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This paper discusses the effect of flaws created by impact damage on the strength of graphite/epoxy panels that were tested as sandwich beams and as skin panels on box beams. A majority of the panels were exposed to random spectrum fatigue loads prior to failing them under static loading conditions. The maximum loads in the fatigue spectrum were scaled to provide a strain equal to the B-basis matrix failure strain for the resin system used. Box beam skin panels were designed to fail by compressive panel buckling in order to evaluate the effect of the impact damage on the buckling strength of the panels. All testing was conducted at room temperature on panels with no moisture conditioning.
Marginally visible damage was found to cause a strength loss up to 35 percent in compression but less in tension. Acoustic C-scan inspections showed that none of the impact damage sizes grew during spectrum loading. Panel stability was unaffected by localized impact damage. The most severe impact damage caused strength losses comparable to drilled fastener holes.
graphite/epoxy, composite materials, impact damage, strength testing, nondestructive tests
Senior technical specialist, Northrop Corp., Aircraft Group, Hawthorne, Calif