linearity and nonlinearity of a materials

Dear Colleagues!
What we imply, when we speak about linearity and nonlinearity of a polymeric material. If to speak with reference to deformation process does it mean that
the value of the module of elastic module remains constant without dependence from deformation? At the same time, under the high speeds of deformation appears a viscous component, which gives rise to mathematical nonlinearity and we receive the nonlinear equation of deformation. Is it necessary to separate the mathematical and physical nonlinearity?
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