Instabilités de déformation pendant l'étirage des polymères solides

The forming processes of solid polymers (thermoforming, stamping, film and fiber stretching) are affected by plastic instability phenomena. These instabilities correspond to different strain localization mechanisms : a) at the microscopic scale (nm scale), cascades of conformational changes in macromolecules ; b) at the mesoscopic scale (μm scale), shear bands, intra-spherulitic inhomogeneities and crazes ; c) at the macroscopic scale (mm scale), diffuse necking and kinking. These phenomena are interpreted in terms of the intrinsic constitutive equation of the material which expresses the influence of strain and strain rate on plastic flow stress. This equation reflects the structural modifications undergone by the material during the deformation. Furthermore, it gives the necessary information for the prediction of strain instabilities by means of computer modelling such as the Finite Difference Method. Two model cases are investigated with a particular attention : the uniaxial tension of High Density Polyethylene cylinders on one hand, and the plane strain stretching of Polyethylene Terephtalate thin films on the other hand.Les procédés de mise en forme des polymères à l'état solide (thermoformage, emboutissage, étirage de films et de fibres) sont affectés par des phénomènes d'instabilités plastiques. Ces instabilités correspondent à différents mécanismes de localisation de la déformation : a) à l'échelle microscopique (quelques nm), cascades de changements conformationnels des macromolécules ; b) à l'échelle mésoscopique (quelques μm), bandes de cisaillement, hétérogénéités intra-sphérolitiques et microcraquelures ; c) à l'échelle macroscopique (quelques mm), strictions diffuses et pliages. Ces phénomènes sont interprétés sur la base de la loi de comportement intrinsèque du matériau qui exprime l'influence de la déformation et de la vitesse de déformation sur la contrainte d'écoulement plastique. Cette loi reflète directement les modifications structurales subies par le matériau dans les conditions de l'essai. Par ailleurs, elle donne les informations nécessaires à la modélisation des instabilités à l'aide de techniques de calcul telles que la Méthode des Différences Finies. Deux cas modèles sont étudiés avec une attention particulière : la traction uniaxiale de cylindres de Polyéthylène à Haute Densité d'une part, et l'étirage en déformations planes de films minces de Polytérephtalate d'Ethylène Glycol d'autre part

Elastic-plastic indentation creep of glassy poly(methyl methacrylate) and polystyrene: characterization using uniaxial compression and indentation tests

International audienc

Physical analysis of the state- and rate-dependent friction law: Static friction

International audienc

Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests

Samples of high-density polyethylene have been subjected to tensile tests and creep experiments by means of a video-controlled testing system (VidéoTraction ©). The evolution of specific volume in this semi-crystalline polymer is determined versus true strain. In the elastic stage, we measure a hydrostatic expansion and then, in the plastic stage, we observe a competition between a compaction effect and a dilatation phenomenon. Although compaction is probably overestimated in the present testing technique, it represents a pertinent mechanism that is ascribed to the orientation of the amorphous chains during stretching. This phenomenon is characterized by X-ray diffraction measurements that show a reduction of average distance between amorphous chains. Dilatation process is explained by the diminution of crystallinity and by the formation, growth and coalescence of crazes inside and between spherulites. Electron microscopy reveals these defects. The competition between compaction and dilatation, controlled by the mobility of the amorphous phase, depends on temperature and time

The effect of strain rate on the die-drawing of polyoxymethylene at elevated temperatures

In this paper we describe the intrinsic deformation behaviour of polyoxymethylene in uniaxial tension, shear and plane strain compression at 160$^{\circ}$C for strain rates from 10$^{ - 3}$ to 1 s$^{ - 1}$. In tension and shear, the deformation was determined by a novel video-controlled testing system. There was a significant evolution of volumetric strain in tension, indicating that damage mechanisms play a key role in the plastic deformation behaviour. Post-mortem analysis on the shear and compressed samples showed no evidence of dilatation. In uniaxial tension, significant strain hardening was observed at high strains while only a small increase of stress was observed in plane strain compression. In shear, the plastic deformation occurred at constant stress. For the specific case of die-drawing of polyoxymethylene, it has been shown with the aid of small angle X-ray scattering that voids are nucleated at the die-entrance by inhomogeneous shear deformation under negative (compressive) hydrostatic stress. The growth of the voids is then aided by the tensile stresses that are predominant in this process at die-exit. Extensive voiding occurred in the oriented sample produced at high strain rates, as revealed by the scanning electron microscopy, which affects the fracture strength and hence the viable production speeds of the die-drawing process

Some Observations of the lamellar Morphology in Isotactic Polypropylene Spherulites by SFM

The lamellar morphology of intruded bulk samples of isotactic polypropylene has been investigated by scanning force microscopy. It is shown that SFM operated in the tapping mode is a powerful tool to characterize the texture of $\alpha$ and $\beta$ spherulites at the lamellar level. Structure, orientation and thickness of the lamellae have been determined in both cases.La microscopie de force atomique a été utilisée pour étudier la structure lamellaire d'échantillons massifs de polypropylène isotactique obtenus par intrusion. La microscopie de force atomique utilisée en tapping mode se révèle être un outil performant pour caractériser la texture des sphérolites $\alpha$ et $\beta$ à l'échelle lamellaire. La structure, l'orientation et l'épaisseur des lamelles ont été déterminées dans les deux cas