Experimental study on the mechanical behavior of carbon/epoxy composites with a carbon nanofiber-modified matrix

Material property characterization tests were performed on satin weave carbon/epoxy composites, where the epoxy resin was modified with 2 wt% carbon nanofibers prior to infusion into a continuous carbon fiber preform. Uniaxial tension tests, tension-tension fatigue tests, and fracture tests were initially performed on 0-3 wt% carbon nanofibers-reinforced epoxy specimens in order to determine the carbon nanofiber weight fraction leading to the optimal mechanical properties of the modified epoxy matrix. In general, the elastic modulus of the modified epoxy increased with increasing carbon nanofibers weight fraction. The ultimate tensile strength, fatigue life, and fracture toughness also increased significantly with increasing amounts of carbon nanofibers and reached maximal values at a carbon nanofiber weight fraction of 2 wt%. The improvement in tensile properties of carbon nanofiber-modified epoxy became more pronounced for specimens loaded at higher strain rates. Further increases in nanofiber content, however, resulted in a relative decrease in nanocomposite strength and fatigue life; this is likely due to local stress concentrations associated with poorly dispersed carbon nanofibers. Vacuum-assisted resin infusion molding was used to fabricate hybrid composite panels consisting of woven carbon fabric and epoxy resin modified with 2 wt% carbon nanofibers. Uniaxial compression, open-hole compression, and short beam shear tests were performed to assess the effect of carbon nanofibers on matrix-dominated composite properties. Hybrid composites containing 2 wt% of carbon nanofibers in the epoxy matrix resulted in compressive strength, open-hole compressive strength, and interlaminar shear strength values that were 19.8%, 27.8%, and 15.8% greater, respectively, than those for woven fabric composites prepared with neat epoxy. Quasi-static uniaxial tension tests and tension-tension fatigue tests of hybrid composite specimens also led to similar enhancements in the composite ultimate strength and fatigue life relative to composites specimens infused with neat epoxy. Scanning electron microscopy images of composite micro-fracture surfaces indicated that randomly distributed carbon nanofibers provide some crack bridging, reduce crack opening, and lead to crack turning for small cracks. Such mechanisms are likely responsible for the improvement in mechanical properties.