Page 64 - Handbook of Properties of Textile and Technical Fibres

Testing and characterization of fibers 45

The fiber is held horizontally between two clamps. One clamp is connected to a
movable crosshead that also contains the load cells. A displacement transducer records
the total movement of the crosshead during a test.
The steady load is measured by one load cell and the cyclic loads, during a fatigue
experiment, are monitored by a sensitive load transducer. The loading conditions of
interest are preselected and an electronic servo system controls the distance between
the jaws and so regulates the load conditions on the fiber.
The tensile strength and modulus of a fiber are determined by straining the fiber in
tension until failure. The strain rate used is often adjusted to result in fiber failure after
approximately 20 s. The loadeelongation curve for the fiber is recorded by a computer
or on a curve plotter. The fiber’s failure stress and strain, yield strength and strain,
initial modulus, secant modulus, and work of rupture may be determined from this
experiment.
In the absence of sufficiently sensitive equipment, pultruded specimens of unidirec-
tional composite composed of strands of the fibers embedded in a matrix can be tested
to failure in tension. The failure load of the specimen is divided by the number of fibers
in the strand. This technique is often used and can give slightly different results from
those found with single fibers. This is because the strength of fibers varies, on average,
with gauge length, and often an average fiber diameter is used, which in practice is
rounded down to the nearest micron. This leads to an overestimation of fiber properties
as the calculation of strength and elastic modulus requires dividing breaking load by
the square of the diameter, and even a reduction of a fraction of a micron on the
real diameter can result in a significant increase in the calculated values.

2.4.1.3 Automated tensile tests
To determine single fiber strength distributions and applied descriptive or inferential
statistics, a sufficient number of tests must be conducted. Similarly, from an industrial
point of view, it is generally necessary to check the quality of a production through
numerous mechanical characterization tests. Accurate, repeatable, and fast methods
are thus required. In the context of quality control, it is also desirable to remove the
biases that could arise from the intervention of different “testers,” even when a stan-
dard exists. Many instruments have been developed to obtain the tensile properties
of single fibers. Most of these instruments are not designed to automatically test a
series of fibers. The following paragraphs describe two of the most advanced and
currently available instruments for conducting automated single fiber tensile tests. It
also provides some explanation about the fiber preparation procedures that could
not be easily automated depending on the nature of the tested fibers.
Dia-Stron Ltd is an English company specialized in delivering test instruments and
measurement solutions especially for fibers. Applications include hair, wool, silk and
other protein-based fibers, cotton, hemp, leaf and other vegetable fibers or polyester,
carbon, aramid, and other synthetic fibers. Dia-Stron Ltd provides a modular system
of manual and automated units, for tensile testing, dimensional properties, failure anal-
ysis, and bending moment. Dia-Stron’s initial brief was to develop instrumentation for
the measurement of skin and hair properties for dermatology and personal care. From