Page 23 - Fiber Fracture

8 K.K. Chawla

fibers as well as to the presence of microvoids and the skin-core structure of these
fibers. It should be pointed out that poor properties in shear and compression are,
however, also observed in other highly oriented polymeric fibers such as polyethylene
and poly@-phenylene benzobisoxazole) or PBO fibers, which are not based on rigid-rod
polymers. A correlation between good compressive characteristics and a high glass
transition temperature (or melting point) has been suggested (Northolt, 198 1; Kozey and
Kumar, 1994).
Thus, with the glass transition temperature of organic fibers being lower than
that of inorganic fibers, the former would be expected to show poorer properties in
compression. For aramid and similar fibers, compression results in the formation of kink
bands leading to eventual ductile failure. Yielding is observed at about 0.5% strain. This
is thought to correspond to a molecular rotation of the amide carbon-nitrogen bond
from the normal extended trans configuration to a kinked configuration Tanner et al.,
1986). This causes a 45" bend in the chain, which propagates across the unit cell, the
microfibrils, and a kink band results in the fiber.
Efforts to improve the compressive properties of rigid-rod polymer fibers have
involved introduction of cross-linking in the transverse direction. There is a significant
effect of intermolecular interaction or intermolecular cross-linking on compressive
strength. A polymeric fiber (PPD) with a compressive strength of 1.6 GPa has been
reported (Jenkins et al., 2001). This high compressive strength is ascribed to bi-
directional, intermolecular hydrogen bonding. A high degree of intermolecular covalent
cross-linking should result in higher compressive strength, as compared to systems in
which only hydrogen bonding is present (Jenkins et al., 2001). However, cross-linking
may also result in lower tensile strength and increased brittleness of the fiber. Cross-
linking by thermal treatment may result in the development of internal stresses. Other
cross-linking methods (e.g. via radiation) should be explored in greater detail. One
would expect radiation to result in a different cross-linked structure than that obtained
by thermal treatment. Here it is instructive to compare the behavior of some carbon
fibers. Highly graphitic, mesophase pitch-based fibers show a fibrillar fracture and
poor compressive properties. PAN-based carbon fibers, which have some linking of thc
graphitic planes in the transverse direction, show better properties in compression and
not a very fibrillar fracture. Of course metallic and ceramic fibers show little fibrillation
during a tensile or compressive failure.

Environmental Effects on Polymeric Fibers
Environmental factors such as humidity, temperature, pH, ultraviolet radiation,
and micro-organisms can affect the strength and the fracture process in polymeric
fibers. Natural polymeric fibers are more susceptible to environmental degradation than
synthetic polymeric fibers. Cellulose is attacked by a variety of bacteria, fungi, and
algae. Micro-organisms use cellulose as a food source. Natural fibers based on protein
such as wool, hair, silk, etc., can also be a food source for micro-organisms, but such
fibers are more prone to degradation due to humidity and temperature. Polymeric fibers,
natural or synthetic, undergo photo degradation when exposed to light (both visible
and ultraviolet). Physically this results in discoloration, but is also accompanied by a