The effect of physical aging on the creep response of a Radel X/IM7 thermoplastic composite was studied. Momentary tensile creep tests were conducted at increasing aging times following a rapid quench from above the glass transition temperature (T_g) to a sub-T_g aging temperature. As the aging time increased, the creep response of the material significantly decreased. The tensile creep compliance data for each aging time were fit to the empirical equation for the creep compliance D(t) D(t)=Doe{(t/to)m} where D_o, t_o, and m are fitting parameters determined using a nonlinear fitting program based on the Levenberg-Marquardt finite difference algorithm. The short-term creep compliance curves, obtained at various aging times, were then shifted to form a momentary master compliance curve. The double-logarithmic aging shift rate, μ, and its dependence on sub-T_g aging temperature were determined. The aging characterization process was conducted on unidirectional specimens with 0-, 90-, and 45-degree fiber direction orientations. This permitted the calculation of the complete principal compliance matrix for the composite material.

The effect of physical aging becomes more apparent during long-term tests when creep and aging occur simultaneously. This results in a gradual stiffening and decrease in the creep response with increased time. Predictions based solely on the Time-Temperature Superposition Principle would significantly over-predict the creep response if physical aging effects were ignored. Theoretical predictions for long-term creep compliance were made using an effective time theory and compared to long-term experimental data for each fiber orientation.