Low-velocity Impact Properties of Interply Hybrid Carbon/Epoxy Unidirectional Composites Strengthened with Pitch-based Low Modulus Carbon Fiber

Abstract

Carbon fiber/epoxy unidirectional laminates exhibit low flexural strength in contrast to their high tensile strength. This is because the flexural failure is dominated by fiber microbuckling at the compression side during bending. For the same reason, the Charpy impact properties cannot be improved only by increasing the tensile strength of the fibers. This paper examined the effect of hybridization on the flexural impact properties of a PAN-based carbon fiber/epoxy unidirectional laminate strengthened with pitch-based, low modulus carbon fibers. PAN-based carbon fiber layers, with fiber modulus of 230 GPa, were used as the core, which was sandwiched by thin outer layers of pitch-based carbon fibers with a lower modulus of 55-155 GPa and a larger compressive failure strain than the PAN-based fibers. An instrumented Charpy impact test showed that the hybrid laminates had higher energy absorption than the monolithic PAN-based laminate. SEM fractographs indicated that the ratio of the tensile failure region in the failed surface increased with the increase in the content of pitch-based low modulus fibers in the hybrid laminates, from 20 percent in the monolithic PAN-based fiber laminate to a maximum of 54 percent in the hybrid laminates. The increased tensile failure region, with suppressed compressive failure, resulted in the higher impact performance of the hybrid laminates. The large compressive failure strain of the pitch-based low modulus fibers was inferred to have the effect of restraining fiber microbuckling at the compression side and improving the impact resistance of the PAN-based fiber composites.