Page 28 - Fiber Fracture

FIBER FRACTURE: AN OVERVIEW 13

1990). In contrast, high-modulus mesophase pitch-based carbon fibers deform by a
shear mechanism leading to kink bands at 45" to the fiber axis.

METALLIC FIBERS

Metallic filaments represent a fairly mature technology. Steel and other metals are
used routinely in the form filaments for reinforcement of tires, surgical purposes, bridge
cables, overhead transmission cables, etc. The tire cord is a major application of steel
filaments, called cord. The driving force behind the research for high-strength steel
wire for tire reinforcement is to make a fuel-efficient car. Increasing the strength of
steel reinforcement wire allows a thinner wire to be used for the same strengthening
effect. Cabled steel filaments (Le. twisted bundles) are used in tires as well as in other
applications. The reversed loading of a tire occurs at ground contact during rotation.
Thus, the behavior of steel filaments in torsion and fatigue becomes an important item.
Also, the twist given to the bundles coupled with the state of residual stress in the steel
filaments can induce longitudinal splitting in the filaments (Paris, 1996).
Strength levels of up to 5 GPa can be obtained in steel filaments. Conventional
galvanized steel wire for bridge cable has a tensile strength of 1.6 GPa. Nippon Steel
researchers raised this strength to 1.8 and 2.0 GPa for steel filaments used in the
world's longest bridge, the Akashi Strait Bridge (Tarui et al., 1996). Such filaments
must withstand static and dynamic loads under a variety of loading conditions. The
strand wire rope has a complex geometry. Even when a strand wire rope is loaded
in simple tension, it can put individual fibers in tension, torsion, and bending. In
particular, bending and torsion can lead to splitting delamination (Brownrigg et al.,
1984). Tungsten filaments are used for incandescent lamps. Aluminum and copper are
used as electrical conductors, but their ability to withstand static (creep) and dynamic
loadings is also important.
Control of inclusion content is of critical importance in metallic filaments. In
particular, in steel, microsegregation and interstitial content are also important. C, Mn,
and Si are required for strength in steel filaments but their microsegregation must be
controlled. P and S must be eliminated. Very large drawing strains are involved in the
processing of metallic filaments. The inclusions typically consist of carbides as well
as oxides such as alumina, mullite, spinel, and calcium hexaluminate. If the inclusions
remain undeformed during the drawing process, decohesion of the matrix/inclusion
interface occurs and void(s) are produced. If the inclusion fractures, void(s) form. It
is important to avoid the formation of hard microconstituents such as martensite and
bainite. Inclusions such as C, Mn, P, etc., tend to segregate to the center of the wire.
An important processing-related point in regard to the manufacture of steel cord is
the number of breaks per ton of production in the drawing process (Takahashi et al.,
1992). Of course, the smaller such breaks the better. Not unexpectedly, the frequency of
breaks is a function of the size of nonmetallic inclusions in the wire. The filament break
frequency tends to zero for inclusions of a diameter of 5 pm or less (Takahashi et al.,
1992). This serves to emphasize a general principle about the importance of inclusion in
control.