Investigation of the Mechanical Behavior of Carbon Fiber- Carbon Nanofiber Composite for Energy Storage Application in Flywheel

Abstract

The main goal of this study is to unravel the mechanics of hybrid composite flywheels with carbon microfibers and carbon nanofibers (CNFs) reinforcements under centrifugal forces and evaluate the role of nanoscale fillers in delaying failure. This work is driven by the desire to more efficiently store energy in a flywheel in which the maximum energy density is limited by the ability of the material to withstand centrifugal forces. The limiting factor for flywheel energy storage is material strength since the flywheel will burst due to centrifugal stresses if spun at too high of angular velocity, yet its stored energy is proportional to the square of the rpm. In a typical flywheel in which the fibers are placed in the hoop direction for ease in manufacturing, the energy storage and the maximum RPM is limited not by fiber failure, but by matrix-dominated failure modes especially in the hoop direction. To avoid such failures, our study is focused on improving the resin strength via nanomaterials, and to enhance the fracture toughness of hybrid composites in mode I. In this thesis, we have studied the role of electrospun carbon nanofibers (CNFs) in the region of experiencing high radial stress in the composite flywheel. Three major objectives of this research study are I) identification of the trade-offs between surface functionalization of CNFs as a means to enhance CNF-matrix load transfer and the mechanics of individual carbon nanofiber, II) investigation of the mechanics of CNF/epoxy nanocomposite and hybrid nano/micro fiber- reinforced composites as a function of dispersion, surface chemistry and mechanics of individual CNFs, and III) investigation of the failure mechanisms of flywheel subjected to centrifugal forces as a function of the nanofillers content. The first two objectives were mainly carried out via experimentation, including material processing and characterization at multiple length scales, while the last objective was mostly carried out via finite elements analysis based on input from the prior objectives. Our studies pointed to an increase of at most 25% in the interlaminar fracture toughness of the carbon fiber composite laminate due to the introduction of CNFs mat interleafs. The study also points to distinct difficulties associated with increasing crack initiation and growth fracture toughness, as related to the least resistant crack growth path. Our computational results to pointed to an increase in the energy density of the flywheel to about ~14% by introduction of the CNFs mat in the interlayer regions.