Page 90 - Handbook of Properties of Textile and Technical Fibres

Properties of wool 71

tensile tester or a dedicated ATLAS instrument (Australian Wool Testing Authority).
The peak force is normalized by the linear density of the staple to give a result in
N/ktex. Typical values are between 15 and 55 N/ktex with wool below 30 N/ktex
referred to as “tender” and discounted at auction. Staple strength does not correlate
well with the average strength of individual fibers (Thompson et al., 1995; Peterson
et al., 1998; Thompson, 1998); rather it is a measure of the reduced number and diam-
eter of fibers at a distinct point in the staple and the degree of alignment of individual
fibers in the staple (Schlink et al., 2000). It nevertheless remains commercially very
important because it is used (along with measures of staple length, the proportion of
midbreaks, and mean diameter) in prediction equations such as TEAM (1988), which
enable exporters and topmakers to objectively estimate the average length of fiber in
the wool top and the amount of short fibers that are removed during processing.

3.4.2 Yarn

Yarn strength can readily be measured on a standard tensile tester, but because of the
commercial importance it is routinely tested using an automated dedicated tester such
as the Tensorapid (Uster Technologies AG) or Statimat (Textechno Herbert Stein
GmbH & Co. KG). Most of these tests involve long gauge lengths where the gauge
length is much greater than the fiber length. The consequence of this is that the yarn
strength is heavily dependent on the structure and integrity of the yarn. Twist in the
yarn is critical to developing transverse pressure and hence axial friction which allows
the fibers to be gripped. At low twist levels (low helix angle), fibers mostly pull out of
the assembly leading to low strength. As the helix angle increases, strength increases to
a maximum before dropping again at high levels of twist. While extremely important
commercially, yarn strength is more about the yarn structure than the material proper-
ties of the wool and hence is largely outside of the scope of this chapter.

3.4.3 Fiber bundles
Although single fiber tests are readily done on a standard tensile tester, many fibers
must be tested to get a meaningful average. An alternative method is to test bundles
of fibers. While much quicker, the method introduces additional complications in
bundle preparation, particularly in crimped fibers such as wool (Yang et al., 1996).
Bundles can be prepared and tested on a standard tensile tester or a dedicated bundle
tester such as the Sirolan-Tensor developed by the Commonwealth Scientific and In-
dustrial Research Organisation (CSIRO). Gauge lengths are always short, typically
0e5 mm. The shorter the gauge length the less accurate the strain data, with the
data becoming completely meaningless at a nominal zero gauge length.
The mean strength of a fiber bundle is expected to be less than the sum of the
strengths of individual fibers constituting the bundle. This is because individual fibers
in the bundle have slightly different lengths and also break at different strains (Yang
et al., 1996). Thus when the peak load in the bundle curve occurs, some fibers may
have already broken and some will not yet have reached their maximum load. The
different fiber lengths generally arise because of different levels of crimp in the fiber