Modeling Creep Relaxation of Polytetrafluorethylene Gaskets for Finite Element Analysis

Abstract

A typical flange joint consists of one or more gaskets to arrest the leak between the two ends. The important requirements of such gasket is its durability and sealing capacity during its service under operational loads. Many materials qualify for sealing purpose but, Polytetrafluorethylene (PTFE) gaskets have a very high durability and can be used due to its chemical inertness. However, a PTFE gasket will not maintain a long-term seal capability. Once compressed, PTFE gradually relaxes over a period of time to a no load condition, where there is no residual sealing force. This situation results in leakage. Thus there is a need to study the interactions between assembly configuration, initial torque, etc. to relaxation behavior of the gasket as a means to reduce the dwell period (the time between initial torque and re-torque). With an advancement in computational methods it is possible to predict the joint behavior using Finite Element Method (FEM) approach. FEM based study of such complex assembly will be useful only if PTFE gaskets are represented with proper material model. Present paper illustrates a mathematical model, viz., Burger's model to accurately capture the stress relaxation of viscoelastic behavior of a PTFE gasket. The same model is approximated and evaluated in FEM package to determine the predictability of relaxation behavior of PTFE gasket.