Mode I and mode II interlaminar fracture toughness of CFRP laminates toughened by carbon nanofiber interlayer

Interlaminar fracture toughness for mode I and II deformation were investigated for carbon fiber (CF)/epoxy laminates toughened by a carbon nanofiber/epoxy interlayer. Vapor grown carbon fiber (VGCF) and vapor grown carbon nanofiber (VGNF) were chosen as the reinforcement for the interlayer. To illustrate the effect of the interlayer on the fracture toughness of the laminates, several types of carbon fiber reinforced plastics/carbon nanofiber (CFRP/CNF) hybrid laminates were fabricated. Each laminate was composed of unidirectional carbon/epoxy prepregs with carbon nanofiber varying the interlayer thickness. Mode I interlaminar fracture toughness was evaluated by a standard double cantilever beam (DCB) test. Mode II interlaminar fracture toughness was evaluated by an end notched flexure (ENF) test using a three point bending test. The experimental results of the DCB test confirms that the mode I interlaminar fracture toughness for hybrid laminates is about 50% greater than the base CFRP laminates. Furthermore, the results of the mode II fracture toughness test confirms that the interlaminar fracture toughness for hybrid laminates is 2–3 times greater than base CFRP laminates. In addition, the results revealed the recommended range of CNF interlayer thickness is between 100 and 150 μm (approximately 20 g/m2 carbon nanofiber area density). The difference in the effect of the interlayer fracture properties under mode I and II deformation was discussed on the basis of fractographic observations derived from scanning electric microscope.