Nonlinear viscoelasticity and fatigue of glassy polymers

Constitutive equations were derived for the uniaxial loading of glassy polymers in the sub-yield region. The model correctly predicts experimental data in tensile relaxation tests for virgin polypropylene specimens and for samples subjected to cyclic preloading. The preloading induces dramatic changes in the initial Young modulus and the rate of rearrangement R. Fatigue results in work hardening at relatively small strains and leads to a decrease in the rigidity of relaxing regions at larger strains.