Characterization of heat sources due to deformation in unfilled natural rubber
Résumé
This paper deals with the calorimetric analysis of deformation processes in natural rubber. Infrared thermography is first used to measure the temperature evolution of specimens under quasi-static uniaxial loading at ambient temperature. Then the heat sources produced or absorbed by the material due to deformation processes are deduced from the temperature variations by using the heat diffusion equation. Heat sources are a more relevant quantity than temperature variations because the latter are influenced by the heat exchanges with the outside environment of the specimen. Different results are obtained from cyclic tests and relaxation tests. First, the heat source due to thermoelastic (entropic) coupling increases with the stretch ratio at constant stretch rate. Second, no mechanical dissipation (intrinsic dissipation) is detected during the material deformation. Third, strain-induced crystallization leads to significant heat production, whereas the melting of crystallites absorbs the same heat quantity with different kinetics. This difference in kinetics explains the hysteresis loop observed in terms of the strain-stress relationship. Finally, relaxation tests on unloading show that crystallite melting does not systematically occur instantaneously. can be detectable or analyzable with the abovemen-tioned techniques. For this purpose, infrared (IR) thermography seems to be an appropriate technique to detect heat sources from measured temperature variations. Indeed, IR thermography has proved over the last twenty years to be a relevant technique to provide information of importance on the deformation processes in materials such as steels, aluminium alloys and composites. Moreover, various studies previously carried out by Chrysochoos and co-workers have shown that heat sources produced by the material itself were more relevant than tem
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