Séminaire 13.01.2014 à 14h

Résumé

We have studied the evolution of mechanical properties during fatigue of neat and glass fiber reinforced polyamides until the final breaking of the materials, and characterized the microscopic mechanisms responsible for the progressive damaging of the samples. We have studied the life-time as function of the applied stress, at different temperatures, and the evolution of the modulus as a function of time during the experiments. Our results show that the life-times fall on a master curve when plotted as functions of the applied stress normalized by the elastic modulus. Our study show that fatigue damage evolves in 3 phases. During the first step the material’s temperature increases and the damage is negligible. The second step is characterized by a stable temperature and a slow decrease of the apparent stiffness (called dynamic modulus in fatigue). The sharp drop of stiffness during the third step is followed by the breaking of the sample. We show that the lifetime is controlled by the duration of the second phase during which the material’s modulus decreases linearly with the logarithm of the time. Characterizations by Ultra Small X-ray scattering, and electronic microscopy enabled us to study the fatigue damage from the nanometric scale to the micronic scale of the semi-crystalline structure of the reinforced polyamide and of the neat polyamide as a reference. We show that the slow decrease of mechanical properties during the second step is due to the nucleation of cavities in the polymeric matrix at the nanometric scale, and to the increase of the number and of the size of these cavities up to the micron scale. The size distribution, density and form factor of the cavities have been quantified. The logarithmic decay of the elastic modulus can be explained by the broad distribution of stress amplification at the microscopic level, or, equivalently, by a broad distribution of the energy barriers for cavitation in the material.