Effects of strain rate, temperature and thermomechanical coupling on the finite strain deformation of glassy polymers

The effects of strain rate and temperature on the inelastic response of a glassy polymer have been studied. Deformation tests in uniaxial compression to strains of −1.0 were conducted on polymethylmethacrylate (PMMA) over a range of temperatures at a strain rate of −0.001/s providing nearly isothermal test conditions and thus documenting the temperature dependence of yield, strain softening, and strain hardening. The specimen surface temperatures were monitored using an infrared detector. Room temperature environment tests were then conducted over a range in strain rates and revealed a significant temperature rise at the strain rates of −0.01/s and −0.1/s. The increase in temperature has a dramatic effect on the stress-strain behavior producing a thermal softening of the material. The moderate rate tests thus underline the importance of understanding the effects of thermo-mechanical coupling during polymer deformations as occurs during impact loading condiyions and deformation processing. The experimental results have been simulated using a fully three-dimensional constitutive model of the large strain inelastic response of glassy polymers in conjunction with a thermo-mechanically coupled finite element analysis. The strain rate and temperature dependence of initial yield is included in the material model as well as temperature dependence of evolving anisotropy and its associated strain hardening. The material model considers that part of the work of inelastic deformation responsible for strain hardening to be stored as an internal back stress and therefore is not dissipative. the remaining dissipative plastic work acts as a heat source in the test simulations where conduction between the specimen and steel platens is modelled as well as convection with the surroundings. Excellent agreement between simulation and experiment is found where the stress-strain curves and temperature-strain curves are well predicted over a range in strain rate and temperature.