An Experimental Approach to the Auxeticity of the Composites Reinforced With Weft Knitted Fabrics: Impact of Fabric Structure and Matrix Type

ABSTRACT

This paper undertakes the effect of fabric structure and matrix type on the auxeticity of the composites reinforced with weft knitted fabrics. Four topologies—Star, Foldable Rectangular, zigzag, and Ribbon (large and small unit cell)—were fabricated on an electronic flat‐knitting machine and characterized in both dry and resin‐consolidated states. The most corrugated topology, zigzag, produced the largest dry‐fabric NPR (−0.60), consistent with its greater out‐of‐plane folding (“kinematic reservoir”); flatter Ribbon geometries reverted to conventional (positive) Poisson behavior after limited strain. To assess how matrix rheology governs transfer of this geometric auxetic effect into a solid composite, foldable rectangular preforms were consolidated with three resins spanning roughly three orders of magnitude in stiffness: rigid Epoxy (2.2 GPa), Unsaturated Polyester (1.5 GPa), and a hyper‐elastic carboxylated styrene‐butadiene rubber (SBR, ~6 MPa). Rigid thermosets constrained geometric unfolding and failed by brittle matrix cracking near 10% strain, attenuating the composite NPR to −0.15 to −0.20. The SBR matrix, by contrast, offered minimal mechanical resistance and preserved a composite NPR of −0.40—close to the −0.42 value of the unconsolidated fabric—while extending strain‐to‐failure to roughly 45%. Charpy impact testing (50 J) showed that, with one exception among the weakly auxetic Ribbon samples, energy absorption broadly tracked NPR magnitude, ranging from 12.7% to 5.5% for the flat Ribbon composites to 45% for Zigzag/epoxy and 68% for rectangular/SBR, consistent with a strain‐induced densification mechanism beneath the impact zone. These findings suggest a design framework for impact‐mitigating textile composites for protective and energy‐absorbing applications.