Fabrication and Mechanical Properties of Multidimensionally (X-D) Braided Composite Materials

This paper describes research concerning the resin impregnation and characterization of multidimensionally braided fiber-reinforced composite materials. These materials are an alternative to traditional laminated structures, having the potential for being more damage tolerant. Three graphite fiber systems were used in this investigation, and three processes were investigated for resin impregnation of the multidimensionally braided material using vacuum or pressure. Two were resin transfer techniques and the third was a resin film lamination technique. While all three methods are presented, the latter technique was chosen for impregnating the test specimens due to the consistently low void content and superior surface quality achieved by this method. Three variables having an important bearing on the performance of braided materials were investigated. These included the effect of braid pattern, tow size, and edge condition on the tensile, compressive, flexural, and interlaminar shear properties. The properties were obtained in the braid direction only. The cutting of the specimen edges substantially reduced both tensile and flexural strengths and moduli. Of the three braid patterns investigated (1 × 1, 1 × 1 × 1/2F and 3 × 1), the 3 × 1 braid showed superior tensile performance and the 1 × 1 × 1/2F pattern exhibited superior flexural properties. Variation in fiber tow size caused variations in tensile, flexural, and short-beam shear properties. The 12K tow size specimens exhibited the best performance. All braided composite materials in the uncut edge condition showed significant improvements in their short-beam shear strengths, being equal to or greater than unidirectional laminated composites. This latter characteristic may be one of several indicators that multidimensionally braided composites are inherently damage tolerant.