Yield Stress Master Curves for Various Polymers below Their Glass Transition Temperatures

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Abstract
Yield stress master curves are presented for poly(methyl methacrylate), polystyrene, polyvinyl chloride, and polyethylene terephthalate. Tensile yield stress was measured at strain rates varying from 0.003 in./in./min to 300 in./in./min and at temperatures varying from approximately 15°C above the glass transition temperature T0 to at least 100°C below T0. The resultant yield, or in the brittle temperature range, failure stress, when plotted as a function of logarithm of strain rate, has been shifted laterally to construct a yield stress master curve similar in concept to the well-known stress relaxation master curve. These master curves cover from 12 to 18 decades of shifted strain rate. The master curve for each material has a characteristic slope which leads, in each case, to a simple equation relating yield stress to strain rate and temperature and is applicable for temperatures from T0 to approximately 100°C below T0. The temperature-dependent shift factors aT of each material were determined for both yield stress and stress relaxation over the temperature range. Comparison of these aT curves shows that in both the ductile and brittle temperature ranges of each material, the shape of the yield stress and stress relaxation aT curves are very similar. However, the absolute magnitude of the change with temperature can be substantially different. Two possible explanations for this difference are offered.

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