The Effect of Stitching with Conductive and Nonconductive Materials on the Mode I Interlaminar Fracture Toughness of Carbon Fiber Composites

This study presents the effects of the stitching with seven threads, having a variety of mechanical properties from brittle to highly ductile behavior, on the delamination resistance/interlaminar fracture toughness of stitched carbon fiber‐epoxy laminated composites. Conductive Copper, Titanium, and Kevlar metallic, as well as non‐conductive Dyneema‐13 Kg, Dyneema‐T90, Kevlar‐10 Kg, and Kevlar‐13 Kg threads were tested for their mechanical properties. Next, laminates of unidirectional carbon fiber‐epoxy composites stitched with these fibers were fabricated using a vacuum assisted resin transfer molding process. Double cantilever beam tests were conducted on samples from the stitched and unstitched composites for comparison. Subsequent fractographic analysis using scanning electron microscope and optical microscope were conducted to reveal the relation between the stitch type and the energy release rate. Experimental results showed that during the crack initiation stage, composites stitched with Kevlar metallic exhibited an average 393% increase in Mode I critical energy release rate, G_IC, the largest increase of the seven thread material types studied. At the fully developed crack propagation stage, composites stitched with Dyneema exhibited a 984% increase in G_IC. However, composite stitched with copper and titanium exhibited 375% and 497% increase in G_IC at the propagation stage, respectively. SEM and optical microscopic analysis revealed a strong relation between the inherent features of the thread materials and the delamination resistance. POLYM. COMPOS., 40:E1252–E1262, 2019. © 2018 Society of Plastics Engineers