Time-to-failure of polymers, and the actual failure mode, are influenced by stress,temperature, processing history, and molecular weight. We show that long term ductilefailure under constant load is governed by the same process as short term ductilefailure at constant rate of deformation. Failure proves to originate from the polymer’sintrinsic deformation behaviour, more particularly the true strain softening after yield,which inherently leads to the initiation of localized deformation zones. In a previousstudy we developed a constitutive model that includes physical ageing and is capableof numerically predicting plastic instabilities. Using this model the ductile failureof polycarbonates with different thermal histories, subjected to constant loads, is accuratelypredicted also for different loading geometries. Even the endurance limit,observed for quenched materials, is predicted and it is shown that it originates fromthe structural evolution due to physical ageing that occurs during loading. For lowmolecular weight materials this same process causes a ductile-to-brittle transition. Aquantitative prediction thereof, is, however, outside the scope of this paper and requiresa more detailed study.