Determination of the elongational properties of polymers using a mixed numerical-experimental method

Abstract : The biaxial properties of materials such as rubbers or polymers are often difficult to identify, and generally a simple isotropic behaviour derived from classical 1D tensile test is considered. However, biaxial properties are useful for the simulation of plastic-processing operations such as blow moulding [1] or thermoforming. As previously reported, the rheological behaviour and the mechanical properties of rubbers and polymers can be obtained by using a bubble inflation rheometer [2-4], or multi-axial tensile test [5]. In this work, experimental data provided by optical measurements on tensile tests and bubble inflation tests are coupled with Finite Element Method simulations for identifying the rheological behaviour. This work is actually based on natural rubber and will be extend to thermoplastic (PP and PET) materials. A bubble inflation rheometer has been developed in the laboratory. It allows to blow under r r controlled pressure rubber or thermoplastic membranes [2]: a circular membrane, clamped at the rim, is inflated by applying air pressure to its bottom face (see Fig. 1, left). In the case of thermoplastic, a heating step is necessary before applying the pressure. The heating can be performed by air convection, by conduction (heating cartridge at the rim) and by IR radiation. FIGURE 1. Bubble inflation rheometer (detail, left) and shape contour extraction (right). Two experimental optical techniques based on non-contact measurement by CCD cameras have been developed for in situ measurements: (i) images acquisition of the 2D projection of the bubble is done during the inflation process, giving shape contours versus inflated pressure (see Fig. 1, right); (ii) Digital Image Stereo-Correlation (DISC) [6] is applied using a calibrated stereo rig in order to obtain the three-dimensional description of the strain fields on the surface of the bubble. f To perform DISC directly on the rubber bubble, several difficulties are to be solved (large level of deformation, semi-transparent aspect of the materials, t lighting, etc.). In addition, tensile tests have been performed using DISC due to the high level of strain. Tensile test performed on standard specimen give, on the one hand, the stress/strain curve. On the
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Laurent Robert, Samia Hmida Maamar, Vincent Velay, Fabrice Schmidt. Determination of the elongational properties of polymers using a mixed numerical-experimental method. ICEM13 - 13th International conference on experimental mechanics, Jul 2007, Alexanroupolis, Greece. 8 p., ⟨10.1007/978-1-4020-6239-1_331⟩. ⟨hal-01818874⟩

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