Effect of interleaved non-woven carbon tissue on interlaminar fracture toughness of laminated composites: Part I - Mode II

A study on the Mode II interlaminar fracture characteristics of non-woven carbon tissue (NWCT) interleaved composites was carried out using the end-notched-flexure (ENF) test. The NWCT interleaved composites were made by interleaving NWCT-prepreg between CFRP layers. CFRP specimen [0₂₄] as well as NWCT interleaved specimen [0₁₂/T/0₁₂] were prepared to compare the characteristics. The symbol "/T/" means that the NWCT layer is located at the 0/0 mid-plane of the interleaved specimen. Three kinds of interlaminar cracks, which have different positions in the NWCT layer, were introduced to the interleaved specimen. The ENF testing results were analyzed using the normal distribution. The mean G_IIC of the interleaved specimen with an upper crack is about 26% higher than that of the CFRP specimen due to a blunt shape of crack tip by Teflon film in the NWCT layer. The mean G_HC of the interleaved specimens with a center crack and a lower crack is about 260 and 205% higher than that of the CFRP specimen due to the short fiber bridging from in-plain random short carbon fibers in the NWCT layer, respectively. The fracture surfaces of the specimens and the fracture process on the specimen side-sections were observed with an optical microscope and a scanning electron microscope, and then they were analyzed for the failure mechanism. The size and spacing of hackles on the fracture surface are affected by both the length and fiber volume of short carbon fibers in the NWCT layer.