Comparative Evaluation of Structure–Processing–Property Relationships in Flax, Hemp, and Cattail Discontinuous fiber Thermoset Composites

ABSTRACT Replacement of glass fiber mats, utilized in large and complex composite parts in the transportation industry, with natural fiber mats requires knowledge of natural fiber mat design and its impact on manufacturing and properties of composites. The effect of needle punch density and fiber type on structure–processing–properties relationships was evaluated by comparing data for flax, hemp, cattail, and flax–hemp hybrid composites manufactured using vacuum‐assisted resin transfer molding (VARTM). The average tensile modulus and specific modulus were maximum for hemp fibers and minimum for flax fibers, with those for cattail fibers in between. The zero‐punched (0‐P) nonwoven mats exhibited the least fiber volume fraction ( V fo ) (3.5%–6.6%) and highest transverse permeability ( k z ) (2.3–5.2 × 10 −11 m 2 ); however, as punch density increased, V fo increased (11.4%–15%), and k z (0.5 × 10 −11 m 2 ) decreased due to differences in fiber‐packing and orientation. At VARTM pressure, all 0‐P composites exhibited similar longitudinal tensile modulus due to similar V f , while the modulus increased in the order of flax, cattail, hybrid, and hemp beyond 101 kPa due to differences in consolidation behavior for different fiber types. The needle‐punched mat composites exhibited higher V f , tensile modulus, and strength at VARTM pressure than those of 0‐P mat composites, suggesting the effect of punch density on consolidation behavior regardless of the fiber types. The impact of mechanical interlocking caused by needle punching was evident in the fractographs.