Effect of Stitching on Impact and Interlaminar Properties of Graphite/Epoxy Laminates

Abstract

Effects of through-the-thickness stitching on the impact damage resistance, impact damage tolerance, and interlaminar fracture toughness (Mode I and Mode II) of plain woven and uniweave textile graphite/epoxy laminates were investigated. The uniweave textile was formed by weaving dry carbon-fiber tows with fiberglass fill tows. The content of fiberglass fill tows was 2.5% by weight. The plain woven laminates were manufactured using resin infusion molding and the uniweave laminates by resin transfer molding. Kevlar and glass yarns of various yarn numbers were used for stitching. Static Indentation-Flexure, Compression-After-Impact, Double Cantilever Beam and End-Notched Flexure tests were conducted. Stitching did not have any effect on the onset of impact damage. 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 energy release rate (G,Ic) was found to increase by at least an order higher (15-30 times) than the unstitched laminates. Mode II fracture toughness (GIIc) increased by 5-15 times over the unstitched laminates. New methods to estimate the Mode II critical strain energy release rate in the stitched laminates are presented. The stitched textile advanced composites are considered potentially superior to prepregs for high-volume, low-cost and high-performance structural materials.