The impact of loop length on the mechanical performance of epoxy composites based on auxetic weft knitted polypropylene fabrics

Knitted fabrics exhibiting a Negative Poisson’s Ratio (NPR) show significant potential for advanced engineering applications, including personal protective equipment and industrial sectors such as aerospace, automotive, marine engineering, and biomedical devices. In this study, auxetic weft knitted fabric structures were designed by systematically varying loop length using polypropylene filament yarn. The knitted fabrics were subsequently incorporated into an epoxy matrix using the Resin Transfer Molding (RTM) process to form fabric-reinforced composite laminates. Tensile tests were conducted to investigate the load–extension behavior and auxetic response of the knitted fabric architecture in both wale and course directions. The results demonstrate that loop length has a pronounced effect on the auxetic behavior of the knitted structure, with increased loop length leading to higher extension and displacement, while shorter loop lengths exhibited greater load bearing capacity. The composite fabrication process was found to preserve the structural integrity of the auxetic fabric architecture. Failure analysis revealed matrix cracking, surface buckling, delamination, and fiber fracture as the dominant damage mechanisms. The findings highlight the effectiveness of structural design in controlling auxetic behavior in knitted fabric–based reinforcements for composite applications.