Thermal effects accompanying the deformation of natural rubber

This paper deals with the thermal effects associated with deformation processes in unfilled natural rubber. Temperature variations are measured by infrared thermography during cyclic uniaxial mechanical tests at ambient temperature. Results show that natural rubber mainly exhibits entropic behaviour: the material produces (resp. absorbs) heat during loading (resp. unloading). The thermal responses obtained provide complementary information regarding the mechanical analysis of changes in the microstructure, especially strain-induced crystallization. The crystallization of the polymer chains under tension leads to a temperature increase of the order of several degrees Celsius. If crystallization occurs, a hysteresis loop is observed in terms of the strain-stress relationship. Moreover, stress relaxation tests show that the thermal signatures of crystallization and of crystallite melting are different. Indeed, if the strain is maintained fixed during loading, the temperature continues to increase for a few seconds before returning to the ambient temperature. This reveals that crystallization continues during relaxation. On the contrary, if the strain is maintained fixed during unloading, the specimen seems to return instantaneously to the ambient temperature. Throughout this paper, the effect of heat exchanges with the outside of the specimen (non-adiabaticity) on the temperature variations is taken into account for the analysis.