TORSIONAL MODULUS OF VAPOR-GROWN CARBON-FIBERS

The torsional modulus (G) of vapor-deposited carbon fibers of both as-grown and annealed samples were determined with a torsion pendulum. G is found to be both stress and size dependent. The extrapolated zero-stress value of torsional modulus is 105 +/- 5 GPa for a 5.4-mu-m diameter as-grown fiber and 200 +/- 10 GPa for a 8.4-mu-m diameter annealed fiber. G decreases with increasing diameter. These unusually high values of torsional moduli, compared to other types of carbon fibers, are believed to be the results of the tree-ring structural morphology and the degree of graphitization of these vapor-deposited carbon fibers.