The meaning and consequences of linearity The development of the mathematical theory of linear viscoelasticity is based on a "superposition principle".This implies that the response (e.g.strain)at any time is directly proportional to the value of the initiating signal (e.g.stress).So,for example,doubling the stress will double the strain.In the linear theory of viscoelas- ticity,the differential equations are linear.Also,the coefficients of the time differentials are constant.These constants are material parameters,such as viscosity coefficient and rigidity modulus,and they are not allowed to change with changes in variables such as strain or strain rate.Further,the time derivatives are ordinary partial derivatives.This restriction has the consequence that the linear theory is applicable only to small changes in the variables