Page 266 - Handbook of Properties of Textile and Technical Fibres

240 Handbook of Properties of Textile and Technical Fibres

importance of understanding the dynamic tensile behavior of cotton fibers as will be
discussed later in this chapter.

7.11.1 Key aspects related to cotton fiber strength testing

In this section of the chapter, the two main critical aspects related to cotton fiber tensile
behavior are:
1. The type of testing: single-fiber strength versus bundle (beard) strength
2. The moistureestrength relationship of cotton fibers

7.11.2 The type of testing: single-fiber strength versus bundle
strength
As indicated earlier, the bundle strength (or beard strength) test is the most commonly
used method for testing cotton fibers. It has received a wide acceptance among cotton
producers and textile technologists over the years, particularly after the introduction of
the HVI system, which allows an ease of testing, fewer samples, and a semiautomated
testing method. In comparison with single-fiber strength testing, bundle strength
testing provides higher consistency and less variability in the values of fiber strength.
This consistency has much less to do with the inherent nature of cotton fiber strength
and a lot to do with the way fibers are tested. It is a direct result of many reasons,
including:
1. A bundle or a beard of fibers typically contains thousands of fibers (4000 to 6000 fibers per
beard). This large number of fibers will certainly result in less between-beard strength
variation in comparison with single-fiber testing, particularly if fiber samples are selected
randomly from the cotton bale.
2. The fiber beard is selected at random points of the fibers. This action randomizes many of
the potential testing noises, including the differential surface effect and the extent of
deconvolution discussed earlier.
3. The fiber specimen is automatically combed and brushed in the HVI specimen preparation
station.
4. The gauge length used, being 1/8 in (3.175 mm), associated with the random gripping of
fibers, masks a great deal of noise that could result from interfiber displacement across the
fiber beard, the presence of short fibers or fiber fragments in the beard, and the need to
decrimp the fibers before testing.
5. The use of calibration cottons for the HVI calibration with different levels of fiber strength
largely sets artificial boundaries on the values produced by the HVI.
In a single-fiber strength test, the critical issue is the sample size or the number of
fibers that should be tested to obtain reliable results. This point will be discussed later
in this chapter. In addition, dealing with single cotton fibers requires a great deal of
sample preparation and technician’s skills. Indeed, straightening a single cotton fiber
that has initial significant crimp can be challenging. In addition, the geometrical effects
resulting from testing a noncylindrical fiber and the fiber tapering effect make it more
difficult to avoid a great deal of testing noise. A calibration for a single-fiber testing
may seem easier than that for bundle strength testing because it can be done using a