The carbon fiber used in vehicles, bicycles, and aircraft is a composite. Extremely thin carbon filaments are arranged in layers and bonded by a polymer matrix, usually epoxy resin. The thermal behavior of the material depends on this internal architecture. The fiber itself shows high thermal stability during brief exposures. The resin reacts first to heat and may char at the surface while the overall structure retains its shape. Mechanical integrity, however, depends on the adhesion between fib