Hydrostatic pressure dependence in mechanical behavior of polymers is studied for the constitutive modeling with the yield surface described by the first invariant and the second invariant of stress and the nonassociated flow rule satisfying incompressible hypothesis. An internal variable theory of rate-independent plasticity is presented incorporating isotropic hardening as a function of accumulated plastic strain. After determination of material constants under uniaxial tension and compression, the model shows that the von Mises type effective stress-plastic strain curves under multiaxial load are quite different from those under uniaxial load. The model is compared with the experimental results in uniaxial tension and compression by Spitzig and Richmond and in torsion by Silano et. al under high pressure.
