Non Linear Viscoelastic Behavior of Polypropylene and Glass Reinforced Polypropylene in Creep

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Abstract
The non-linear visco-elastic behavior of various thermoplastic materials has long ago been established. A vast field, however, remains unexplored, firstly, in finding out workable theories for actual engineering applications with their limits of applicability specified, and secondly, in understanding the physical mechanisms which control the manifestation of non-linear visco elastic behavior in these polymeric materials. An attempt to understand these phenomena has been made by performing 6 hrs isothermal creep tests on unreinforced and glass-fibre reinforced polypropylene samples under uniaxial tension. In the case of the homopolymer, experiments were performed under different stress levels and at various temperatures up to 60 °C. Stress levels were chosen in such a way that maximum strain level always remained below 3%. This was done in order to achieve complete recovery druing the successive recovery cycle. Validity of an equation of the form ε = εo + ε+tn has been tested. It is found that ε o in the above equation is approximately a linear function of stress and is weakly dependent on temperature, whereas ε+ is a non-linear function of stress and is strongly temperature-dependent. The exponent n, however, is a constant independent of both stress and temperature. With incorporation of glass fibers appreciable increase in the modulus and slowing down of time dependent deformation were achieved.
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