Flexi Says: Carbon fiber is a material consisting of fibers about 5-10 nm in diameter that are composed mostly of carbon atoms. You can get an idea of how thin the fibers are by comparing a carbon fiber with a human hair in the image below (the carbon fiber is the smaller strand in the figure). The carbon atoms in carbon fibers are bonded together in crystals. The crystal alignment gives the fibers a high strength-to-volume ratio. In other words, the fibers are very strong for their size.