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The swelling behavior of a graphite/epoxy (AS/3501-5) is shown to be described by a model which is based on the assumption that the swelling is negligible until the moisture concentration reaches a threshold value and then increases linearly thereafter. The model also provides a means of measuring the volume fraction of voids. The analytical results, which take into account the curing stresses, are compared with the experimental data for the transverse and thickness strains of symmetric laminates as well as for the warpage of unsymmetric laminates. The initial absorption in virgin specimens facilitates the subsequent diffusion, and the residual stresses appear to be responsible for the absorption being slower than the desorption during the early stage of moisture conditioning. Warpage of unsymmetric laminates is discussed in light of the cracks found in constituent plies. Finally derived is a temperature-relative humidity relationship which renders the graphite/epoxy composite laminates free of residual stresses.
swelling, residual stress, graphite/epoxy, composite materials, absorption, desorption, hygrothermal strain
Materials research engineer, Air Force Materials Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio
Research engineer, University of Dayton Research Institute, Dayton, Ohio