Page 264 - Handbook of Properties of Textile and Technical Fibres

238 Handbook of Properties of Textile and Technical Fibres

7.10 The toughness of cotton fibers

The toughness of cotton fiber is reflected by the energy or the work to break (work of
rupture), which is determined by the area under the loadeelongation curve and the
units are in cN$cm or Jules. The specific work of rupture is determined by the area un-
der the stressestrain curve and the units are in cN/tex or Pa. In general, a fiber can be
strong, but not very tough (e.g., long vegetable fibers such as linen). This means that
although the fiber is strong, it may fail easily under excessive external stress applied in
a short period (e.g., impact force). When cotton fibers are compared to wool fibers, one
will find that cotton is significantly stronger but considerably less tough than wool
fibers. A typical value of the specific work of rupture of a cotton fiber is in the range
from 0.5 to 1.5 cN/tex (7.5e22.5 Pa), whereas a typical value for wool fibers is in
the range from 2.7 to 3.8 cN/tex (35e50 Pa). These values are far less than those typi-
cally found in staple-moderate strength polyester fiber, which is about 12 cN/tex
(155 Pa).

7.11 Cotton fiber elastic recovery

Fiber elasticity is determined by the extent of recovery of a fiber after being subjected
to loading and unloading action, as shown in Fig. 7.7(a). The key parameter of
elasticity is the so-called elastic recovery, which is determined by the following
equation:

Elastic extension
Elastic RecoveryðERÞ ¼
Total extension
An elastic fiber is one that exhibits full dimensional recovery on removal of loading.
This will lead to an elastic extension equal to the total extension, or elastic recovery of
one (or 100% if elastic recovery is expressed in percentage). The cotton fiber exhibits
only partial elasticity; meaning it will likely suffer some permanent elongation on
loading and then unloading. Following, the well-accepted elastic recovery values
that were reported early by Meredith (1945) and Elmogahzy and Chewning (2001)
developed the stresseER and straineER relationships shown in Fig. 7.7(b) and (c).
These relationships can be used as a general guideline for such relationships for
most cotton varieties under static tensile testing conditions. A simple interpretation
of these relationships indicates that a small tensile stress amounting only to 0.1 or
0.2 cN/tex (less than 1% of the breaking stress) applied on the fiber can result in about
0.5 elastic recovery, and a small tensile strain amounting only to 3% can result in 0.6
elastic recovery. In other words, cotton fiber is prone to some permanent deformation
on loading and unloading using small levels of stress or strain, which is a serious
matter that has received little attention from the makers of textile machinery. The
deficiency in elastic recovery of cotton fibers also shed a great deal of light on the