3D printing Kevlar fiber layer distributions and fiber orientations into nylon composites to achieve designable mechanical strength

The designable strength and reinforcing direction of continuous fiber 3D printed nylon composites were studied by printing different fiber layer distributions and fiber orientations. According to three different fiber layer arrangements, four layers of fiber were printed in the samples. They are concentrated four layers (fiber in layer [2/3/24/32]), halve four layers (fibers in layer [2/3/38/39]), and quartering four layers (fibers in layer [8/16/24/32]). Results show that composites with quartering four layers performed the highest tensile strength of 88.18 MPa. Meanwhile, composites with halve four layers, which fiber layers symmetrically distributed near the surface reached the highest stiffness of 3140 MPa. Different tensile strength indicates that designers can optimize the distributions of fibers in their composite to achieve some specific mechanical properties. Then the effects of fiber arrangement angles on the tensile properties were studied. As the increase of fiber angles, the tensile strain at the first stress peak increased while the tensile stresses decreased at the first stage and then tended to be consistent at the final stage up to fracture. Two empirical models were successfully established to predict the mechanical behavior of the different fiber angles composites with reasonable errors. The model presented could be used to predict the mechanical properties of composites with different fiber angles and then help the researchers to design their composites. Additionally, by subjecting the composites to autoclave process to improve the properties, we found that the mechanical strength of the composites greatly increased by 2.41 times higher than the original strength with ultrahigh stiffness up to 20.11 GPa, which indicates a brittle fractured mode due to enhanced interfaces.