Resistance to transverse tension of the unidirectional carbon fiber/Poly(ether ether ketone) (PEEK) composites was examined by uniaxial tensile and cleavage tests. Three types of unidirectional car bon fiber/PEEK composites were fabricated from: (a) prepreg sheet, (b) plain weave cloth (warp: comming led yarn of 1800 d of carbon fiber with PEEK fiber, weft: PEEK yarn of 70 d), or (c) 5-shaft sateen weave cloth (warp: carbon yarn of 1800 d, weft: PEEK yarn of 900 d). The initial Young's modulus and the breaking strength measured by the transverse tensile tests were the highest on the specimen (a). Those of the specimen (c) were the lowest because of the lack of the carbon fiber/PEEK interfacial adhesion. On the other hand, the mode I intralaminar fracture toughness estimated from the cleavage test was the highest a on the specimen (b). The interfacial adhesion between the carbon fiber and PEEK resin was also investigated by scanning electron microscopic observation of the fractured surface after transverse tensioning. For specimen (b) the fracture surface was rough and irregular, and the fibers were twisted with one another. The fracture surface of specimen (a) was flat and smooth, and the fibers were parallel to one another. It was concluded that the undulation and twisting of the carbon fibers enhanced the fracture toughness oft specimen (b).

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