Constitutive equations for non-linear viscoelastic materials are cz;express
ed first using a caused operator which relates a response function to any l
oading history The corresponding tangent linear equations are then defined
through the Frechet derivative of this operator This definition is used to
propose a step-by-step treatment in view of the derivation of the overall b
ehaviour of non-linear non-ageing viscoelastic heterogeneous materials. (C)
Academie des sciences/Elsevier Paris.