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There is strong evidence that weaker composites, regardless of fiber content, tend to cleave on a single plane, while stronger specimens exhibit extensive fracture zones as a result of cumulative damage. This type of behavior is the same before and after thermal cycling of specimens up to 500 cycles.
Assessment of the mechanical and thermal cycling behavior of engineering composites has been extended to include transverse tensile, compressive, and flexural properties, as well as the longitudinal tensile properties and short-beam shear strength. After cycling, the 55 volume percent 0-deg tensile modulus exhibits a five percent loss. In 25 volume percent specimens, the effect is less because of the greater bulk of the resin. Compressive strength of 25 and 55 volume percent specimens shows a significant decrease for higher fiber content, while the lower fiber content retains about the same strength after thermal cycling. Flexural strength in high-fiber content specimens shows an increase with increased thermal cycling. Similar behavior is noted for 25 volume percent up to 200 cycles; however, there is a drastic reduction after 200 cycles. There was little change in interlaminar strength with the number of cycles.
composite materials, carbon fibers, epoxy resins, mechanical properties, interfaces, fracturing
materials engineering specialist, Boeing-Vertol Company, Philadelphia, Pa.
metallurgist, General Electric Research and Development Center, Schenectady, N.Y.