Out‐of‐plane compression properties and failure mechanism of gradient 3D woven composites

Abstract Gradient three‐dimensional (3D) orthogonal preforms with low‐ and high‐density regions were manufactured through the flexible oriented 3D woven process (FO3DWP) at different fiber volume fractions and z ‐yarn contents. The layer porosity distribution of preforms was regulated by modifying fiber tension and measured using micro‐computed tomography (micro‐CT) technology. After impregnation with epoxy resin, the out‐of‐plane compression properties of gradient 3D woven composites were evaluated. The results indicated that the out‐of‐plane compression strength could be improved by the gradient design. In particular, the gradient composite with a fiber volume fraction of 0.46 and z ‐yarn volume fraction of 0.05 exhibited the highest compression strength at 470 MPa. Furthermore, failure morphologies were analyzed using micro‐fracture photographs and scanning electron microscopy (SEM). The gradient design efficiently protected the interface between z ‐directional fibers and the matrix by minimizing z ‐yarn bending. However, the compression strength of 3D woven composites decreased as the z ‐yarn content increased from 0.05 to 0.15, due to the increased bending of the z ‐directional fibers during the compression test.