Biaxial stress relaxation in glassy polymers: polymethylmethacrylate

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Abstract
Biaxial stress relaxation studies were performed on glassy polymethylmethacrylate in combined torsion-tension strain fields using a specially designed apparatus with exceptionally high stiffness and low cross talk between the torsional and tensile load measuring transducers. It was found that at low strain levels uniaxial tension relaxation is slower than pure torsion relaxation; tensile-component relaxation rates are unaffected by the level of torsional strain; torsional-component relaxation rates decrease as tensile strain is increased; uniaxial tension relaxation rates approach the pure torsion rates at higher strains (∼2%). A phenomenological treatment is presented which shows that relaxation rates can be coupled to the strain fields in which they are observed and yet be consistent with the concepts of linear viscoelasticity and the Boltzmann superposition integral. It is concluded that the mean normal strain, and not the octahedral shear strain, is the principal factor that governs both the effect of strain field on relaxation kinetics and the onset of nonlinear viscoelastic behavior. A mechanistic interpretation based on long-term relaxation processes in glassy polymers is offered to explain the effects of mean normal strain on relaxation kinetics.