Page 257 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 231

7.4 Testing methods of the tensile behavior of cotton
fibers

The tensile behavior of cotton fibers can be characterized using two main types of
testing (Elmogahzy and Chewning, 2001; ASTM D1445-05; ASTM D5867-05,
2005; ASTM International, 2007; Booth, 1968; Hamby, 1949): (1) static testing and
(2) dynamic testing. Key differences between these two categories of testing include
the following:
1. Static tensile testing is the most common type of testing in quality control departments and
research laboratories, and it is typically performed for determining the inherent tensile
characteristics of cotton fibers for cotton classing purposes, comparing different types of
cotton, or establishing thresholds and criteria of the different basic tensile parameters of
cotton fibers.
2. Dynamic tensile testing is less common and is often restricted to research laboratories for the
purpose of testing the tensile behavior of cotton fibers under special applications such as
repeated loading and unloading or under impact effects. Key tensile parameters obtained
from this type of testing are the dynamic elastic modulus, fatigue, and yield point.
3. Static testing always involves mechanical means via gripping the fibers at two ends and
stretching them to the breaking point; a process that results in a classic stressestrain curve.
4. Dynamic testing may be performed using mechanical means or other indirect means such as
sound waves or oscillating mechanical signals.
5. Static testing is always destructive as it ends with fiber breakage, while dynamic testing may
be achieved nondestructively.
6. Dynamic testing is always performed at higher speeds than static testing.

7.4.1 Static testing of the tensile behavior of cotton fibers
The basic principle of testing the tensile behavior of a fiber is by simply gripping the
two ends of a single fiber at a predetermined gauge length between the two clamps.
Using a tensile testing device, this process can lead to a loadeelongation curve that
can be converted into a stressestrain curve from which all necessary tensile parameters
can be calculated. Because fibers are never represented in practice in a single form,
there is also a need to test and characterize the tensile behavior of a fiber bundle or
a group of fibers to simulate the straining behavior during processing or the contribu-
tion of fiber strength to the durability of the textile product. Fig. 7.3 illustrates the
general principle of single-fiber and bundle tensile testing.
In practice, the flat bundle test for cotton fiber strength has received wide
acceptance since its introduction in 1953. For many years, two testing instruments
dominated the research laboratories of cotton fiber strength (ASTM D1445-05): the
Pressley fiber-bundle strength tester and the Stelometer tester. These systems are still
used in some countries, and they are considered as references to some of the more
advanced instruments because of their basic manual setup.
The Pressley fiber-bundle strength tester was developed in 1939 by Dr. E. H. Press-
ley, who was a cotton breeder at the University of Arizona. This tester operates based
on the inclined plane principle to test a flat bundle of parallel fibers clamped between a
set of jaws at zero-gauge length or at a gauge length of 1/8 in (3.175 mm). The tester
utilizes a mobile weighing trolley, which is released to go down the inclined plane to