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Abstract

Graphite fiber-reinforced resin composite materials are widely used in commercial applications where high strength and low weight are critical factors. In order to predict the fracture toughness of a composite material from the constitutive properties of the resin and fibers, experimental methods for the analysis of microscopic displacements and strain fields that develop at the fracture crack tip within the composite material are required. Information derived from measurement of displacements, and calculation of strain fields can then be used to test micromechanical models of matrix-dominated fracture. A method was developed in which it is possible to conduct near real-time fracture analysis of epoxy-based composite materials, and to subsequently obtain micrometer-scale measurements of displacements in the region of the crack tip. A map matrix was generated on the surface of test specimens in an SEM equipped with a tensile stage, along with an x-ray spectroscopy and image analysis system. A 40 × 40 point digital map was introduced onto the surface of the specimen using the digital x-ray mapping function of the x-ray analysis system which produced a surface matrix with point spacing of 10 μm. The quality of maps varies with test specimens and therefore it is necessary to optimize microscope operation parameters for each resin tested. Reproducible results were obtained with both neat resins and graphite/epoxy composites. In situ analysis of a region of a propagating crack tip grown using the tensile stage reveals a deformation zone ahead of the crack tip and images of the stages of microcracking were captured by the image analyzer for subsequent measurement of displacement. Direct measurement of crack tip displacements from SEM electron beam-induced reference matrices provide an important tool in characterizing the fracture behavior of both neat resin and composite materials.