Papers by Author: Noelle Billon

Abstract: Heat dissipation during mechanical testing can disturb experimental characterisation of polymers. In this work it is demonstrated that these effects are not limited to extreme loading conditions such as impacts. A visco-hyperelastic, visco-plastic constitutive model is proposed that accounts for thermo mechanical coupling in a fully 3D thermodynamics approach. Strain-rate and temperature dependencies are coupled using a concept close to the well known time-temperature superposition principle. Constitutive and coupling parameters are identified at the same time using an inverse analysis protocol. An experimental data base is generated for mechanical measurements at different temperatures and strain rates but also for temperatures during tests measured using IR technology. Such a protocol allows investigation on the strain-rate sensitivity in a much more relevant manner than classical one and the value of the so-called Taylor-Quinney coupling parameter is discussed.

Abstract: The present work can be regarded as a first step toward an integrated modelling of mould filling during injection moulding process of polymer matrix composites and the resulting material behaviour under service loading conditions. More precisely, the emphasis of the present research is laid on the development of a mechanical model which takes into account the processing-induced microstructure and is capable to predict the mechanical response of the material. In the Part I, a set of experiments which captures the mechanical behaviour of an injection moulded short fibre reinforced under different strain histories is described. Three mechanical testing are conducted: Dynamic Mechanical Analysis (DMA), uniaxial tension and simple shear. Tests show that the material exhibits complex responses mainly due to non-linearity, anisotropy, time/rate-dependence, hysteresis and permanent strain. Moreover, the relaxed state of the material is characterized by the existence of a so-called anisotropic equilibrium hysteresis independently of the prescribed strain rate.

Abstract: Thermo mechanical behaviour of unfilled and filled polypropylenes are studied in tension from 10-4 to 102 s-1. Complementary low velocity compression and shear tests are also performed. A high-speed video camera (up to 2500 frames/s) combined with image analysis, image correlation and an infra red pyrometer allow measuring 3D-strain fields and temperature during tests. Thus, data can be processed without restrictive assumptions. Beside usual (for polymers) temperature and strain rate sensitivities it is found that plastic deformation in these materials does not obey incompressibility assumption. Voiding damage is evidenced in the polymer matrix by SEM observations that result in volume change and significant decrease in Young modulus for both materials. Moreover, an increase in the temperature of more than 10 °C is observed and is likely to modify the behaviour of each material at high strain rates. Shear and compression measurements demonstrate that yield criteria and constitutive equation depend on loading. It is concluded that apparent yield stress in semi-crystalline polypropylene can be a result of a combination of “non strain rate sensitive” “non-cohesive mechanisms” and “strain rate sensitive” “cohesive mechanisms”. Experimental characterisation on polymers should then be revisited as most of the usual assumptions are invalid and non monotonic tests should be generalized.