linearity and nonlinearity of a polymeric materials

But viscoelasticity has both linear and nonlinear formulations. Linear models include the (generalized) Maxwell model (all the various spring-and-dashpot manipulations) while the nonlinear models are necessary to describe normal forces.

The need for a linear vs. nonlinear formulation can depend on both the rate of deformation as well as the actual extent of deformation. If you can get away with using a linear model, do so. Put the extra energy elsewhere.

John

Ok, is there the exact definition, for example The material is linear, if the Boltzman superposition principle is valid for it. Or Material is non-linear if, at the recalculation of the stress-strain curves received under various conditions of experiment (deformation rate etc.) in axises stress vs. time or strain vs time,. they will not coincide.

Bird, Armstong and Hassager limit linear models to steady state and "small deformations", which usually means that the Boltzman superposition applies.

John