Molecular Dynamics Modeling and Characterization of Complex Nano-Structure of PAN-Based Carbon Fibers

Abstract The microstructure of typical carbon fiber reinforced polymer composite (FRPC) materials is composed of 6 μm diameter fibers, embedded in polymer resin. Because the fibers are the main load bearing constituents, the mechanical performance of the overall laminate is inherently linked to the nanostructure of individual fibers. In-situ characterization of individual fiber properties at the micro and nano-scales poses challenges due to the small size-scales associated with testing. In addition, the complex nanostructure of typical PAN-based carbon fibers, which is composed of folded graphitic sheets oriented along the fiber axis, poses modeling challenges. To address this, the following study employs molecular dynamics (MD) to arrive at the complex nanostructure of PAN-based carbon fibers through process modeling, in conjunction with kinetic Monte Carlo (KMC) simulations, which will set the stage for obtaining the constitutive properties through virtual loading. In future studies, the KMC simulation aims to replicate the carbonization and graphitization manufacturing conditions, leading to the formation of the nanostructure of compact folds of graphitic sheets with internal flaws and voids. The effects of processing parameters, such as temperature, and the details of the bond potential and cutoff distances are shown to affect the resulting nanostructural morphology. The details of the complexity of carbon fiber morphology can allow for the emergence of material nonlinearity and statistical strength distribution. In the present study, the density range of typical carbon fibers is used to validate both the KMC model and the representative volume element (RVE) morphology, which sets the conditions for material characterization through various loading such as tension, compression, and flexure in future studies.