A size effect model for predicting in-plane mechanical isotropy of triaxial woven composite strips

Two-dimensional triaxial woven fabrics (TWF) and corresponding composites (TWFC) exhibit advantages such as in-plane quasi-isotropy, lightweight, and high dimensional stability, making them ideal materials for space-deployable structures. However, the quasi-isotropy behavior of these materials exhibits pronounced size-dependent characteristics at finite dimensions, posing challenges for accurate prediction using conventional unit cell models. This study investigates the size effect on the tensile properties in TWF systems under various loading orientations. By integrating elastic mechanics theory with analytical geometry, a novel mechanical model is established to quantify the relationship among specimen dimensions, tensile modulus and loading angles. The model reveals the mechanical response governed by local constraints from fiber bundles. Additionally, it quantifies the strengthening effects of the woven architecture on macroscopic mechanical behavior. Theoretical derivation reveals a three-stage size effect in the mechanical response, delineated by two critical aspect ratio thresholds. For aspect ratios below $1/\sqrt{3}$, the quasi-isotropic properties virtually vanished in both TWF and TWFC rectangular specimens. Between aspect ratios of $1/\sqrt{3}$ and $\sqrt{3}$, quasi-isotropy progressively improved with increasing aspect ratio. For aspect ratios exceeding $\sqrt{3}$, TWF samples achieved peak quasi-isotropic behavior, whereas TWFC exhibited continuous enhancement. Furthermore, uniaxial tensile experiments and finite element simulations validate the model's precision, showing a deviation of less than 5 %. This model addresses the limitation of the representative volume element analytical method which neglects boundary effects, providing a theoretical tool for the coordinated design of material and dimensions in aerospace deployable structures such as space-deployable antennas.