Criteria to predict the embrittlement of polycarbonate

A direct correlation is found between the time evolution of the yield stress in unnotched tensile bars and that of the impact energy measured using notched tensile bars. In both cases a master curve can be constructed with an Arrhenius type of shift function, using the same activation energy. Combining these experimental findings with numerical simulations lead to a maximum hydrostatic stress as a criterion to predict the onset of failure, σ c h . Where aging kinetics is not dependent on the polymer’s molecular weight, embrittlement is, and for higher molecular weights a higher σ c h is found. Moreover, for high polymers we observe a more stable craze extension and crack propagation after cavitation, causing the computed impact energies to underestimate the experimental ones for all but the lowest molecular weight polymers. Because the maximum in hydrostatic stress under the notch, σh, increases with the polymer’s age, defined by the value of the state parameter Sa, and given the proportionality between Sa and the yield stress, σy, an alternative failure criterion is proposed which is a molecular weight dependent critical value of the evolving yield stress, σy,c. It predicts a product’s increased sensitivity to damage under the influence of progressive aging as enhanced by temperature.