Tensile and Shear Properties of Biaxial Flat Braided Carbon/Epoxy Composites With Dispersed Carbon Nanofibers in the Matrix

In laminated flat braided composites there are no fibers through the thickness direction except at the edges due to the fiber continuity of the braiding technique. A delamination along the interlaminar planes can be propagated because of the lack of fibers in the Z- or third-direction to the composite. The delamination initiates essentially as a result of arising the stresses concentrations around the transverse or matrix cracks that appear due to the mismatch of the thermal expansion coefficients of the fibers and matrix during the fabrication process. The delamination renders low interlaminar composite properties and represents a fundamental weakness of laminated flat braided composites especially with increasing the braiding angle, and thus minimizes the shear stress transfer. In this research, laminated flat braided carbon fabrics were performed via flattening tubular braided fabrics with braiding angle of ±45° by applying carefully compressive loads laterally on the tubular fabrics. Then, carbon fiber reinforced epoxy matrix composites were fabricated from the above-mentioned biaxial fabrics with and without uniformly dispersed carbon nanofibers throughout the epoxy matrix. Three loading percentages of carbon nanofibers (specifically, 0.5, 1, and 2 wt%) were dispersed in the matrix of the composites to enhance the matrix and interlaminar/inter-ply properties. The influence of matrix and interlaminar properties improvements on the in-plane tensile and shear response of the laminated flat braided composites was clarified via conducting of ±45° laminates tensile tests. The experimental results of tensile tests revealed that the tensile and in-plane shear properties as well as the fracture behavior of the composites are substantially influenced by the incorporation of the dispersed carbon nanofibers in the matrix of the composites. A pulsed thermography technique was used to inspect the occurrence of the delamination after the fracture under tensile loadings. The thermal wave image and logarithmic temperature-time curves of the pulsed thermography inspection illustrated that the composites with dispersed carbon nanofibers rendered higher interlaminar properties than that of composites without nanofibers. The main conclusion of this research can be summarized that dispersion of carbon nanofibers through the epoxy matrix of laminated flat braided composites is not only enhanced the matrix properties but also improved the interphase morphology between the composite plies that maximized the stress transfer of the composites. In other words, the fabricated braided composites with braiding angle of ±45° are predominantly by both of matrix and interlaminar properties.