Elastoplastic response of unidirectional graphite/aluminum under combined tension-compression cyclic loading
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 in each 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.