Page 292 - Handbook of Properties of Textile and Technical Fibres

266 Handbook of Properties of Textile and Technical Fibres

Despite the numerous studies of yarn strength, no universal model exists today that
can fully explore or predict the mechanical behavior of staple-fiber yarn under tensile
loading. This is primarily due to the overwhelming stochastic nature of spun yarns
making where it is very difficult to achieve a complete resolution of the different
factors influencing yarn strength.
Early in this chapter, we discussed the concept of loadeelongation (or stressestrain)
curve and the different parameters that can be derived from it. This concept basically
holds for any material subject to tensile loading including textile yarns. Accordingly,
the parameters associated with the curve (e.g., breaking stress, toughness, modulus,
etc.) can be used for characterizing yarn strength. The shape of the curve, however,
varies widely depending on many factors including yarn type (ring, rotor, or air-jet),
twist level, and yarn texture. In practice, the strength of staple fiber yarn is commonly
described using the following parameters: skein strength (lbs), count-strength product
(CSP), single-end strength (cN/tex), strength irregularity (CV strength %), breaking
elongation (%), and work of rupture (cN$in cm).
The skein strength is typically measured by winding a 120-yard (w110 m) skein on
a wrap reel of 1.5 yards (w1.4 m) circumference. The yarn is then removed from the
reel and tested in the form of 80 revolutions of parallel threads at a constant rate of
traverse. When the specimen is subjected to tensile loading, all threads will resist
the loading until a break occurs in one of the threads (the weakest point). The remain-
ing unbroken threads will then support the skein until a second thread breaks. This
process continues through a succession of thread breaks until a total failure occurs.
It is believed, therefore, that the skein strength test provides a combined measure of
the strength of a composite specimen of yarns and the interyarn friction. The parameter
obtained from this test is called the skein or lea strength and is expressed in
pounds.
The skein strength test is commonly accompanied by a yarn count test in which the
same test specimen is weighed to determine the cotton count. The CSP provides a
strength measure commonly known as the skein break factor (Ne$lb). In practice,
this measure is used more common than the absolute value of skein strength. Typical
values of skein break factor for different yarn types are given in Table 7.2. These
values are based on yarn data corresponding to cotton US crops in 1990 and 1991.
The wide range of CSP values is, therefore, a result of the wide range of values of fiber
characteristics used in the make of the yarns.
The single-end strength represents a more fundamental parameter than the skein
strength. Using modern tensile testers (e.g., Uster Tensorapid), strength parameters
can be obtained at a constant rate of extension of 5 m/min and a gauge length of
50 cm. These parameters include breaking load, breaking elongation, loadeelongation
(or stressestrain) curve, yarn tenacity, yield stress and strain, work of rupture, and
tensile modulus. Typical values of strength parameters of different types of cotton
yarns and at different values of yarn count are listed in Tables 7.3e7.5. Another tensile
tester, also developed by Uster, called the Tensojet allows measuring strength
variability from many breaks. This tester operates at a very high rate of extension
(400 m/min). Using this tester, up to 30,000 tests per hour can be performed.