Page 259 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 233

The above methods test cotton fiber strength in a fiber bundle or a beard form. A
more basic understanding of the tensile behavior of cotton fiber requires a single-
fiber strength testing. In practice, there are three single-fiber strength testers that are
commonly used in research laboratories around the world. These are the Mantis
single-fiber tensile tester, the Favimat instrument, and the classic Instron tester. The
Mantis single-fiber tensile tester was developed at Schaffner Technologies, Inc. and
later acquired by Zellweger Uster. It can test up to three automatically fed fibers per
minute and accumulate more than 1000 load-extension data points for each fiber
tested. The Mantis uses a pair of clamps that grip the fiber ends. It operates based
on a constant rate of extension (typically 1 mm per s). An electrooptical system is
used for measuring the projected ribbon width of each fiber prior to breaking. Key
features of the Mantis instrument include (Hebert et al., 1995; Thibodeaux et al.,
1998) the following: (1) Mantis breaking strength decreased with increasing gauge
length, (2) the convolution counts of cotton fibers can be predicted by the electroop-
tical measure of light scattering, (3) Mantis single-fiber strength and elongation
were reported to correlate well with single-fiber strength and elongation measured
on the Instron tensile tester, and (4) Mantis ribbon width was related to fiber fineness
measured by reference methods.
The Favimat instrument (Delhom et al., January 4e7, 2010) (Textechno,
M€ onchengladbach, Germany) also measures single-fiber strength and elongation at a
constant rate of extension and at different gauge lengths. It also measures the fineness
of fibers utilizing the vibroscopic technique (Montogmery and Milloway, 1952)
(Textechno, M€ onchengladbach, Germany). Single-fiber strength testing can also be
performed using the familiar Instron tester at different gauge length and elongation
rates. The displacement imposed by these instruments may be applied by a constant
speed motor, with interchangeable gears to vary the rate of elongation, or from a
computer-controlled step motor. The lower clamp is typically stationary and mounted
on the frame, while the movable upper clamp is attached to a stiff load cell containing a
strain gauge or another form of transducer. Load and elongation signals are transferred
to a computer to allow data analysis and production of different plots.

7.5 Strength characterization: the stressestrain curve

As indicated above, fiber strength and extensibility are measured during a tensile test in
which an extension is applied to a specimen of a single fiber or a bundle of fibers in
axial direction (see Chapter 2). This extension causes a tension to be developed as
the specimen is extended in length. The tension continues to build up until the
specimen breaks. The load at which the specimen breaks provides a measure of fiber
strength, and the corresponding increase in specimen length provides a measure of the
breaking elongation. This process can be described by the loadeelongation curve,
which is a plot that describes the process of stretching a specimen of a length L o by
a force F to a length (L o þ DL) at which the fiber breaks. This curve can be converted
to a stressestrain curve by simply dividing the load by the appropriate normalizing
parameter (area or fineness).