Mines ParisTech

About: Mines ParisTech is a education organization based out in Paris, France. It is known for research contribution in the topics: Finite element method & Microstructure. The organization has 6564 authors who have published 11676 publications receiving 359898 citations. The organization is also known as: École nationale supérieure des mines de Paris & École des mines de Paris.

Polypropylene/natural fibres composites: Analysis of fibre dimensions after compounding and observations of fibre rupture by rheo-optics

Abstract: Fibre size distribution and the way how fibres are breaking during compounding with polypropylene were analysed. Two types of cellulose-based fibres were used: natural flax and man-made (regenerated) cellulose, Tencel. No significant influence of Tencel initial fibre size on its final size after processing was observed. However, the fibre type and the presence of defects have a stronger effect on the rupture. The value of the average final length of flax fibres was found to be similar to the distance between kink bands. In order to understand fibre rupture mechanisms, they were observed using a rheo-optical system which enabled visualization of fibre break-up. All fibres studied broke by fatigue after an accumulation of strain, contrarily to what is known for glass fibres. Flax fibres broke around the kink bands while Tencel® broke after numerous bending.

Rheology of polypropylene in the solid state

Abstract: The tensile behaviour of a commercial grade of isotactic polypropylene was tested in a temperature range between 20 and 150 °C with a video-controlled testing system which is capable of imposing a constant true strain-rate within the neck automatically. The results are displayed in the form of effective stress-strain curves and modelled by a constitutive equation in a multiplicative form. It is thus shown that, for each temperature, the plastic response can be described up to very large strains (ɛ ≃ 2.0) by a set of four parameters. The assumptions introduced in this modelling are critically discussed in order to check the validity of the simplifying hypotheses (strain homogeneity, isochoric deformation, etc.). The constitutive equation thus obtained was utilized in a finite difference code in order to predict the development of stretching instabilities of polypropylene. The simulation gives access to the engineering stress-strain response of the stretched test piece and to the detailed kinetics of the incipient neck. It is found that the severity of the instabilities is less at room temperature than near the melting point because of the decrease of the strain-hardening and of the strain-rate sensitivity with temperature.