Page 263 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 237

influencing it. In addition, a cotton fiber typically exhibits natural crimp, which is
important for fibers to be adhered together during processing. As a result, the initial
loading of a cotton fiber in a single-fiber strength test can be fully consumed in
removing this crimp, leading to initial overestimation of the inherent fiber elongation.
In bundle strength tests, cotton fibers are combed prior to testing, which assists in
removing a great deal of crimp but can also elongate the fibers unnecessarily. Again,
this can lead to an error in testing the actual elongation value of the fiber bundle. These
factors can be multiplied by the high sensitivity of the fiber to testing parameters such
as the degree of tightness of fiber clamps and the initial momentum imposed by the
movement of the clamp to apply the load during testing.

7.8 Stiffness or tensile rigidity of cotton fibers: the
elastic modulus

Fiber stiffness is commonly determined by the initial slope of the stressestrain curve
(tan q in Fig. 7.4). This is commonly known as the initial modulus or the elastic
modulus; the higher the initial modulus, the stiffer the fiber. In practice, the stiffness
of cotton fiber is not routinely measured, and standard values of cotton fiber stiffness
are not well established. This measurement is associated with the same obstacles
discussed above in the context of fiber elongation. The natural crimp of the cotton
fibers can easily provide a misleading value of the inherent stiffness of cotton fiber.
This means that the cotton fiber specimen should be prepared carefully to remove
the crimp prior to testing. A perfect specimen preparation process in which the crimp
is totally removed does not exist. Nevertheless, typical values of cotton fiber initial
modulus reported in the literature (Morton and Hearle, 1975) may range from 350
to 800 cN/tex (5e12 GPa). This range is much higher than that for wool fibers, which
is 210e300 cN/tex (3.2e4.5 GPa), but lower than staple-moderate strength polyester
fiber, which is about 900 cN/tex (13.5 GPa).

7.9 The yield point

Asillustrated inFig.7.4, the yield point isa point onthe stressestraincurvebeyondwhich
the material enters the phase of nonlinear pattern and irrecoverable strain or permanent
(plastic) tensile deformation. The yield point is defined by the corresponding yield stress
and yield strain. In engineering terms, the work-to-yield is approximately determined by
1
/ 2 (yield stress yield strain) because of the rough triangular shape of the stressestrain
curve upto the yield point. In engineering design, the yield point is critical in the construc-
tion of material assemblies. As shown in Fig. 7.5, a typical stressestrain curve of a cotton
fiber will not exhibit a clear yield point. This has led to an analysis in which the yield point
is approximated as shown in Fig. 7.4. For example, Meredith (1945) estimates the yield
point by the point at which the tangent to the curve is parallel to the line joining the origin
and the breaking point, and Coplan and Singer (July 1953) estimates the yield point as the
point occurring at the stress given by the intersection of the tangent at the origin with the
tangent having the least slope.