Strain rate effects on the thermomechanical behavior of polymers

The thermomechanical behavior of two common polymers, polymethyl methacrylate (PMMA) and polycarbonate (PC), subjected to compressive dynamic loading was investigated in this study. The stress-strain response of each material was examined, over a wide range of strain rates (10^-4 to 10³ s^-1), using an Instron machine and a split Hopkinson pressure bar (SHPB). It was found that the compressive yield stress for both materials increases with strain rate increases. For PMMA, the material changes its compression behavior from ductile to brittle as strain rate increases. In the SHPB experiments, simultaneously to stress and strain measurements from the bars, temperature change was monitored using a high speed infrared HgCdTe detector array. The amount of plastic work converted to heat, β, was measured. For PC, this value was found to be within the range of 0.5 to 0.6. It was found that competition between thermal softening and strain hardening dictates the behavior of this material after yielding, in a process similar to that occurring in metals. For PMMA, a value of β could not be defined, because of the brittle nature of the material. However, some heating was observed during the failure of the PMMA specimen, suggesting that the material at this strain rate is not perfectly brittle.