The characterization of surface modified graphite fibers/epoxy composites by mechanical testing and scanning electron microscopy

The effect of gas plasma etched graphite fibers on the interlaminar shear strength(ILSS) of unidirectional graphite fiber/epoxy resin composites with various fiber volume fraction has been studied. The tensile properties and ILSS of composites with various fiber volume fraction have been studied using an Instron Tensile Tester. The etched surface of the fibers and the fracture surface of composites have been studied using scanning electron microscopy(SEM). The surprising result from the tensile tests is that the percent elongation of AS4/epoxy composites was much higher than that of individual components, i.e., the epoxy and the fiber. The stress-strain curves of AS4 fiber/epoxy composites show yielding. The yielding is interpreted to result from shear stress concentration due to the fiber breakage. Two cases of failure mode were observed in this study; i) ductile plastic deformation by the PAN-based AS4/epoxy composites, ii) brittle catastrophic failure shown by the pitch-based VSB-16/epoxy composites. The tensile strengths and moduli of the composites increased as the volume fraction of fibers increased. AS4/epoxy composites showed greater tensile strength but lower moduli than HMS/epoxy composites. The results of four-point bending tests showed that the ILSS of these composites did not change greatly with etching time. The values of ILSS showed a maximum about at 25% fiber volume fraction. The composites of epoxy/PAN(poly acrylonitrile) based fibers (AS4 and HMS) showed greater ILSS than those of the pitch-based fiber(VSB-16)/epoxy. The SEM investigation on these fibers showed little change in fiber surface with 0-4 min plasma etching. The most characteristic features of interlaminar shear failure surface of AS4/epoxy composites are delamination and formation of hackles. The principal fracture features of AS4/epoxy composites under tension were fiber pull-out (bundles and single fibers) and very rough fracture surface due to the pull-out of fiber bundles with epoxy.