Role of plasticity in heat generation during high rate deformation and fracture of polycarbonate

The role of plastic deformation in generating heat during dynamic compression and fracture of polycarbonate was examined in a combined experimental and numerical investigation. Split Hopkinson pressure bar (SHPB) experiments and opening mode dynamic fracture experiments were performed to measure the thermomechanical response of polycarbonate at various loading conditions. A companion set of dynamic, thermomechanically coupled explicit finite element simulations of the tested geometries was performed to isolate the temperature increase of the polymer due to plasticity effects only, and these were then compared to the experiments to identify the role of plasticity in the overall heating of the polymer. Temperature measurements during the experiments were made with a remote sensing technique that utilizes the detection of infrared radiation. Polycarbonate was observed to have a rate dependent yield and plastic deformation response under uniaxial compression. For the strain rates tested in the SHPB experiments (400 to 3000 s^-1), the fraction of plastic work rate converted to heat was found to be approximately 0.5. The dynamic fracture experiments indicated that the maximum temperature increase in the region surrounding the propagating crack was over 100 K, and the crack tip velocity was limited to approximately 60% of the Rayleigh wave speed. The numerical simulations used the plastic behavior of polycarbonate observed during the compression SHPB experiments as inputs for an incremental plasticity model that uses a rate dependent Mises yield surface and isotropic hardening. Plastic deformation was the only source of material heating in the simulations. Good agreement between the SHPB experiments and the numerical simulations was seen for both stress and temperature during the compression event. Crack growth during the dynamic fracture simulations was imposed by a manual node release technique that duplicated the crack growth speed observed in the experiments. The dynamic fracture simulations which only included the thermoplastic effect significantly under-predicted the temperature increase in the region surrounding the propagating crack. The results indicate that plastic deformation is not the dominating source of heat generation during the dynamic fracture of polycarbonate, it only accounts for about 8% of the measured heating. Additional effects, such as thermofracture coupling, must be considered in the simulations if a more accurate comparison is to be made with experiments.