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 o f a glassy polymer have been studied. Deformation tests in uniaxial co mpression to strains of -1.0 were conducted on polymethylmethacrylate (PMMA) over a range of temperatures at a strain rate of -0.001/s provi ding nearly isothermal test conditions and thus documenting the temper ature dependence of yield, strain softening, and strain hardening. The specimen surface temperatures were monitored using an infrared detect or. Room temperature environment tests were then conducted over a rang e in strain rates and revealed a significant temperature rise at the s train rates of -0.01/s and -0.1/s. The increase in temperature has a d ramatic effect on the stress-strain behavior producing a thermal softe ning of the material. The moderate rate tests thus underline the impor tance of understanding the effects of thermo-mechanical coupling durin g polymer deformations as occurs during impact loading conditions and deformation processing. The experimental results have been simulated u sing a fully three-dimensional constitutive model of the large strain inelastic response of glassy polymers in conjunction with a thermo-mec hanically coupled finite element analysis. The strain rate and tempera ture dependence of initial yield is included in the material model as well as temperature dependence of evolving anisotropy and its associat ed strain hardening. The material model considers that part of the wor k of inelastic deformation responsible for strain hardening to be stor ed as an internal back stress and therefore is not dissipative. The re maining dissipative plastic work acts as a heat source in the test sim ulations where conduction between the specimen and steel platens is mo delled as well as convection with the surroundings. Excellent agreemen t between simulation and experiment is found where the stress-strain c urves and temperature-strain curves are well predicted over a range in strain rate and temperature.