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

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.