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Fatigue damage in (0, ± 45) and (0, ± 45,90) boron/epoxy laminates was studied with X-ray radiography and scanning electron microscopy (SEM). In addition, limited tests for residual strength and stiffness were performed. Initially, fatigue damage in both (0, ± 45) and (0, ± 45,90) laminates occurred as intralaminar cracks around the edge of the hole. Then, whenever further damage developed, intralaminar cracks in the ± 45-deg plies began to propagate from the edge of the hole. Finally, in both types of laminates, primarily ± 45-deg fibers broke (prior to two-piece failure) where intralaminar cracks in the ± 45-deg plies had occurred. In the (0, ± 45) laminates, the 45-deg plies developed intralaminar and transthickness cracks along lines parallel to the loading axis and tangent to the hole in the test specimen. This damage, which was most pronounced under compressive loads, had little effect on either strength or stiffness. In contrast, in the (0, ± 45,90) laminates, the ± 45-deg plies developed intralaminar cracks transverse to the loading axis. This transverse damage, which occurred primarily under tensile loads, affected the residual strength but not the stiffness and frequently resulted in catastrophic failure across the notch. The results of this study suggest that in boron/epoxy laminates the 45-deg plies play a key role in the fatigue process.
composite materials, laminates, fatigue damage, radiography, microscopy
Research engineer, U.S. Army Air Mobility Research and Development Laboratory, Langley Research Center, Hampton, Va
Research engineer, National Aeronautics and Space Administration, Langley Research Center, Hampton, Va