Endochronic viscoplastic material models for filled PTFE Thomas Kletschkowski a, * , Uwe Schomburg a , Albrecht Bertram b a Engineering Mechanics, Universit€ at der Bundeswehr Hamburg, Holstenhofweg 85, D-22043 Hamburg, Germany b Institut f€ ur Mechanik, Otto-von-Guericke-Universit€ at Magdeburg, Universit€ atsplatz 2, D-39106 Magdeburg, Germany Received 18 June 2002 Abstract Todescribethenonlinearmaterialbehaviourofpolytetrafluoroethylene(PTFE),aviscoplasticmaterialmodelofthe overstresstypeisproposedbasedonexperimentaldata.Theapproachstartswitharheologicalmodelwithoutanelastic range,usingarate-independentelastoplasticmodelwithanendochronicflowruleandanonlinearelasticlawinparallel connectionwithanonlinearMaxwellmodel.Thegeneralizationtothreedimensionsispossibleforbothsmallandfinite strainsundertheassumptionthatchangesofthedensityarepurelyelastic.Numericalsimulationsshowthatthemodel can describe the short and long time material behavior of filled PTFE. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Viscoplasticity; Endochronic theory of plasticity; PTFE compound 1. Introduction Due to their tribological characteristics, chem- ical inertness and temperature stability, polytetra- fluoroethylene(PTFE)compoundsareincreasingly used for rotary shaft seals in engineering. In con- trasttoclassicalrotaryshaftseals,whicharemade of elastomeric materials, the engineer has to take into account the inelastic material behaviour of PTFE for the design process. In order to prepare a numerical simulation of thesealingsystem,uniaxialexperimentshavebeen carried out to study the material behaviour of a typical PTFE compound that consists of 90% PTFE, 5% short cylindrical glass fibres and 5% MoS 2 .Theglassfibresareintroducedtoreducethe wearoftheseal.TheMoS 2 isusedtodecreasethe friction between the seal and the shaft in the con- tactzone.Thematerialbehaviourhasbeenstudied in uniaxial relaxation tests, uniaxial tension tests with different strain rates, and retardation tests after unloading (see Pohl, 1999; Kletschkowski et al., 2000). The tension and relaxation tests have been performed with respect to the mechanical straining of the PTFE seal during the mounting and service life on the shaft of the engine. The hoopstrainofthesealduringthemountingresults inapressurebetweenthesealandtheshaft,which is required to prevent leakage. This pressure is caused by the inner hoop tension in the sealing element. In the past, engineers used different constitutive laws for the numerical simulation of different Mechanics of Materials 34 (2002) 795–808 www.elsevier.com/locate/mechmat * Corresponding author. Tel.: +49-40-6541-2282; fax: +49- 40-6541-2822. E-mail address: kletsch@unibw-hamburg.de (T. Kletsch- kowski). 0167-6636/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0167-6636(02)00197-7