Page 189 - Fiber Fracture

174 J.G. Lavin

Fig. 22. Vapor-grown carbon fiber fracture surface. From Endo (1988). (Copyright 1988. Reprinted with
permission from the American Chemical Society.)

Vapor-grown fibers typically have a hollow center and multiple walls, which are
arranged like tree rings, as shown in Fig. 22. However, they may be grown in many
other shapes, as shown in Fig. 23.
There are basically two kinds of processes for producing vapor-grown fibers. The
most common process is the one described above, in which the catalyst is a metal
supported on a ceramic. This process produces long fibers which are tangled together in
a ball which is extremely difficult to break up. A variant of this process is one in which
the catalyst is an organometallic injected into a chamber containing the gas mixture.
These fibers tend to be short and straight. However, they may be aggregated together
and bound by amorphous carbon. In either case, the reinforcing capabilities of the fiber
are restricted.

FAILURE MECHANISMS

Tensile Failure

The most revealing experiments were conducted by Bennett et al. (1983), who
fractured PAN-based carbon fibers in glycol, a medium which absorbed the explosive
energy generated at fiber failure. This allowed meaningful examination of the broken
ends by SEM and TEM. They observed large misoriented crystals in the internal flaws