Page 20 - Fiber Fracture

FIBER FRACTURE: AN OVERVIEW 5

INTRODUCTION

Fracture of a fiber is generally an undesirable occurrence. For example, during
processing of continuous fibers, frequent breakage of filaments is highly undesirable
from a productivity point of view. When this happens in the case of spinning of a
polymer, ceramic or a glass fiber, the processing unit must be stopped, the mess of the
solution or melt must be cleaned and the process restarted. In the case of a metallic
filament, a break means that the starting wire must be pointed again, rethreaded, and the
process restarted. In service, of course, one would like the individual fibers whether in a
fabric or in a composite to last a reasonable time.
Fracture in fibers, as in bulk materials, initiates at some flaw(s), internal or on
the surface. In general, because of the high surface to volume ratio of fibers, the
incidence of a surface flaw leading to fracture is greater in fibers than in bulk materials.
Fractography, the study of the fracture surface, of fibers can be a useful technique for
obtaining fracture parameters and for identifying the sources of failure. In general, the
mean strength of a fiber decreases as its length of diameter increases. This size effect
is commonly analyzed by applying Weibull statistics to the strength data. As the fiber
length or diameter increases, the average strength of the fiber decreases. It is easy
to understand this because the probability of finding a critical defect responsible for
fracture increases with size. This behavior is shown by organic fibers such as cotton,
aramid, as well as inorganic fibers such as tungsten, silicon carbide, glass, or alumina.
In this paper, the salient features of the fracture process in different types of fibers,
polymeric, metallic, and ceramic are described. Points of commonality and difference
are highlighted.

POLYMERIC FIBERS

A very important characteristic of any polymeric fiber is the degree of molecular
chain orientation along the fiber axis. In order to get high strength and stiflness
in organic fibers, one must obtain oriented molecular chains with full extension. An
important result of this chain alignment along the fiber axis is the marked anisotropy in
the characteristics of a polymeric fiber.
Rigid-rod polymeric fibers such as aramid fibers show very high strength under
axial tension. The failure in tension brings into play the covalent bonding along the
axis, which ultimately leads to chain scission and/or chain sliding or a combination
thereof. However, they have poor properties under axial compression, torsion, and in
the transverse direction. Fig. 1 shows this in a schematic manner. The compressive
strength of ceramic fibers, on the other hand, is greater than their tensile strength. The
compressive strength of carbon fiber is intermediate to that of polymeric and ceramic
fibers. This discrepancy between the tensile and compressive properties has been the
subject of investigation by a number of researchers (see Chawla, 1998 for details).
An example of kinking under compression in a high-performance polymeric fiber
derived from rigid-rod liquid crystal is shown in Fig. 2 (Kozey and Kumar, 1994).
Note that this is a single fiber with preexisting striations on the surface. High-strength