The interface properties between surface grown carbon nanotubes carbon fiber (CNTs-CF) and epoxy resin were investigated by different modification methods. The X-ray photoelectron spectrometer probations show that the pristine CNTs-CF has very low contents of both oxygen element and active carbon element. Taking the naked carbon fiber (CF) without CNTs grown as a reference, the interfacial shear strength (IFSS) of CNTs-CF/epoxy is only 5.6% higher. After heat treatment, chemical modification and sizing treatments, the surface chemical activity of CNTs-CF is improved considerably. In contrast with the IFSS of standard CF/epoxy, the interfacial strength of chemical-treated CNTs-CF-E1/ epoxy is increased to 122.8 MPa by 29.7%. The increasing content of the epoxy active groups at the modified CNTs-CF-E1 surface is conducive to forming covalent bonds between the epoxy resin and the CNTs-CF-E1 surface particularly the activated nanotube surface. The enhancement of CNT/epoxy interface is beneficial to the over whole interface property of the composites. Hence, the compressive and torsion strength of sizing CNTs-CF-E1 bundle combined with epoxy resin are increased by 32.4% and 14.1%, respectively. The SEM images show the failure morphologies of different modification methods, and the enhancement mechanism of multiscale interface is further clarified. Compared with chemical treatment, heat treatment can remove the inert amorphous carbon on the CNTs-CF fiber surface and improve the interface adhesion of CNT/epoxy and CF/epoxy, and surface sizing treatment improves the CF/epoxy interface. The enhancement mechanisms of these modified CNTs-CFs/epoxy should be ascribed to the competition and synergetic effects of the multiscale interfaces, including CNT/CF, CNT/epoxy, and CF/epoxy.

This article aims to improve the shear delamination resistance of composite laminate and minimize implantation damage on fiber orientation by using fine z-pins.The z-pins with the diameters as small as 0.1 and 0.2 mm were prepared by using different carbon fibers. The results show that z-pinning method significantly improves the mode II interlaminar fracture toughness, and the G IIC value becomes larger with the decrease in z-pin diameter. Compared with the control sample, the precracked G IIC values of (carbon fiber-reinforced polymer composite) CFRP (0.5CF4XÀpin) , CFRP (0.2CF4XÀpin) , and CFRP (0.1CF4XÀpin) increase by 67%, 142%, and 176%, respectively. As z-pin diameter decreases from 0.5 to 0.1 mm, the failure mode changes from z-pin pullout to shear fracture. The evolution of failure mode greatly enhances the resistance to crack propagation and the final G IIC value. The carbon fiber type of pin (0.5mm) has no obvious influence on the delamination sliding resistance. For pin (0.1mm) and pin (0.2mm) , the strong deformation resistance of z-pin is the dominant factor driving the improvement of the G IIC values though providing high-shear traction load. In addition, the z-pins with large shear deformation capacity can also enhance the crack sliding resistance.

Carbon nanotube (CNT) film possesses excellent mechanical and piezoresistivity, which may act as a sensor for process monitoring and reinforcement of the final composite. This paper prepared CNT/epoxy composite film via the solution dipping method and investigated the electrical resistance variation (ΔR/R0) of CNT/epoxy composite film during the curing process. The temperature dependence of electrical resistance was found to be closely related to resin rheological properties, thermal expansion, and curing shrinkage. The results show that two opposing effects on electrical resistivity occur at the initial heating stage, including thermal expansion and condensation caused by the wetting tension of the liquid resin. The lower resin content causes more apparent secondary impregnation and electrical resistivity change. When the resin viscosity remains steady during the heating stage, the electrical resistance increases with an increase in temperature due to thermal expansion. Approaching gel time, the electrical resistance drops due to the crosslink shrinkage of epoxy resin. The internal stress caused by curing shrinkage at the high-temperature platform results in an increase in electrical resistance. The temperature coefficient of resistance becomes larger with an increase in resin content. At the isothermal stage, an increase in ΔR/R0 value becomes less obvious with a decrease in resin content, and ΔR/R0 even shows a decreasing tendency.

An experimental investigation on the resistance welding of carbon-fiber-reinforced polyetheretherketone (PEEK) composite laminate using three types of stainless steel (SS) meshes with different sizes and electrical resistances as heating elements is reported. The objective of this study is to determine the influence of the metal mesh on the welding process and performance at different power densities ranging from 29 to 82 kW/m2. Resistance welding equipment is used to monitor the temperature and displacement along the thickness of the laminate. The results show that the power density determines the welding time and heat concentration. A large power density results in a short welding time, but also increases the temperature gradient at the joining interface (almost 50 °C) and causes an obvious deformation of a contraction of more than 0.1 mm along the thickness of the laminate. A SS mesh with low resistance has a strong welding capability, i.e., a high welding efficiency under low power density. A lap shear strength of approximately 35 MPa can be obtained with the appropriate power density. The shear strength is affected by the bonding between the metal mesh and polymer, the metal mesh load bearing, and the metal mesh size.