Fatigue life predictions for glassy polymers : a constitutive approach

Long-term failure under constant or cyclic load is governed by the same process as short-term failure at constant rate of deformation. Failure proves to originate from the polymer’s intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study we developed, and validated, a 3D constitutive model that is capable to predict the occurrence of these plastic instabilities, yielding quantitative predictions of the lifetime of polycarbonate under constant load.1 Here we demonstrate that the same approach is also applicable to predict the life span of polycarbonate under cyclic loading conditions, over a large range of molecular weights and thermal histories, with a single parameter set only. The model incorporates the influence of physical aging, accelerated by the applied cyclic stress. For low cycle fatigue, at large stress amplitudes, where failure is thermally dominated, it is shown that the current constitutive model has to be extended to a multirelaxation time expression to properly describe the (evolution of the) energy dissipation.