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H I G H - P E R F O R M A N C E C O M P O S I T E S
TESTING TECH
men mid-thickness (the neutral axis of
bending stress). These screws prevent
axial and lateral movement.
Finally, because the specimen is con-
tinually defecting (bending) during
each cycle, each of the loading/support
clamps must be free to rotate so that
they do not constrain the bending of the
specimen.
An example of a test fxture that meets
all of the above requirements is shown
in Fig. 1. The base and loading beams
are directly attached to the base and
crosshead of a testing machine capable
of applying an axial load in both ten-
sion and compression. The specimen
loading and support span lengths can
be varied by loosening the two bolts at
the outer end of each loading/support
assembly and sliding the assemblies
along the beam to the desired position.
These bolts can be seen at the top of the
image in Fig. 1. As shown, the fxture is
set up to perform four-point loading. For
three-point loading, one of the loading
head assemblies is removed and the
remaining head assembly is moved to
the center of the beam. (The comparative
merits of three- and four-point loading
were discussed in my previous columns.)
A pivoting cradle is mounted in each
loading/support assembly (the bushings
in which the cradle axes pivot appear as
white circles in Fig. 1). These particu-
lar bushings were fabricated of a Tefon
polymer. Oil-impregnated bronze bush-
ings are stronger, are fabricated to closer
tolerances and, thus, are more suitable
for high fxture loadings, but the Tefon
bushings generate little friction and,
therefore, wear very well.
The fxture shown has fat loading and
support pads since it is designed to be
used in sandwich panel testing. Alterna-
tively, cylindrically shaped pads can be
used — these are sometimes specifed for
solid composite laminates. The pads are
clamped against the specimen by tight-
ening the bolt in each clamp half. Using
two bolts permits the user to accommo-
date specimens of different thicknesses
while keeping the specimen centered in
the cradle.
Although there has been some inter-
est in reversed fexural fatigue testing of
composites for many years, there is at
the present time no ASTM standard for
such testing. However, there is a rela-
tively new standard for plastics, ASTM D
7772-12, "Standard Test Method for Flex-
ural Fatigue Properties of Plastics,"
which does include reversed loading.
Further, reversed fexural fatigue testing
of composites appears to be increasing
signifcantly. This is, perhaps, an indica-
tion of this industry's progress toward
maturity — and the need for more realis-
tic service data as applications become
more demanding.
L E A R N M O R E
@
w w w. c o m p o s i t e s w o r l d . c o m
Read this article online at short.
compositesworld.com/RLFlex.
Dr. Adams' commentary on "Flexural testing
of composite materials" can be read in HPC
March 2013 (p. 11) or by visiting
short.compositesworld.com/H85ksD8P.
Dr. Adams' discussion of "Flexural test method
standards for composite materials" can be
read in HPC May 2013 (p. 11) or by visiting
short.compositesworld.com/7hguppDi.
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