Influence of CNT Reinforcement and Fiber Orientation on the Mechanical Performance of Woven Kevlar/Epoxy Composites

Carbon nanotubes (CNTs) are widely used as nanoscale reinforcements in polymer composites because of their high stiffness, high aspect ratio, and ability to enhance interfacial stress transfer. In woven Kevlar/epoxy composites, however, the mechanical benefit of CNT addition depends not only on nanotube presence, but also on whether the fabric Fiber orientation enables the CNT-modified interface to participate effectively in the dominant load path. In this study, woven Kevlar/epoxy composites with and without 4 wt.% multi-walled carbon nanotube (MWCNT) treatment were investigated under three displacement rates, namely 1, 10, and 100 mm/s, for two specimen orientations relative to the woven yarn directions: 0°/90° and ±45°. The 0°/90° Fiber orientation represents a tension-dominant load path, whereas the ±45° Fiber orientation promotes yarn rotation and matrix-shear-dominant deformation. The experimental results show that CNT treatment produces a clear increase in elastic modulus in the 0°/90° composites, with an improvement of approximately 40–50% at the lowest loading rate and continued enhancement at higher rates. In contrast, only limited gains are observed in the ±45° composites. The calculated CNT engagement index reached 0.8667 in the 0°/90° Fiber orientation but remained low or negative in some ±45° loading conditions, indicating that the effectiveness of CNT reinforcement depends strongly on Fiber orientation relative to the woven yarn directions. To interpret this behavior in a design-oriented manner, three Fiber orientation-sensitive comparison parameters are introduced: the CNT engagement index, the Fiber orientation sensitivity factor, and the rate amplification factor. These descriptors distinguish absolute stiffness from actual reinforcement utilization and indicate that modulus improvements depend on specimen orientation relative to the woven yarn directions. This study indicates that fabric Fiber orientation governs whether the CNT-modified interface is effectively mobilized or largely bypassed. This provides a useful framework for selectively deploying CNT reinforcement in woven protective composite systems and for rethinking nanotube reinforcement as a load-path-dependent design feature rather than a universally effective additive.