Mullins' effect and damage accumulation in perfluoroelastomers

Purpose - The purpose of this paper is to develop a constitutive model in cyclic viscoplasticity of perfluoroelastomers that accounts for the Mullins effect and to determine adjustable parameters in the stress-strain relations by fitting observations in mechanical tests. Design/methodology/approach - A perfluoroelastomer with a complicated internal structure is modeled as an equivalent incompressible, permanent, non-affine network of chains with constrained mobility. Its viscoplastic response at finite strains is treated as sliding of junctions between chains with respect to their reference positions. Damage accumulation is associated with acceleration of plastic flow of junctions driven by growth of free volume. Stress-strain relations are derived by using the Clausius-Duhem inequality. Findings - Constitutive equations are developed that correctly describe the mechanical behavior of perfluoroelastomers under cyclic loading with stress- and strain-controlled deformation programs and arbitrary numbers of cycles. Adjustable parameters in the stress-strain relations are found by matching experimental data in uniaxial tensile tests. Numerical simulation demonstrates that the model adequately predicts characteristic features of the Mullins effect. Originality/value - A constitutive model is derived that can be applied for description of the viscoplastic response in perfluoroelastomers at cyclic loading with complicated deformation programs and prediction of their time to failure under fatigue conditions.