The stress heterogeneity effects on the heat build-up of semi-crystalline polymers: Theories and experiments

The heat build-up induced by the conversion of mechanical work into dissipative heat is a common and dangerous phenomenon on polymers subjected to static or dynamic loading. The temperature rise, linked to the strain rate and the local stress state, promotes the microstructural alteration and mechanical properties degradation. Therefore, without consideration of the temperature rise, the components lifetime may be overestimated by the available prediction models. In order to study the coupled heat build-up mechanisms and stress heterogeneity effects on semicrystalline polymers, uniaxial tensile tests were carried out on two materials under different crosshead speeds on notched flat samples whose surface stress state is heterogeneous. During loading, the simultaneous sample surface temperature was recorded using high-accuracy infrared cameras. Image processing was applied to measure the sample geometry evolution, e.g. the notch root curvature radius. Besides, based on Bridgman theory, the analytical stress fields were computed. A good correlation between mechanical responses, geometrical evolution, stress states and heat build-up was obtained regardless of the speed and material. The thermomechanical behavior during the load-softening is particularly remarkable due to the increase of stress triaxiality ratio. The inverted parabola heat build-up profile in thickness reveals a stress heterogeneity in plat sample thickness. The increasing cross-head speed leads to a higher temperature rise due to the heating regime transition from isotherm to adiabatic heating. Further work will investigate the heat build-up and stress heterogeneity effects on pre-cracked specimens of higher stress triaxiality ratio. The development and numerical implementation of a fully-coupled 3D thermomechanical model, accounting for the heat build-up, will be an encouraging challenge