Modelling damage accumulation in filled polymers

A variant of the non‐linear endochronic viscoelasticity theory is developed wherein the reduced‐time function depends on a damage parameter, equivalent strains or stresses, as well as on some functions enabling one to distinguish between loading and unloading processes. Some combinations of the first and second invariants of strain and stress tensors are used as the equivalent strains or stresses. The existence of flaws is taken into account by the introduction of special equivalent stresses which are dependent on principal tensile stresses and the characteristics of the material. As a criterion of damage equivalence in non‐linear viscoelastic materials, the condition of a specific dissipation equality is introduced. Experiments for the determination of material functions entering the constitutive equations are outlined. The proposed variant of the theory has made it possible to realistically explain the results of creep and constant loading rate tensile tests and creep tests to rupture under conditions of two‐step loading sequences acting on filled polymeric materials.