Strain rate, temperature, and their coupled effects on the deformation process of four polycarbonates and a short glass fibre reinforced polycarbonate composite

Polycarbonate is a typical glassy thermoplastic polymer, which has been widely used in engineering applications because of its ductility, transparency and impact resistance. This research presents the thermomechanical responses of four polycarbonate resins: three are homo-polycarbonate with different melt flow rates, one is co-polycarbonate with impact-modified co-monomer; another one is polycarbonate-based short glass fibre composite. Specifically, the interplay between the effects of temperature and strain rate is investigated during the large strain deformation process.
The starting place for this research is to use low-temperature data to simulate the high-rate behaviour of polymer materials, which is inspired by the time-temperature equivalence theory in polymers. In high rate experiments, the strength of polymers increases; however, at the same time, the adiabatic environment-induced self-heating can cause thermal softening during the large strain deformation process. This study considers this trade-off and the combined effects of strain rate and temperature changes when describing the constitutive behaviour of polymer materials. Phenomenological finite element models are used to understand the experimental observations. In order to provide a full set of characterization data, experimental techniques were developed or improved for thermomechanical characterization in tension at low strain rates, combining full-field imaging and in-situ temperature measurements; measurements of temperature rise in polycarbonate during rapid deformation to large strains; application of time-temperature equivalence at large strains; characterization of polymer composites including thermal imaging and phase contrast X-ray imaging.
Experimental results show that the thermomechanical behaviour is consistent with time-temperature equivalence theory at small strains: both an increase in strain rate and a decrease in temperature increase the yield stress. At very low temperatures or high strain rates, this is enhanced by the secondary molecular transition of polycarbonates. In large strain tensile tests, it was shown that using only a global force-displacement curve to validate the constitutive parameters of the numerical model is insufficient because the test describes not only the material constitutive behaviour but also the structural response. For large strain compressive deformation, the temperature rise measurement results allow us to calculate the ratio of plastic work to heat generated, the Taylor-Quinney coefficient, in adiabatic experimental conditions. The value of this coefficient is dependent on both temperature and strain rate, and is affected by both the glass and secondary transitions. For the composite, data show the effect of localised heating on large strain behaviour, in particular causing significant strain localisation and hence weakening of the material. As well as giving a deeper understanding of the material response, the data generated in this study can be used to develop constitutive models over a wide range of temperatures and strain rates.

Keywords: optical measurements
polycarbonate composite
polycarbonates
high rate
temperature and rate dependent
Advisor: Siviour, C