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.
Many mechanical or structural components are subjected to multi-axial, irregular cyclic loading during service. The direction and amplitude of principal stress and strain vary over a period of time results in non-proportional cyclic loading on the component. At geometrical discontinuities, even a monotonic load will result in multi-axial state of stress. In general, the life of the components subjected to multi-axial stress loadings, are evaluated using classical yield theories. The Tresca and von Mises criterions along with Basquin-Coffin and Manson life curve are widely used in commercially available Finite Element Analysis (FEA) tools. These classical methods are conservative and may not yield good experimental correlation at all the loading conditions and this augments the need for robust life estimation methodology.There are many commercially available FEA tools to estimate the multi-axial fatigue life viz. nCODE® which uses Wang-Brown method [1]. However, it has been found that for shear dominated fatigue material Fatemi-Socie criteria is more suitable. So an attempt is made to develop a an algorithm to implement Fatemi-Socie criteria in a commercially available generic FEA software in a cost effective way. This paper discusses how to estimate the life of a sample specimen subjected to multi-axial and non-proportional loading conditions. The classical yield criteria based on von-Mises stress with Basquin-Coffin and Manson equation and critical plane method viz, Fatemi-Socie criteria are implemented in to commercial FEA tool, ANSYS. This paper also attempt to see how these theories compare with experimental data. Results of this study would help in leveraging the established process of implementing custom based life estimation method in ANSYS for the estimation of the life of the mechanical components.
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