Published: Jan 1995
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The fatigue resistance of a multiaxial braided (3-D) graphite/epoxy composite in both unnotched and post-impacted conditions has been evaluated. The material tested is a [±30°/0°] multiaxial braid constructed from AS4/12K tow graphite fibers and British Petroleum E905L epoxy resin. These materials were braided as dry preforms, and the epoxy was added using a resin transfer molding process (RTM). The unnotched and post-impact specimens were tested in compression-compression fatigue at 10 Hz with a stress ratio of R = 10. The unnotched tension-tension fatigue specimens were tested at 5 Hz with a stress ratio of R = 0.1. Damage initiation and growth were documented through the application of radiography and ultrasonic through transmission (C-scans). Visible inspection of surface and edge damage was also noted to describe the initiation and progression of damage in these materials. The mechanisms leading to damage initiation were established, and failure modes were determined. Stiffness and strength degradation were measured as a function of applied cycles. These 3-D braided composite results were compared to strain levels currently used to design primary structure in commercial aircraft composite components made from autoclave-cured prepreg tape.
braided composites, tension fatigue, compression fatigue, low velocity impact, damage mechanisms
Lockheed Engineering and Sciences Company, Hampton, VA
NASA Langley Research Center, Hampton, VA