An experimental and analytical study was performed on the use of tension stress relaxation to characterize the effects of elevated temperature and physical aging on the linear viscoelastic behavior of IM7/K3B. Isothermal stress relaxation tests on a [±45]2s laminate were run over a range of sub-glass transition (Tg) temperatures. The sequenced test method most commonly employed for creep was successfully adapted to the stress relaxation test and from those sequenced tests, material parameters such as aging shift rates and momentary master curve coefficients were developed for use in the analytical model.
The analytical viscoelastic model was based on classical lamination theory, the hereditary integral formulation type constitutive law, and effective time theory. Time-aging time super-position, effective time theory, and viscoelasticity were used to determine the physical aging related material parameters from the relaxation tests. Results were compared to previously measured isothermal creep compliance results via known relationships for the convolution of compliance to modulus. Time-temperature superposition was also used to evaluate master curves and related shift factors. All of the results illustrated that the relative influence of temperature and aging must be considered when assessing long-term performance and that the loading mode may have to be considered when accurate predictions of viscoelastic behavior are required.