Investigating a Creep Behavior of Polypropylene

Abstract

A viscoelasticity consisting viscos and elastic part, is a model defining mechanical property of both elastic and viscous behaviour of a material. A polymer or a material can have both a linear and non-linear viscoelastic behaviour. Linear viscoelasticity of a thermoplastic polymer is a theory explaining a relationship between linear stress and corresponding strain rate at any particular time period. There are several mathematical models of linear viscoelasticity based on the Boltzmann superposition principle, some of a significant principle and theories, that portrays creep deformation are Maxwell, Kelvin-Voigt, Standard linear solid, Burgers model. Creep is a gradual deformation of a material. Factors like time, temperature, material properties, load etc, affects the creep deformation. But only the time dependent deformation or a creep of a polypropylene and stress relaxation under room temperature is analysed using Standard Linear Solid (SLS) model of viscoelasticity. Using a Testometric machine, step tensile test of a polypropylene as a thermoplastic material was conducted. A sample piece (dog-bone) of a polypropylene was used as a specimen, different parameters were set for each test and a test lasted for approximately 2 hours each. Basically, test for creep and stress relaxation were conducted using a step tensile method. As a creep test, initial stress was applied, and the same stress was held over time to get a creep curve and the rate of strain over time. For the stress relaxation, initial strain was applied, and the same strain was kept throughout a test to get a relaxation curve and a rate of change of stress. Under SLS model a stiffness or young’s modules of a linear spring and a coefficient of viscosity of a linear viscous dashpot were calculated from the result. A time dependent stress and strain equation were developed based the constitutive equation and resulted parameters from the experiment. The result compromised that creep or deformation varies linearly over time for PP as a thermoplastic material and also verified that a constitutive model of linear viscoelasticity based on Boltzmann Superposition is very much applicable to analyse a time dependent creep or deformation under a room temperature. As expected, the experimental work is very much adequate to the past experiments and theory of viscoelasticity.