Many mechanical tests of composite materials require the compression
loading of a test fixture. These include not only compression test
methods, but also many shear, flexure, curved-beam strength, fastener
pull-through, fracture toughness and other test methods. Often, these
fixtures are simply placed between two flat surfaces and loaded in
compression. In a universal testing machine, these flat surfaces are
provided by essential accessories called compression platens.
basic types of compression platens are used, fixed and spherical seat.
Both types are represented in Fig. 1. Those shown are 150-mm/6-inch
diameter platens, a popular size because it represents a good tradeoff
between capacity, weight and cost. The 150-mm/6-inch diameter spherical
seat platen (top center) weighs about 14 kg/31 lb. Weight and cost
increases rapidly as the diameter is increased. Smaller diameters can
be used if the application permits.
The platens at left and
right in Fig. 1 are fixed platens. As indicated in the photograph, an
attachment method is provided to rigidly connect the platen to the
testing machine. For example, at the lower right corner is an adapter
that can be threaded into the platen. Its other end is a smooth stud
that can be held in the V-grips of the testing machine. The platen in
the top center of Fig. 1 and those in Fig. 2, are spherical seat
platens. These typically consist of two thick disks held in contact
with each other by springs, as shown. One of the contacting surfaces
has a shallow hemispherical cavity in its surface. The other has a
mating hemispherical protrusion. Thus, one plate can be fixed to the
testing machine, leaving the other plate is free to swivel.
very shallow concentric grooves on the working faces of the platens are
intended as a visual aid when positioning the specimen or test fixture
in the center of the platen. The working faces must be very hard so
that the object under compression does not damage them. Thus, platens
are typically fabricated of case-hardened, or even better,
through-hardened alloy steel. It also is customary for commercial
suppliers to chrome plate the entire platen. Although plating is
unnecessary in terms of damage prevention if the platens are already
hardened, it does add rust- and corrosion-resistance.
are specific applications where only one compression platen is
required, for example, to provide a solid base upon which to support a
test fixture attached directly to the testing machine at its upper
end. A fixed platen would typically be used in this case.
the application requires two platens, top and bottom, with the specimen
or a test fixture compression loaded between them, two fixed platens
might be preferred. If these two fixed platens are well aligned in an
adequately stiff testing machine, they provide loading surfaces that
remain parallel to each other and perpendicular to the axis of the
testing machine, the desired conditions for compression loading. An
alternative is to use one fixed platen (usually on the bottom) and one
spherical seat platen. Then the spherical seat platen can swivel as
required to make full contact with what it is loading, making initial
alignment less critical. Which combination is best usually depends on
the application and testing machine characteristics. However, a
significant, nontechnical consideration is that spherical seat platens
typically cost more than three times as much as fixed platens. When
neither combination is suitable, because of testing machine
deficiencies, it might be desirable to add a compression subpress.
compression subpress is a self-contained unit, placed between two
platens within the testing machine. This device is designed to provide
loading surfaces that will remain parallel to each other and
perpendicular to the specimen’s loading axis, independent of the
testing machine characteristics. The subpress must be sufficiently
rigid so that it does not distort unacceptably under load. Also, the
loading surfaces (that is, the surface at the lower end of the loading
rod and the surface of the base anvil) must be fabricated of very hard
materials so that they are not indented or otherwise deformed by the
specimen when concentrated loads are applied.
One such test
fixture is shown in Fig. 3. The spherical cap on top of the
spherical-end loading rod (shown removed in the photo) ensures that any
misalignments of the testing machine are not transmitted to the
subpress. A linear (ball bearing) bushing in the upper portion of the
subpress frame maintains the alignment of the loading rod during axial
movement, minimizing friction by providing rolling rather than sliding
contact with the rod. Compression subpresses are available in various
sizes. The fixture in Fig. 3 has a 25-mm/1-inch diameter loading rod
and anvil, fabricated of steel, with hardened tool steel loading
In Fig. 4, a 25-mm/1-inch diameter subpress like
the one shown in Fig. 3 is shown to the left of a 50-mm/2-inch model.
Obviously, the latter can apply much greater loadings to larger
specimens, but it is much heavier, more awkward to work with, and more
expensive. In addition to size, the general configuration of the
subpress can be modified, for example, to accommodate very tall
specimens or short specimens while keeping the length of the loading
rod as short as possible.