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During this investigation, methods were developed for determining the fracture directions of failed graphite fiber/epoxy resin composite test specimens and structures. Notched test specimens with 90-deg unidirectional fiber orientations were failed in tension by delamination. The resulting fracture surface features consisted of the graphite fiber and epoxy components. Epoxy fracture surface topographic features were identified with a scanning electron microscope (SEM). These features, designated as “hackles,” were correlated with the fracture direction. The direction of hackle overlap or tilt reversed when the fracture direction changed. This characteristic can be used to isolate fracture origins on failed test specimens or structures.
Test specimens having cracks with known fracture directions were sectioned, mounted, and polished using standard laboratory techniques and were examined with an optical microscope. A crack tip was detected which provided evidence to support a proposed theory for delamination crack initiation and propagation in graphite/epoxy structures.
Finally, graphite/epoxy test specimens and structural test articles that had been tested to failure in tension and fatigue were examined, and the hackle features were correlated with the fracture directions and origin locations within the specimens.
composite materials, fracture (materials), fractography, failure analysis, delamination, fracture direction, graphite/epoxy, nondestructive tests
Lead engineer, Materials and Physics Laboratories, McDonnell Aircraft Co., St. Louis, Mo