Approche expérimentale du comportement mécanique des polymères en sollicitation rapide

In the more recent applications of technical polymers, they are frequently submitted to high strain rates. Being viscoelastic by nature, their mechanical properties are rate dependent. It is therefore important to investigate their properties over a wide range of strain rates. The aims of this study are twofold: i) to propose an experimental technique applicable to a large domain of strain rates, included those covering impact tests, and ii) to apply this technique to the study of the rate dependence of the mechanical properties of some technical polymers. In the first section, the deformation mechanisms of polymers, and the way in which they are influenced by strain rate, are reviewed. Some basics of fracture mechanics are also briefly presented. A detailed study of our original approach to the high rates tests then follows. This technique is based on the reduction of dynamic effects which often affect the measurements at high loading rates. Rather than impact tests, our technique is a quasi-static tests at high rates. This results from the use of a damper as coupling element between the servohydraulic piston and the specimen, together with the specific design of load and displacement detectors. The transient acceleration, responsible for the dynamic effects is decreased, but the high loading rates are maintained. The second section is devoted to the investigation of the rate effect on the tensile and rupture properties of polyoxymethylene (POM) and impact modified polymethylmethacrylate (PMMA). It is shown that high molecular weight POM is less sensitive to the increase of the loading rates than for lower molecular weights. The physical structure, which depends on the processing conditions, plays an important role as well. Results show greater embrittlement by increasing the loading rate than by lowering the temperature. Finally, a molecular model of fracture is proposed. PMMA modified by different fractions and morphologies of the rubbery phase are submitted to tensile and fracture tests at room temperature. Several stages in the micromechanisms of deformation are identified by transmission electron microscopy. The progressive disappearance of some of these stages leads to ductile to brittle transitions in some of the materials studied. The velocity at which these transitions occur depends on the fraction and on the morphology of the modifying phase. It is also shown that a spherical morphology of the rubbery phase is not necessarily required to improve the toughness, but that this latter depends on the interactions between the phases.

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