AbstractIn the bulk, the nonlinear viscoelastic response of glassy polymers is due to the irreversible work done on the body by the surroundings. The source of the irreversibility is plastic flow of material near distributed shear bands or microcracks in the polymer. Shear bands and microcracks also form new traction free boundaries in the body. The presence of these new boundaries diminish the load bearing capacity of the polymer. These changes in polymer lattice structure are a mechanism that promotes the release of stored strain energy. If the release of stored strain energy is stress controlled, then at sufficiently high levels of stress to cause a permanent structural arrangement of the polymer chains, polycarbonate and LaRC‐TPI behave as nonlinear viscoelastic materials. If the current stress is less than the maximum stress the polymer has experienced, then the current energy release rate for the propagation of shear bands, crazes, etc., is less than the critical energy release rate. In this instance, damage production is a constant and the material can be modeled using linear viscoelasticity. It will be shown that the stress‐induced nonlinear shift factors are a measure of the rate of damage production in glassy polycarbon