A test fixture for combined tension-compression cyclic testing of unidirectional composites was
designed and characterized using 606l-O aluminum specimens. The elastoplastic response of
graphite/aluminum l5° off-axis and 90° specimens under tension-compression cyclic loading was
subsequently investigated at three temperatures, -l50°F, room temperature and 250°F. The test
results showed that the tensile response was predominantly elastoplastic, whereas the compressive
response could not be characterized exclusively on the basis of the classical plasticity theory. Secondary
dissipative mechanisms caused by inherent voids in the material’s microstmcture had an
apparent influence on the elastoplastic behavior in compression. At different test temperatures, the
initial yield stress in tension and compression were translated in the tension direction with increasing
temperature. This is believed to be caused by residual stresses induced inieach phase of the composite.
The micromechanics model proposed by Aboudi was subsequently employed to correlate
the experimental and analytical results at room temperature. A semi-inverse methodology was incorporated
to determine the in-situ properties of the constituents. Comparison between the analytical
and experimental results showed good agreement for monotonic tensile response. For
tension-compression cyclic loading, fairly good correlation was obtained for l5° specimens, but
poor for 90° specimens. The major cause of the discrepancy is suggested to be caused by the secondary
dissipative mechanisms.
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