Nonlinear viscoelastic constitutive equation in consideration of permanent strain in vinylester resin

This paper presents the characterization of nonlinear viscoelastic behavior in vinylester resin. The description of nonlinear viscoelastic behavior is based on well-known Schapery's constitutive equation. Cardon proposed the method that can estimate nonlinear parameters in Schapery's constitutive equation, separately by creep-recovery tests in consideration of permanent deformation nonexistent in original Schapery's constitutive equation. However, formulation of permanent deformation has almost never been discussed in these articles, In this study, the irreversible strain is formulated and nonlinear viscoelastic constitutive equation taking into account the permanent deformation is proposed. It is found that the behavior of irreversible strain is nonlinear and it has a threshold stress value between the linear behavior and the nonlinear behavior same as nonlinear parameters in Schapery's constitutive equation. Tensile tests in which crosshead speed is changed as time-dependent factor is performed to verify that the proposed constitutive equation can predict time-dependent behavior. Fine agreement between the experimental results and prediction is observed and the validity of the constitutive equation is confirmed.