Physical aging in graphite/epoxy composites

Abstract Matrix‐dominated mechanical behavior of a graphite/epoxy composite has been found to be affected by sub‐T_g annealing. Postcured (±45°)_4S specimens of Thornel 300 graphite/Narmco 5208 epoxy were quenched from above T_g and given a sub‐T_g annealing at 140°C for times up to 10⁵ min. The ultimate tensile strength, strain‐to‐break, and toughness of the composite material were found to decrease as functions of sub‐T_g annealing time. No weight loss was observed during the sub‐T_g annealing. The time‐dependent change in mechanical behavior is explained on the basis of free‐volume changes that are related to the physical aging of the nonequilibrium glassy network‐epoxy. The results imply possible changes in composite properties with service time.