Page 323 - Fiber Fracture

FRACTURE OF NATURAL POLYMERIC FIBRES 305

INTRODUCTION

A Traditional Kew of Natural Fibres

Natural polymeric fibres have (literally) supported the development of human civil-
isation since its prehistoric beginnings. A particularly prominent role has been played
by cellulose, a polysaccharide which is one of the world’s most abundant and versatile
fibrous polymers. Cellulose fibres are the reinforcing component of wood, a natural
composite that can be fashioned into devices used for shelter, transportation, agriculture,
war, communication, ornament and recreation. Cellulose fibres have been woven into
clothing, twisted into ropes and bowstrings, and processed into papyrus and paper.
Fibrous proteins, especially keratin (wool, mohair), collagen (hide, parchment, catgut)
and silk also have a rich history and an assured future as useful materials.
There is an extensive literature on the properties - including the fracture character-
istics - of fibrous polysaccharides and proteins. Most is written from the perspective
of textile science, where traditionally the greatest practical and financial interest in these
materials has been concentrated. Analysis of the failure of textile fibres is subject to the
following considerations.
(1) Individual natural filaments are too fine and/or too short to be easily used on
their own in the weaving of cloth or the reinforcing of compositcs. Instcad, bundles of
filaments are combined into macroscopic yarns.
(2) The bundles are twisted to help distribute load among the filaments (Hearle et al.,
1980; Warner, 1995). This is necessary because the filaments have polydisperse fracture
characteristics: some are weaker than others, so an efficient load transfer mechanism
must be in place to compensate for prematurely broken filaments. Increasing the twist
leads to enhanced friction and transfer of load within the yarn, and may also increase
strength by inactivating defects in the filaments. The effect of twisting on friction and
defects can be modelled empirically, phenomenologically, or statistically.
(3) In an axially loaded yam, the individual twisted filaments are not themselves
loaded axially; in other words, the filaments are not loaded along their strongest
direction. Therefore, although some consequences of increasing the twist will tend to
increase the yam strength, other consequences will tend to decrease the strength. The
net result is that maximum strength is achieved with moderate twist (Warner, 1995).
(4) Failure and other mechanical properties do not only depend on structure at or
above the length scales of individual filaments. Structure at smaller length scales is
important too.
When native natural fibres are used in conventional textile yams, the manufacturer
has control over the macroscopic degree of twist imparted to the filaments, and (within
limits) the length of filaments used. However, (s)he at best has only partial control
over structure and properties at length scales smaller than that of the filaments. At
these smaller length scales, nature controls the structural variables that will dictate
fibre strength: the primary structure (monomer sequence) of the polymer chains,
the conformation (shape) of the chains, and the supramolecular organisation of the
chains. Often the chains adopt hierarchical helical structures, exemplified by those in
keratin (Fig. 1). Combined with the macroscopic twist in yams, the molecular and