Strain rate effects on the thermomechanical behavior of polymers

The thermomechanical behavior of two common polymers, polymethyl methacryla te (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 10(3) s(-1)), u sing an Instron machine and a split Hopkinson pressure bar (SHPB). It was f ound that the compressive yield stress for both materials increases with st rain rate increases. For PMMA, the material changes its compression behavio r from ductile to brittle as strain rate increases. In the SHPB experiments , simultaneously to stress and strain measurements from the bars, temperatu re change was monitored using a high speed infrared HgCdTe detector array. The amount of plastic work converted to heat, beta, was measured. For PC, t his 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 be havior of this material after yielding, in a process similar to that occurr ing in metals. For PMMA, a value of beta could not be defined, because of t he brittle nature of the material. However, some heating was observed durin g the failure of the PMMA specimen, suggesting that the material at this st rain rate is not perfectly brittle. (C) 2001 Elsevier Science Ltd. All righ ts reserved.