Simulation and Measurement of the Self Heating and Thermal Stability of Polymers Under Fatigue Sollicitations

Polymer materials are widely used in structural applications. It is known that the mechanical work of plastic deformation transforms partly into heat, which can cause a noticeable temperature rise in the material. Hence it is very important to control their self heating and the rate of conversion during their sollicitations. This paper concerns the analysis of the deformation-induced self heating in a cylindrical specimen, made of a polymeric material under a fatigue test. A 3 D numerical model, based on Finite Elements Method, is built in order to compute the heat transfers in segmented polyurethane formulations, PP. We also develop experimental measurements, and assess the extent to which agreement with theoretical analyses has been attained. The viscoelastic behaviour of polyurethane formulations leads to a dissipation of the supplied mechanical work during a dynamic mechanical test. Part of this dissipated work is converted into heat and leads to a self-heating of the sample during the test, and to a decrease in the material modulus. Temperature distributions are then calculated in the specimen, and the existence and stability of steady-state distributions of temperature are analyzed. The results show a good agreement between the calculated and the measured temperatures, by adjusting only one parameter, leading to identify the ratio of the thermal to mechanical power conversion. The effects of the loading frequency on the steady-state solutions are discussed, and a quantification of the rate of energy conversion in both the two different material is analysed and discussed: For the PP formulation, part of the dissipated energy is assigned to structural modifications at the beginning of the test. At the end of the sollicitation, all the mechanical work is converted into heat. For the flexible material, the totality of energy is converted to heat source throughout the test. The conversion rate during tests at 10 Hz gives a good discrimination between the mechanical work dissipated in heat form and mechanical reorganizations. For the tests at 27 Hz, the conversion rates are approximately 50%, for various volumes and formulations of samples.Copyright © 2010 by ASME