Carbon-Nanofiber-Reinforced Syntactic Foams: Compressive Properties and Strain Rate Sensitivity

The current study is focused on exploring the possibility of reinforcing syntactic foams with carbon nanofibers (CNFs). Syntactic foams are hollow, particle-filled composites that are widely used in marine structures and are now finding applications in other modes of transportation due to their high stiffness-to-weight ratio. The compressive properties of syntactic foams reinforced with CNFs are characterized over the strain rate range of 10(-4) to 3,000 s(-1), which covers seven orders of magnitude. The results show that despite lower density with respect to neat epoxy, CNF/syntactic foams can have up to 7.3% and 15.5% higher quasi-static compressive strength and modulus, respectively, for the compositions that were characterized in the current study. In addition, these properties can be tailored over a wide range by means of hollow particle wall thickness and volume fraction, and CNF volume fraction. The compressive strength of CNF/syntactic foams is also shown to generally increase by up to a factor of 3.41 with increasing strain rate when quasi-static and high-strain-rate testing data are compared. Extensive microscopy of the CNF/syntactic foams is conducted to understand the failure and energy absorption mechanisms. Crack bridging by CNFs is observed in the specimens, which can delay final failure and increase the energy absorption capacity of the specimens. Deformation of CNFs is also noticed in the material microstructure. The deformation and failure mechanisms of nanofibers are related to the test strain rate and the structure of CNFs.