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Results are presented of an inelastic finite-element analysis of the residual microstress state in a unidirectional graphite/epoxy composite subjected to temperature cycling from room temperature (294 K) to 405 K or a combined cycle of changing matrix moisture content from a saturated condition (5.6 percent water by weight) to 2.0 percent moisture content with a simultaneous temperature change from room temperature to 339 K. In the analysis, the graphite fibers are modeled as being transversely isotropic, and assumed temperature- and moisture-dependent properties of modulus and strength for epoxy are utilized.
The results indicate that relatively large residual microstresses result from the cooldown of the composite from the curing temperature (450 K) to room temperature, and that in portions of the matrix material the residual stresses nearly exceed the yield strength of the material for a 60 percent volume composite. It is shown that moisture saturation of the matrix causes matrix yielding in both 40 and 60 percent fiber volume composites. It is noted that subsequent thermal cycling, or hygrothermal cycling, promotes definite changes in these microstress states which cause dimensional instability of the composite.
hygrothermal cycling, composite materials, temperature-dependent properties, moisture-dependent properties, microstress, finite-element analysis, dimensional instability
Member of technical staff, Sandia Laboratories, Albuquerque, N.M.
Professor, University of Wyoming, Laramie, Wyo.