Viscoelastic properties of semi-crystalline thermoplastic polymers: dynamic analysis and creep Dan-Andrei Şerban 1, a , Henry Hanson 2, b , Liviu Marşavina 1, c and Vadim V. Silberschmidt 2, d 1 Department of Strength of Materials, ‘‘Politehnica’’ University of Timişoara, 1 M. Viteazul Blvd., Timişoara, 300222, Romania 2 Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Leicester-shire LE11 3TU, UK a serban.andrei85@gmail.com, b h.n.hanson@lboro.ac.uk, c lmarsavina@yahoo.com, d v.silberschmidt@lboro.ac.uk Keywords: Semi-crystalline thermoplastic polymer; Viscoelastic properties; DMA test; Creep Abstract. When subjected to external loading, polymeric materials behave in a manner intermediate between elastic solids and viscous fluids. Their mechanical properties depend on a material’s viscous flow, which, in turn, is influenced by (i) temperature, with its different magnitudes determining a ductile or brittle behaviour and (ii) time, through the effect of a deformation rate and long-term relaxation. Short-term viscoelastic properties (loss and storage moduli) of a studied semi-crystalline thermoplastic polymer were obtained using Dynamic Mechanical Analysis, while its long-term viscoelastic properties (compliances) were determined using creep tests. 1. Introduction In many practical applications materials of various components and structures are considered to be sufficiently characterized by simple laws (elasticity, plasticity, viscosity, etc.) in order to reduce computational complexity of their simulations. In reality, no solid exhibits perfect elasticity and no fluid exhibits pure Newtonian viscosity: deformation of every type of material has an elastic part and a flow part [1]. This behaviour is more evident in the case of polymers (due to the fact that their microstructure is composed on long molecular chains [1]), where, apart from the elastic/hyperelastic stress-strain response [2], a viscous-flow behaviour can be observed in the form of stress relaxation/creep as well as temperature- and strain-rate-dependent properties [1-4]. In many cases, it is impossible to separate permanent damage induced to the microstructure from the reversible damage. The molecular basis for stress relaxation and creep accounts for several types of mechanisms that are responsible for softening of the material: chain scission, viscous flow, bond interchange, molecular relaxation, etc [1]. The goal of this study is to collect information about the viscoelastic behaviour of the studied semi-crystalline thermoplastic polymer in order to develop a virtual-material model for finite element analysis. Previous tests performed on the studied material determined strain-rate- and temperature-dependency of its mechanical properties [2] but no quantitative description of the viscoelastic properties was obtained. A practical way at looking at viscoelasticity is to consider the material as a mathematical model that features both elasticity and viscosity. Several models were developed throughout the years, such as the Maxwell fluid (a spring and a dashpot in series), the Kelvin solid (a spring and a dashpot in parallel). The mathematical models used in specialized software packages use combinations of these simple models, such as the generalized Maxwell fluid, the generalized Voigt-Kelvin solid or the Burgers model [1, 3-5], which can provide more accurate descriptions in terms of material behaviour. The softening behaviour of viscoelastic materials is modelled with the help of Prony series. (0) 8 pages © (0) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 79.114.75.14-30/01/12,18:26:17)