Constitutive equations are derived for the nonlinear viscoelastic behavior of amorphous glassy polymers. The model is based on the theory of cooperative relaxation in a version of the trapping concept. Stress-strain relations are applied to fit experimental data for polycarbonate in the sub-yield and post-yield regions. Fair agreement is demonstrated between observations and results of numerical simulation. It is revealed that yielding causes substantial changes in the energy landscape of amorphous polymers.