Predictive modelling of the properties and toughness of polymeric materials -: Part I: Why is polystyrene brittle and polycarbonate tough?

The brittleness of polystyrene (PS) and the toughness but notch sensitivity of polycarbonate (PC) have been studied by the detailed finite element analyses of the stress and strain fields in a notched tensile bar with a minor defect. The defect represented a flaw or imperfection, generated during the test specimen production. The large-strain mechanical responses of both materials were approximated by an accurate elasto-viscoplastic constitutive model with appropriate material parameters. It was assumed that failure occurs instantaneously once the dilative stress exceeds a certain critical craze-initiation stress. The analyses show that the unstable post-yield mechanical response of both materials results in localisation of stresses and strains near the defect at a very low macroscopic strain (0.16%). As a result, a strong dilative stress concentration is formed just below the surface of the defect. For the polystyrene specimen, the critical stress is reached at the defect. For the polycarbonate, however, the effect of the stress concentrating defect was counteracted by a higher craze-initiation stress and stronger strain hardening. The PC craze-initiation resistance, however, did not suffice to overcome the dilative stress concentration raised by the notch tip. (C) 2000 Kluwer Academic Publishers.