Advanced composites technology Page: 4 of 17
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to fail at half the launch pressures that it has seen in numerous firings. A recent survey of the
available studies on the effects of triaxial compression stresses on compression and shear
strengths shows that results vary significantly.1 The disparities are most certainly due to
variations in materials, test specimens, and test methods used in the different studies. Many
of the tests utilized nontraditional methods to perform tests in high-pressure cells designed
for studying isotropic materials. While strength changes due to superimposed high pressure
have been measured, it is clear in many of these studies that the failure modes were quite
different from pure compression or shear.
In order to make the next leap in improving sabot performance, we need to have a
predictive capability using existing computer simulations. The proposed design concepts
incorporate new fiber architectures, structural shapes, and materials. To study the numerous
combinations of these new approaches in a purely empirical manner would be impractical.
We are developing a scientific approach to these issues by conducting the critical
experiments to validate or develop a proven failure model. For this purpose, we have
designed and constructed a triaxial compression test system that utilizes standard composite
test specimens. The hardware normally used for uniaxial tests has been adapted to fit within a
high-pressure cell capable of maintaining constant hydrostatic pressure up to 100 ksi during
uniaxial testing. During the past year we have made additional upgrades to the system and
improved testing procedures to the point where high-pressure testing can be conducted in a
reliable and facile manner. The features of this test system and initial results for the effect of
pressure on compression strength of isotropic and fiber composite materials will be
System Design and Upgrades
In adapting proven composite compression test methods and fixtures for use in the
confined spaces of a high-pressure vessel, a limitation is the size of the specimen relative to
the bore of the vessel. The hardware required to prevent erroneous failure modes such as end-
brooming and longitudinal splitting takes up a significant percentage of the cross-section.
Consequently, the reaction loads generated due to pressure are nearly an order of magnitude
greater than the uniaxial force required to fail even the strongest composite specimens.
Furthermore, due to the relatively low compressibility of the liquid pressure medium, small
fluctuations in pressure occur as the servohydraulic supply pressure unit tries to maintain a
constant pressure while the piston is lowered into the vessel. A 500-psi fluctuation (which is
less than 1% of the higher superimposed pressures) results in over 600 lb of reaction force on
an external load cell. This amounts to approximately 10% of the failure load for some
specimens and therefore can potentially mask any real changes in strength due to the triaxial
stress state on the specimen. Compounding this problem with measuring load externally is
the added frictional force at the high-pressure seals in the piston packing. Our original design
for circumventing these problems using an internal load cell was based on measuring the
deflection of a hollow piston with a Bourns potentiometer. The body of the potentiometer
was mounted above the piston and the pressure seal in a hollow stand-off. In this
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DeTeresa, S J; Groves, S E & Sanchez, R J. Advanced composites technology, report, October 1, 1998; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc671614/m1/4/: accessed November 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.