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A candidate approach for improving the fracture toughness of graphite fiber epoxy composites was investigated in which coatings of tough epoxy based adhesives were applied to the graphite fibers and subsequently overcoated with a 350°F (177°C) service epoxy laminating resin. Composites were fabricated from the coated fiber/epoxy matrix prepreg and from prepreg made with uncoated fibers and the same matrix. Tests on the composites included Mode I interlaminar fracture GIc by the double cantilever beam method, 0° compression, and 90° tension. Scanning electron microscope photomicrographs of fracture surfaces were made. Comparison of GIc values for adhesive coated fiber composites with those of uncoated fiber composites was inconclusive because of anomalies in specimen behavior during testing. However, photomicrographs of fracture surfaces of the GIc specimens indicated that the coated fiber composites had much greater fracture surface area and deformation in the polymeric constituents than did the uncoated fiber composites. Although there were no major differences in 0° compressive strength and 90° tensile strength between the coated fiber composites and the uncoated fiber composites, in some cases the coated fiber composites were slightly superior.
graphite fiber composites, coated fibers, adhesives, toughness, delamination resistance, fractography
Materials research engineer, Air Force Wright Aeronautical Laboratories, AFWAL/MLBC, Wright-Patterson AFB, OH
Associate research chemist, University of Dayton Research Institute, Dayton, OH