Improved unit cell model and elastic constant prediction method of 3D four-directional braided composites

An improved unit cell model for 3D four-directional braided composites was proposed, considering the cross-section shape variation of interior fiber bundles along their center line in manufacturing process of jamming action, and this model was further used to predict equivalence elastic constant. Firstly, based on analyzing the braiding process, interior fiber bundle distribution of unit cell was established; then, the geometric position coordinates for fiber bundle jamming regions of improved unit cell model were derived with alternately changed circular and elliptical fiber bundle cross-section, assuming the original circular cross-section is squeezed into oval shape ellipse in pinch regions, thus curling the fiber bundle paths. Mathematical relationship between braiding parameters and unit cell geometric dimensions was also deduced, with a relative calculation error for braiding pitch length less than 4% compared with test data, which is better than those models ignoring fiber bundles' deformation. Finally, equivalence elastic constants were predicted, further used to investigate the effects of braiding angle and fiber volume fracture to elastic constants.