This work aims to explain the mechanisms of weak bond formation in CFRP epoxy adhesive joints by studying aqueous NaCl mist contamination. First, a mechanism of epoxy ring consumption by Cl^－ ion was proposed to explain the reduction in covalent bond density at the adhesion interphase, as a function of pH alkaline drift. Second, double cantilever beam test (DCB test) was applied to CFRP joint samples with aqueous NaCl mist contamination at the adhesion interphase to prove a weak bond triggered by Cl^－ ion contamination. Third, contamination layer thickness was estimated by substituting the data into the drift function. Obtained nanometer-thin thickness indicates that the stirring flow of the adhesive dissipates the layer to form a standard joint, while steady flow keeps the layer to form a weak-bond joint. Therefore, 1) Cl^－ ion contamination at the adhesion interphase and 2) the handling in the adhesion process explain the statistical aspect of weak bond formation in the production line of CFRP adhesive joints.

An energy director (ED), a sharp, flat- or triangular-shaped resin bead, is usually inserted between CFRPs during ultrasonic welding. In this study, a two-dimensional model was used to conduct finite element analyses for CFRP ultrasonic welding. The effects of the ED shape on the temperature increase, deformation history, and dissipated energy behavior are discussed. The results of the numerical simulations show that the triangular ED more easily increases the temperature than the flat ED does, and thus its use consumes less energy and time than using the flat ED. However, this study indicates that a triangular ED is not always better than a flat ED. In the ED, the temperature is distributed significantly; that is, the temperature range between points is vast. There is the possibility that unexpected chemical reactions such as oxidation occur. It is found that an abrupt temperature increase is caused by a synergic effect. That is, the increase in temperature causes the viscoelastic and frictional dissipated energy to be remarkable, and an increase in dissipated energy increases the temperature. Consequently, it is difficult to optimize the parameters, such as overall pressure, frequency, and welding time, for ultrasonic welding.

In this study, we used the acoustic emission (AE) method to evaluate the integrity of a carbon fiber-reinforced plastic (CFRP) single-lap joint. We fabricated single-lap joints with different adherends, surface roughness and contamination conditions. For these joints, AE measurements were conducted under tensile shear adhesion strength tests to investigate the possibility of evaluating the state of adhesion. The shear adhesion strength for unidirectional CFRP was higher than that for textile CFRP. Surface roughening effectively improved the strength and contamination degraded the strength at the tips of the joint. AE measurement results suggested the possibility of integrity monitoring for CFRP joints by focusing on whether the Kaiser effect is effective.

This study investigated the effect of vacuum ultraviolet (VUV) light irradiation on the thermal fusion bonding strength of carbon fiber/thermosetting polyimide composite laminates (CF/PI) using a thermoplastic polyimide (TPI) film as a hot-melt adhesive. The adherend (CF/PI) and/or adhesive (TPI) surfaces were irradiated by the VUV light (72nm) for 4 min in a low oxygen atmosphere (0.2±0.1%), followed by the thermal bonding of two CF/PI specimens with TPI film adhesive under 320ºC. The bonding shear strength was observed to be improved considerably from 18.6 MPa to 36.2 MPa through the VUV light irradiation process. In addition, the fracture behavior changed from an interface fracture mode into a cohesive or adherend fracture mode. To elucidate the surface modification, chemical analyses of the CF/PI and TPI surfaces were carried out based on the X-ray photoelectron spectroscopy. Experimental results revealed that carbonyl and hydroxyl groups were introduced on the CF/PI and TPI surfaces by VUV light irradiation. It was suggested that these functional groups may contribute to improve the bonding shear strength.

Low-temperature environments may cause strength degradation of composite bonded joints. Regardless, the method employed for synthesizing these joints has the potential to be effective. In this study, experimental and numerical investigations were conducted to determine the influence of a low-temperature environment on the strength properties of double-lap composite bonded joints. The strength properties and fracture modes of these joints at different temperature levels, ranging between the room and cryogenic temperatures, were studied using a cryogenic testing system with a refrigerator. The experimental results indicate that the strength of the bonded joints is significantly reduced as the temperature decreases. Finite element analysis (FEA) of the double-lap composite bonded joints was conducted to elucidate the cause of strength reduction of the bonded joints at low temperatures. In the FEA, the temperature dependency of the elastic constants, thermal contractions, and elastoplastic properties of the adherend and adhesive materials were considered. These factors were then used to calculate the stress distribution in the adhesive layer and energy release rates at the interface between the inner adherend and the adhesive layer. Numerical results revealed that the mode I energy release rates at the interface significantly increased with a decrease in plastic behavior, owing to the low-temperature environment. A reduction in the plastic characteristics of adhesive materials is, thus, a dominant factor for the strength reduction of double-lap composite bonded joints in a low-temperature environment.

The need for developing technology for joining dissimilar materials, particularly carbon fiber reinforced thermoplastics (CFRTPs) and metals for the progress of multi-material structures, is imperative. In this study, differently shaped surface nanostructures were fabricated by anodizing and etching processes using aluminum alloy A5052. A silane coupling treatment was also performed, and then CFRTP laminates with PA6 (as a matrix resin) and an aluminum alloy with a nanostructure surface were heat-welded using a hot press to prepare single lap joint specimens. Static tensile shear tests evaluated the influence of the surface nanostructure’s shape on the bonding strength and fracture morphology. The results clarified that the nanostructure of the interface improved the bonding strength and, from fracture surface observation results, changed the fracture morphology from brittle to ductile. Furthermore, crack growth observation results confirmed that the crack growth rate decreased in the specimens with a nanostructure, and the fracture toughness of the bonding interface improved, resulting in improved bonding strength.

This study aimed to evaluate the bonding strength in the shear and tensile directions of composite plates bonded by a heat press. These composite plates composed of a carbon fiber reinforced thermoplastic (CFRTP) and an aluminum alloy (A5052). A microstructure was generated on the surface of the aluminum alloy by chemical treatment. We measured the surface free energy associated with the dimple structures of both nano-order and micro-order sizes; we also observed the cross section and bonding interface under a microscope. The effect of the interfacial microstructure on the bonding strength was studied from these results. A strong anchor effect was observed on treatment with large dimples, resulting in a high shear strength. However, the contribution of the anchor effect to the tensile strength was negligible. High tensile strength and surface free energy were observed on treatment with small dimples.