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Biaxial tests to determine stiffness and strength properties have been carried out on carbon/epoxy material systems using tubular specimens. The tests have included AS4, IM7, and T800 carbon fibers in a variety of layups that include fibers in [0/ ±60] layups, [0/ ±45/ 90] in quasi-isotropic layups, and layups with extra fibers in the loading or off-axis directions, as well as layups in which the loading directions were at an angle with respect to the fiber directions, and specimens with 2-D triaxial braids. Loadings included biaxial tension, biaxial compression, mixed tension and compression, and compression under superposed pressure. The tests show a number of features that can be interpreted on both a macroscopic and a micromechanics level. The results for specific layups and materials show that ultimate laminate failure can be accurately correlated by means of a critical fiber direction strain. However, relating the laminate failure values to the properties of the fiber and matrix requires a more detailed examination at the micromechanics level. Ultimate fiber direction tensile strain values apparently depend on the details of the interaction of matrix cracking and fiber-matrix inter phase strength, and thus in situ fiber strength in a laminate differs from that in a tow test.
carbon fiber composites, composites, biaxial tests
Professor, University of Utah, Salt Lake City, UT