EFFECT OF STITCHING ON IMPACT AND INTERLAMINAR PROPERTIES OF GRAPHITEEPOXY LAMINATES/

Effects of through-the-thickness stitching on the impact damage resist ance, impact damage tolerance, and interlaminar fracture toughness (Mo de I and Mode II) of plain woven and uniweave textile graphite/epoxy l aminates were investigated. The uniweave textile was formed by weaving dry carbon-fiber rows with fiberglass fill tows. The content of fiber glass fill tows was 2.5% by weight. The plain woven laminates were man ufactured using resin infusion molding and the uniweave laminates by r esin transfer molding. Kevlar(R) and glass yarns of various yarn numbe rs were used for stitching. Static Indentation-Flexure, Compression-Af ter-Impact, Double Cantilever Beam and End-Notched Flexure tests were conducted. Stitching did not have any effect on the onset of impact da mage. However, stitching leads to significant improvement (25-40%) in impact damage tolerance as measured by CAI strength and impact damage area. Mode I fracture toughness as characterized by critical strain en ergy release rate (G(Ic)) was found to increase by at least an order h igher (15-30 times) than the unstitched laminates. Mode II fracture to ughness (G(Ilc)) increased by 5-15 times over the unstitched laminates . New methods to estimate the Mode II critical strain energy release r ate in the stitched laminates are presented. The stitched textile adva nced composites are considered potentially superior to prepregs for hi gh volume, low-cost and high-performance structural materials.