Constituent level examination of an ebeam cured graphite/epoxy composite

The relationships between the mechanical properties of the resin, interface, and composite laminate were investigated for an ebeam cured composite consisting of graphite fibers (IM7) in a toughened cationic epoxy resin (ORNL 3K). The goal was to gain insight into the relative contributions of the resin and interface characteristics to the poor transverse strength of this composite. Resin and composite properties were compared with two thermally cured graphite/epoxies and one thermally cured graphite/bismaleimide - AS4/3501-6, IM7/977-3, and IM7/5250-4, respectively. The tensile strength and strain-to-failure of 3K were significantly lower than the average for the three thermal cure resins while the fracture toughness and tensile modulus were nearly equivalent. Differences between the ebeam and thermally cured composites were also observed with respect to the mode of failure and the amount of fiber pullout on fractured laminates. To partially isolate the contribution of the resin a thermal postcure was us ed that significantly modified the resin mechanical properties while only slightly altering the resin cure shrinkage. The results indicate that it is unlikely that improvements to the resin mechanical properties alone would be sufficient to reach the laminate transverse strength of the thermally cured composites used as a baseline.