To identify a delamination crack in a CFRP laminate wirelessly, we proposed an electric resistance change method with oscillating circuit in the previous study. Although the method detects the delamination creation, it cannot monitor condition of applied strain before delamination creation because the electric resistance change due to strain changing is quite small. In the present study, a bridge circuit, two amplifiers and voltage-controlled oscillator are added to sensing circuit so that it can monitor very little change of the electric resistance change. Using proposed strain sensing system, the electric resistance change and oscillating frequency change due to strain changing are experimentally measured. As a result, the method is found to successfully monitor the applied strain.

Authors’ group has been done researches on the damage monitoring of the laminated Carbon Fiber Reinforced Plastic (CFRP) using the electrical resistance change method. The method adopts reinforcement carbon fibers as sensors, and it is called self-sensing method. Using thin CFRP laminates, delamination crack location and dimension are identified with the electrical resistance change method. However, the electrical resistance decreases when a delamination crack is made in a thick CFRP beam. The present paper experimentally investigates the effect of the dent for the measurements of electrical resistance change. Indentation tests are performed for the thick CFRP plates. The effect of the dent is calculated compared with the effect of piezoresistivity caused by the residual stress relief. As a result, the effect of the dent is larger than the effect of the residual stress relief for the thick CFRP. For the thick CFRP, dents cause the significant reduction of the electrical resistance in the thickness direction, and that causes electrical resistance decrease.

Abstract. To achieve uniform curing of resin, internal heating with the addition of carbon nanotubes (CNTs) has attracted considerable attention. Numerical simulations of the residual stress in CNT-filled resins cured by an electric field were carried out in the present study, taking into account the CNT dispersion within the resins. The simulations were based on an unsteady-heat-transfer equation and the cure reaction; the residual stress due to the thermal expansion and cure shrinkage was calculated using a finite element method. In addition, microscope images of actual CNT-filled resins were used for modeling the inhomogeneous electrical conductivity due to CNT aggregates. The simulation results show that, compared to external heating, Joule heating, or resistive heating, in which a conductive material itself generates heat from the passage of an electric current, enables more uniform curing and generally suppresses the residual stress. However, high local residual stress was observed around the high-electrical conductivity region in the model with inhomogeneous electrical conductivity. The present results thus highlight the need to take into account the inhomogeneity of CNT-filled resins for accurate evaluation of the residual stress.

Since composite materials have high specific strength and stiffness, they are used for many fields such as aerospace and marine structures. According to such utilities, joining method between composites and metals must be developed. In this study, dimple treatment is carried out as a new reinforcing method for FRP/metal co-cured joint. Dimple treatment is applied to the adhesive surface of metal so that resin of FRP permeates into dimples and the strength of joints increases. It is revealed that dimple treatment achieves as high bonding strength as chemical surface treatment.

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