Page 24 - Fiber Fracture

FIBER FRACTURE: AN OVERVIEW 9

worsening of mechanical characteristics. A striking example of environmental leading
to failure of aramid fiber was the failure of the tether rope for a satellite in space. The
rope made of aramid fiber failed because of friction leading to excessive static charge
accumulation, which led to premature failure of the tether rope and the loss of an
expensive satellite.
In a multifilament yam or in a braided fabric, frictional force in the radial direction
holds the fibers together. Such interfiber friction is desirable if we wish to have strong
yams and fabrics. However, there are situations where we would like to have a smooth
fiber surface. For example, for a yarn passing round a guide, a smooth fiber surface
will be desirable. If the yam surface is rough, then a high tension will be required,
which, in turn, can lead to fiber breakage. In general, in textile applications, frictional
characteristics can affect the handle, feel, wear-resistance, etc. In fibrous composites,
the frictional characteristics of fiber can affect the interface strength and toughness
characteristics.

CARBON FIBERS

Carbon fibers are, in some ways, similar to polymeric fibers while in other ways
they are similar to ceramic fibers. A characteristic feature of the structure of all carbon
fibers is the high degree of alignment of the basal planes of graphite along the fiber axis.
The degree of alignment of these graphitic planes can vary depending on the precursor
used and the processing, especially the heat treatment temperature used. Transmission
electron microscopic studies of carbon fiber show the heterogeneous microstructure of
carbon fibers. In particular, there occurs a pronounced irregularity in the packing of
graphitic lamellae as one goes from the fiber surface inward to the core or fiber axis.
The graphitic basal planes are much better aligned in the near-surface region of the
fiber, called the sheath. The material inside the sheath can have a radial structure or
an irregular layer structure, sometimes termed the onion skin structure. The radial core
and well aligned sheath structure is more commonly observed in mesophase-pitch-based
carbon fibers. A variety of arrangement of graphitic layers can be seen in different
fibers. In very general terms, the graphitic ribbons are oriented more or less parallel to
the fiber axis with random interlinking of layers, longitudinally and laterally (Jain and
Abhiraman, 1987; Johnson, 1987; Deurbergue and Oberlin, 1991). Fig. 4 shows a two-
dimensional representation of this lamellar structure called turbostrutic structure. Note
the distorted carbon layers and the rather irregular space filling. The degree of alignment
of the basal planes increases with the final heat treatment temperature. Examination of
lattice images of the cross-section of carbon fiber shows essentially parallel basal planes
in the skin region, but extensive folding of layer planes can be seen in the core region. It
is thought that this extensive interlinking of lattice planes in the longitudinal direction is
responsible for better compressive properties of carbon fiber than aramid fibers. In spite
of the better alignment of basal planes in the skin region, the surface of carbon fibers can
show extremely fine scale roughness. A scanning electron micrograph of pitch-based
carbon fibers is shown in Fig. 5. Note the surface striations and the roughness at a
microscopic scale