showed obvious relaxation behavior,The leakage rate decreased with the increase of gasket stress in the experiment of static relaxation and small deformation amplitude relaxation.With the extension of experiment time,the leakage rate became larger.The results showed that the gasket stress was the main factor to determine the leakage rate. |
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