Page 68 - Handbook of Properties of Textile and Technical Fibres

Testing and characterization of fibers 49

with U 1 the transverse displacement (mm), F the applied load normalized by the
1
compressed length (N mm ), and R the fiber radius (mm).

2.4.3 Four-point bending technique to determine compressive
properties

A four-point bending beam method has been used to determine the compressive prop-
erties of several different types of fiber. Raman spectroscopy has been used to follow
the molecular deformation of aramid, carbon, and alumina-zirconia fibers (Young
et al., 1996). Certain Raman bands have been found to be sensitive to the applied stress
and shift to lower frequencies under tension and to higher frequencies on compression
(Andrews et al., 1996; Vlattas and Galiotis, 1994). This behavior reflects the deforma-
tion of the polymer backbones or other atomic bonds in response to the applied stress.
It is necessary to know the Raman band shift as a function of stress of the fiber so that a
stressestrain curve can be determined (Colomban, 2002). The fiber to be studied is
placed on the surface of a rectangular polymethyl methacrylate (PMMA) beam and
covered with a solution of PMMA/chloroform to seal it into the surface. The beam
is loaded in four-point bending, and the strain gauges measure the strain of the concave
surface. As described previously in Section 2.3.4, the shift in frequency of the Raman
peak then gives a direct measurement of the strain of the fiber at the molecular level.

2.4.4 Elastica loop test
The loop test was originally described for obtaining the tensile properties of fibers
(Sinclair, 1950; Jones and Johnson, 1971). However, in this type of test, most organic
fibers will yield in compression by developing shear bands known as kink bands. The
fiber is twisted into a loop and the size of the loop reduced until the first kink band is
observed at the bottom of the loop where the radius of curvature is smallest. Fig. 2.19
shows the experimental arrangement as described by Fidan et al. (1993). The test is
usually conducted under a microscope with the fiber specimen positioned in an oil
film, to aid observation, between two glass slides or in an SEM. When the first kink
band is observed the loop size is recorded and the radius of curvature measured or
calculated so as to obtain the critical compressive strain ε cr , which is given by:

(2.19)
ε cr ¼ d=2R m
y

MT MT
T T
Y Rm L

x
Figure 2.19 Elastica loop test. The fiber is twisted into a loop that is reduced in diameter until
the plastic deformation or failure occurs.