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A test program has been conducted in which the effect of specimen thickness on the fatigue lives of quasi-isotropic, graphite/epoxy lamintes was investigated. Three replicate specimens for each notched and unnotched geometry at 16, 32, and 64-ply thicknesses of a [90/45/0/–45]ns and [45/0/–45/90]ns laminate were tested in compression-compression fatigue. The fatigue lives of the notched specimens did not appear to be a strong function of laminate stacking sequence or specimen thickness. The stress concentration at the hole dominated over the interlaminar stresses at the straight free edge. The unnotched specimens of the [90/45/0/–45]ns laminate with tensile interlaminar normal stresses delaminated more readily than did the [45/0/–45/90]ns laminate with compressive interlaminar normal stress. The life of the 16-ply unnotched specimens was lower than was the life of the 32 and 64-ply specimens. Delaminations were located at the interface where the maximum τxz shear stress occurred regardless of the sense of magnitude of the interlaminar normal stress. Finally, a specially designed antibuckling fixture was found to be effective in preventing out-of-plane motion without overconstraining the specimen.
composite material, graphite/epoxy, fatigue, thickness effect, interlaminar stresses
Assistant professor, Texas A&M University, College Station, TX
Professor, Virginia Polytechnic Institute and State University, Blacksburg, VA