Page 45 - Handbook of Properties of Textile and Technical Fibres

26 Handbook of Properties of Textile and Technical Fibres

the case of natural fibers. In this case, and in the case of too “soft” fibers, the polishing
can indeed induce deformations of the observed section. Other techniques should
therefore be preferred.

2.2.4 Variabilities along the length of a fiber

There is often a considerable scatter in fiber diameters between fibers taken from a
bundle. However, with many fibers their diameters can also vary along their length,
which makes conversion of the breaking load into failure stress more difficult and in-
creases the dispersion of the results. A technique described by Morimoto et al. (1998)
allows the diameter of a fine fiber to be measured at several sections along its length.
Two rectangular flat glass slides are separated from each other by two short lengths of
glass fiber, between the slides, at one end and one short length of glass fiber placed
between them at the other end. The glass fibers are placed parallel to the longest
side of the slides and positioned to form an isosceles triangle. The fiber to be tested
is then placed at right angles over the single short length of glass fiber so that its thick-
ness at the point of contact lifts the one slide. The angle between the glass slides is
measured precisely using the optical interference technique with a HeeNe laser,
and the diameter of the fiber is simply calculated knowing the distance between the
spacing glass fibers. A maximum error of 0.1 mm is claimed for this technique.
A particularly interesting technique for measuring fiber “apparent” diameter, both at
a given point on the fiber and for scanning along the length of the fiber, is provided by
the Japanese company, Mitutoyo. Their apparatus is suitable for fibers with diameters
from a few microns up. A laser beam scans across the fiber, which is held perpendic-
ular to the laser beam between two grips. The time of occlusion of the light is measured
by a light cell and the diameter calculated. The measurements should be verified
initially, for each fiber type, by comparison with results from a scanning electron
microscope (SEM), but after this is done, measurements of fiber diameter can be
made rapidly and with great accuracy. Verification with results from an SEM is advis-
able as the light from the laser may interact differently with different fibers, so that
refraction at the surface of transparent fibers can occur and differences in surface
roughness between different types of fibers can also modify the results. The equipment
can be arranged so that the length of the fiber can be scanned and variations of diameter
easily determined. It is also possible to rotate the fiber on itself to determine an estimate
of the cross section in the case of noncircular fibers. This efficient dimensional analysis
solution is supplied by Dia-Stron Ltd.
For carbon fibers or ultrafine fibers, a greater resolution could be obtained by using
laser interferometry. This technique employs a low-power laser beam (<0.5 mW), for
instance, of the HeeNe type. A sensor is placed normal to the beam, and the fiber is
held in front of the sensor, perpendicular to the laser beam source. The interference
pattern varies in intensity as is shown in Fig. 2.2. The diameter of the fiber (d) is given by

s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2L

d ¼ nl 1 þ (2.8)
DZ n