In recent years, carbon fibre reinforced thermoplastics (CFRTP) are expected to be used as lightweight structural materials for mass-produced vehicles. CFRTP with thermoplastics as matrix allows us to weld them using melting of matrix by heating. We have been developing a direct resistance heating method, which uses carbon fibres as the resistance heating element. Carbon nanotube (CNT) is expected to be used as additive to FRP and we reported that the fibre/matrix interfacial shear strength was improved by grafting CNT on the surface of carbon fibres and tensile lap-shear strength was improved by using CNT grafted carbon fibre as the heating element for welding. For the practical use of CFRTP for structural parts, flexural strength is also necessary to be evaluated. In this study, flexural test was carried out to clarify the effect of CNT deposition time to the surface of carbon fibres on flexural strength of resistance welded CFRTP using CNT grafted carbon fibre as the heating element. The highest flexural strength was obtained when CNT10, for which CNT is grafted on the carbon fibres for deposition time of 10 min, was used for the heating element of resistance welding. In the case of CNT deposition time of 60 min, the lowest flexural strength was obtained because of the poor impregnation of the resin into the carbon fibre due to the excess CNT on the carbon fibres.

Carbon Nanotube (CNT) grafted carbon fibers were used for the heating element of resistance welding, and the effect of CNT deposition time at deposition temperature of 600ºC on the tensile lap-shear strength of resistance welded CFRTP was evaluated. The longer the deposition time became, the longer the length of CNT became. The highest tensile lap-shear strength was obtained when CNT grafted carbon fibers grafted at the deposition time of 30 minutes were used for the heating element of resistance welding because it is reported that fiber matrix interfacial properties of the carbon fiber was improved by CNT deposition to the surface of carbon fiber. In the case of the CNT grafted carbon fibers grafted at the deposition time of 60 minutes, lower tensile lap-shear strength was obtained because of the poor impregnation of the resin into the carbon fiber due to the excess CNT on the carbon fibers. The length of CNT should be controlled to obtain good mechanical properties.