Equations are developed to predict the influence of external stresses on the stress–strain relationships of glassy polymers. Different stress fields applied to a polymer result in different domain sizes of those polymer segments which have to move as a whole in order for plastic deformation to occur. These domains are the components of an activation volume tensor, which is a key parameter in describing the nonlinear relaxation time. The shape of the stress–strain curves depends on the time scales in which the solid polymer is relaxed and measured. The yield stresses, yield strains, and stress–strain curves of polystyrene in tension, compression, and shear are calculated and discussed.

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