The tensile response of 3D-printed Onyx–Kevlar composites was systematically characterized to quantify the effect of Kevlar volume fraction (5–35%) and fiber orientation on mechanical performance. Specimens were fabricated via fused filament additive manufacturing with continuous Kevlar layers in [±45 \(^\circ\) ] layups and Onyx layers in [0 \(^\circ\) , 90 \(^\circ\) ] and [±45 \(^\circ\) ] configurations, while ultimate load capacity increased monotonically with fiber content and was consistently higher in the [0 \(^\circ\) , 90 \(^\circ\) ] configuration due to direct load transfer measured Young’s modulus diverged significantly from values predicted by the theoretical rule of mixtures using manufacturer data (errors ranging from 100 to 300%). To address this, it is introduce a modified rule of mixtures that incorporates the actual effective fiber cross section (accounting for voids and non-uniform fiber placement) and recalculates constituent properties as a function of off-axis orientation. Application of this correction reduces the discrepancy to less than 25% between predicted and experimental Young’s modulus. These findings demonstrate that accurate modeling of additively manufactured composites requires explicit inclusion of void fraction and fiber orientation effects, providing a robust framework for the predictive design of Onyx–Kevlar structures.
