The characterization of uniaxial graphite fiber/epoxy composites by mechanical testing and scanning electron microscopy

Abstract

AbstractThe effect of the constituent material on the mechanical properties and on the overall crack propagation behavior of uniaxial graphite/epoxy composites was studied using two different types of graphite fibers, i.e., PAN precursor and pitch precursor fibers in combination with an epoxy known for its toughness. The tensile properties and interlaminar shear strength (ILSS) of composites with various fiber volume fraction were studied using an Instron Universal Testing Machine. The fracture surface of composites was 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 causes of failure mode were observed in this study: (1) 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. The principal fracture features of AS4/epoxy composites under tension were fiber pullout (bundles and single fibers) and very rough fracture surface due to the pullout of fiber bundles with epoxy, whereas VSB‐16/epoxy composites under tensile and bending loads showed only tensile (or compressive) failure. The composites of epoxy/PAN (polyacrylonitrile)‐based fibers (AS4) showed greater ILSS than those of the pitch‐based fiber (VSB‐16)/epoxy. The most characteristic features of the interlaminar shear failure surface of AS4/epoxy composites are delamination and formation of hackles.