Page 186 - Fiber Fracture

FRACTURE OF CARBON FIBERS 171

described by Fain et al. (1988) and Robinson and Edie (1996). However, the stiffness
aspects of modified fiber cross-section could be adversely affected while thermal
and adhesion responses may be improved. Packing densities of individual fibers in fiber
assemblages may also be changed. Processing continuity and part fabrication costs could
be critical aspects influencing adoption of this technology to modify product responses.
A more detailed discussion of fiber manufacture will be found in Bahl et al. (1998).
Commercially useful fibers are made from mesophase pitch at heat treatment
temperatures of 1600°C and above. As heat treatment temperatures are increased, the
modulus of mesophase pitch fibers increases, and modulus values close to the theoretical
modulus of graphite (1 TPa) are possible. The term ‘graphitization’ is frequently applied
to heat treatment above 2500°C. However, this does not mean that the structure is
converted to graphite. Most carbon fibers, even those with a modulus above 700 GPa,
are mostly made of turbostratic carbon with small graphitic domains. The inert gases
used in carbonizing furnaces are nitrogen and argon. Nitrogen is preferred because of
cost. However, above about 2000°C significant quantities of cyanogens are produced by
the reaction of nitrogen with the graphite of the furnace, so argon, which is completely
inert, is sometimes used instead.

Fig. 18. Pitch-based carbon fiber fracture surface.