Page 258 - Handbook of Properties of Textile and Technical Fibres

232 Handbook of Properties of Textile and Technical Fibres

Common testing criteria:
• Cotton variety information
• The reliability and validity of data
• Testing environment
Output: stress-strain curve
Stress

Single-fiber strength Bundle strength
Strain
Pressley ®
HVI ®
Favimat ®

Instron ® Stelometer ®
Figure 7.3 The two major categories of cotton fiber tensile strength testing.

pull the fiber bundle until breakage. The breaking load is read on a graduation of the
inclined plane. The two sections of the fiber bundle are then weighed on a sensitive
scale, and the fiber strength is expressed using the so-called Pressley index calculated
as the force-to-break in pounds divided by the bundle weight in milligram. The
Stelometer uses the Pressley jaws and a pendulum principle to apply a constant rate
of loading on the cotton fiber being held at 1/8 in (3.175 mm) gauge length. Unlike
the Pressley tester, the Stelometer can measure both the breaking strength and the
breaking elongation.
In the early 1980s, a rapid fiber testing system called the high-volume instrument
(HVI) was developed (ASTM D5867-05, 2005). This system is produced by Uster
Technologies to test several key fiber properties including fiber length, micronaire,
color, trash area, and fiber strength (ASTM D-4605). In the length/strength station
of the HVI, the fiber beard, which has been tested for fiber length is repositioned
and clamped between two jaws at a gauge length of 1/8 in (3.175 mm); then, it is
subjected to the tensile test at a constant rate of elongation. With the assumption
that the linear density is constant across length groups, HVI estimates the mass of
the specimen using optical sensor and the micronaire value. Fiber tenacity is then
displayed in gram force per tex; (g f /tex ¼ 0.981 tenacity in cN/tex). In the early
generation of the HVI strength tester, problems associated with fiber beard clamping
system resulted in a low precision of the values of HVI breaking elongation as well
as an artificial dependency of these values on the fiber breaking strength. However,
the work by Roger Riley of Uster Technologies (Roger Riley Jr, 1997) in the late
1990s solved a great deal of these problems. A third significant difference between
traditional testers and the HVI strength tester is the rate or speed at which the fibers
are broken. The HVI systems break the fibers about 10 times faster than the laboratory
methods.