Page 277 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 251

intrinsic nature, which reflects the internal structures of materials. Indeed, engineers
prefer comparing different materials via their elastic modulus as it sets useful
thresholds of material classification into rigid, ductile, or flexible structures by virtue
of the material structure and bulk density.
Dynamic tensile testing can be performed using direct or indirect methods (Woo
and Postle, 1974, 1975, 1978; Wakeham and Honold, January 1951; Weyland,
1961; Lotmar and Meyer, 1937; Fujino et al., 1955; Hamburger, 1948; Woo, 1975;
Rayan and Postle, 1981). In the direct method, the dynamic modulus is derived
from the stressestrain relationship produced by the testing instrument. In 1951,
Wakeham and Honold (January 1951) reported the use of a cam-generated mechanical
vibration of 1 Hz to apply a cyclic load to the fiber specimen and obtained the elastic
modulus of single cotton fibers. The authors used the original specimen test and cross-
sectional area in their analysis. In the same year, Balls (Weyland, 1961) reported the
use of an impact loading test by employing the pendulum loading principle to break a
fiber bundle. The obvious limitation in this method is that it does not provide contin-
uous loading and unloading as a result of the need to break the specimen in every test.
The indirect methods aim directly at determining the dynamic elastic modulus
without the need to develop a stressestrain curve. Three main methods were used
for cotton fibers: (1) the free-vibration method, (2) the forced-vibration method, and
(3) the acoustic method. In the free-vibration method, the fiber is caused to oscillate
using an electromagnetic impulse (15e30 Hz) via a spring of known stiffness and a
loading weight. In this case, the dynamic modulus is determined by measuring the
periods of the oscillation with and without the fiber, or by measuring the frequency
with the aid of a photocell. In the forced-vibration method, a metallic cantilever
oscillator is used, and a strained specimen is attached to the free end of the cantilever.
The dynamic elastic modulus is then determined as a function of the fiber strain using
the natural frequency of the cantilever (Lotmar and Meyer, 1937).
The acoustic method deserves some attention because it can be applied to fibers,
yarns, and fabrics. The idea of the acoustic method is that the velocity of propagation,
V, of sound waves through a specimen can be measured. This velocity was found to be
highly related to the dynamic elastic modulus of the material, E dynamic . The general
equation relating these two parameters is (Woo and Postle, 1974):
2
E dynamic ðGPaÞ¼ r V
3
where r is the fiber density in g/cm and V is the velocity in km/s.
Using the conventional mass per unit length, or tex, the above equation can be
simplified to:
2
E dynamic ðcN=texÞ¼ 100 V

Woo and Postle (1974) classified the acoustic methods based on the sound wave
propagation pattern into two types: continuous and pulse waves. In the continuous
propagation pattern, the frequency of the vibration is usually fixed, and the distance