Friday, October 5, 2012
Linear and Non-linear Viscoelastic Behavior of Rigid Polymers
Wolfgang Knauss, Professor Emeritus, GALCIT, Caltech
The effect of the dilatational stress component on the yield-like behavior of rate dependent polymers is examined via the example of PMMA. Starting with uniaxial data published in 1973 by C Bauwens-Crovet and describing the dependence of yield (maximum) stress in compression and in tension, the then offered reduction or 'shift' scheme is reviewed, in particular, the fact that tension and compression data lead to different data reductions. Following the more recent recognition that the dilatation affects the time dependence of polymeric deformation processes much like temperature affects temporal scaling, the same data is then subjected to an analysis based on volumetric changes accompanying loading. Although it is recognized that dilatational effects may not completely described all nonlinear viscoelastic material behaviors in general, it is found that such a treatment unifies the compression and tension data into a single master curve to the extent that temperature and loads, as well as time-temperature shift data from the two deformation modes are congruous. For situations where purely dilatational effects are insufficient to effect unification of the data -- because of the influence of large shear stresses or deformations -- a molecular mechanics model is offered that demonstrates the influence of shear amplitude on the yield-like behavior of polymers. The model is consistent with typical laboratory data.