Aging during elevated temperature stress relaxation of IM7/K3B composite

An experimental and analytical study was performed on the use of tension st ress relaxation to characterize the effects of elevated temperature and phy sical aging on the linear viscoelastic behavior of IM7/K3B. Isothermal stre ss relaxation tests on a [+/-45](2s) laminate were run over a range of sub- glass transition (T-g) temperatures. The sequenced test method most commonl y employed for creep was successfully adapted to the stress relaxation test and from those sequenced tests, material parameters such as aging shift ra tes and momentary master curve coefficients were developed for use in the a nalytical model. The analytical viscoelastic model was based on classical lamination theory, the hereditary integral formulation type constitutive law, and affective t ime theory. Time-aging time superposition, effective time theory, and visco elasticity were used to determine the physical aging related material param eters from the relaxation tests. Results were compared to previously measur ed isothermal creep compliance results via known relationships for the conv olution of compliance to modulus. Time-temperature superposition was also u sed to evaluate master curves and related shift factors. All of the results illustrated that the relative influence of temperature and aging must be c onsidered when assessing long-term performance and that the loading mode ma y have to be considered when accurate predictions of viscoelastic behavior are required.