Thermal and calorific effects accompanying the stress softening
Résumé
This study investigates thermomechanical effects in filled rubbers under cyclic uniaxial tensile loading at ambient temperature. More especially, it focuses on the calorific response of crystallizing (NR) and non-crystallizing (SBR) rubbers during stress softening, which occurs during the first mechanical cycles. Temperature changes were measured by infrared thermography. Heat sources produced or absorbed by the material due to deformation processes were deduced from these temperature changes by using the heat equation. Heat source variations during each mechanical cycle were analysed and the mechanical dissipation produced in each cycle was deduced. For both materials, the relative contribution to mechanical dissipation of dissipative mechanisms involved in stress softening and viscosity was determined. now, the Mullins effect has only been investigated from a mechanical point of view, while its thermal and calorimetric signatures might provide information of paramount importance. In this study, we propose to measure the temperature changes during the deformation of rubbers by using Infrared Thermography (IRT). It can be noted that in the tests performed in the present study, stress softening in rubbers is accompanied by heat production and heat exchanges with the outside. Moreover, during the first cycles, the temperature evolution is not stabilized. Consequently, the analysis of the temperature does not provide a clear understanding of the thermomechanical phenomena. For this reason, temperature variation cannot easily be used to study the thermal effects accompanying stress softening. This is the reason why we use the framework of the Thermodynamics of Irreversible Processes (TIP) and the heat diffusion equation to measure the total heat source produced or absorbed by the material. This approach is applied in the present study in order to identify the calorimetric signature of the Mullins effect, i.e. the mechanical dissipation due to stress softening.
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