Mechanical properties and damage tolerance of multiaxial warp-knit composites

Multiaxial warp-knit fabric is a relatively new textile material form that is being considered as an alternative to biaxial woven and unidirectional broadgoods. The paper presents the results of an experimental characterization of composite materials reinforced with four different knitted carbon fabrics and compares the results with conventional prepreg tape and uniweave fabric composites. Tension and compression mechanical property tests, open hole tension and open hole compression tests, and compression after impact tests were conducted on fabric-reinforced carbon/epoxy composite specimens to assess material performance. The fabrics were produced on Liba and Mayer multiaxial warp-knitting machines. Two different generations of Liba machines were used by Hexcel and Saerbeck and a Mayer machine was used by Milliken to produce 4-ply multiaxial fabrics. Composite test laminates were fabricated by two resin transfer molding methods with two different epoxy resin systems. Test results indicate that high quality knitted carbon fabrics can be produced and resin transfer molding can be used to produce aerospace quality composite materials. Tension and compression strength reductions for the Hexcel and Milliken fabric composites ranged from 20 to 30% compared to prepreg tape laminates. However, the knitted fabric composites exhibited compression after impact strengths up to 80% higher than the strength of comparable prepreg tape laminates. The performance of the Saerbeck knitted fabric was comparable to uniweave fabric for all the tests conducted.