A non-linear viscoelastic model is introduced by employing a new formulation where the relaxation time of rate processes is directly dependent on the induced deformation state applied to the material. The plastic flow of amorphous polymers is considered as a continuous structural change, and the yield phenomenon is modelled through the corresponding viscoelastic constitutive equations. All material constants necessary to predict the yield stress, the rate and temperature dependence are non-adjustable parameters depending on the particular deformation mode. To verify the results of the proposed model, two independent series of experiments have been performed on specific polymeric materials, and have been proved to satisfy adequately the proposed analysis.