Stress and Strain Dependence of Viscoelastic Properties

In tensile creep of glassy polystyrene under high stresses, of the order of 10 dynes/cm , the rate of deformation at a given time after loading increases with something like the fourth power of the applied stress, instead of being directly proportional. Similar nonlinear stress dependence for shear creep of poly(methyl methacrylate) below Tg was found by Lethersich, and the creep and creep recovery functions were very different in shape, even though the total deformation (of the order of 5%) was fully recoverable. 

In stress relaxation of glassy polycarbonate at tensile strains up to 4%, Yannas and Haskell found that the time-dependent tensile stress ot could be expressed by a power series  

Nonlinear oscillatory viscoelastic behavior of glassy polycarbonate and polymethyl methacrylate has been investigated in a thorough study by Davis and Ma-cosko with very nearly homogeneous shear strains up to 4%. The analog of equation 8, derived in the same manner from the extension of the Boltzmann su- 

In dynamic longitudinal deformations at low audio frequencies, polystyrene was found by Sauer and collaborators to exhibit an energy dissipation per cycle which increased rapidly with the amplitude of vibration. 

Nonlinear phenomena such as these have been discussed in terms of transition-state kinetics by Schwarzl. A substantial asymmetry of a potential barrier to molecular motions, imposed by high external stress, leads to a shift of the viscoelastic time scale to shorter times as well as a broadening of the retardation spectrum. The nonlinear behavior of fibers in transient loading experiments has been interpreted in this way by Eyring and collaborators.  

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