SUBLAMINATE BUCKLING AND COMPRESSION STRENGTH OF STITCHED UNIWEAVE GRAPHITE EPOXY LAMINATES/

Effects of through-the-thickness stitching on the sublaminate buckling and residual compression strength (often referred as compression-afte r-impact or CAI strength) of graphite/epoxy uniweave laminates are exp erimentally investigated. Primarily, three stitching variables: type o f stitch yarn, linear density of stitch yarn and stitch density were s tudied. Delaminations were created by implanting teflon inserts during processing, The improvement in the CAI strength of the stitched lamin ates was up to 400% compared to the unstitched laminates. Stitching wa s observed to effectively restrict sublaminate buckling failure of the laminates. The CAI strength increases rapidly with increase in stitch density. It reaches a peak CAI strength that is very close to the com pression strength of the undamaged material. All the stitch yarns in t his study demonstrated very close performance in improving the CAI str ength. It appears that any stitch yarn with adequate breaking strength and stiffness successfully restricts the sublaminate buckling.