Papers by Author: Akira Todoroki

Abstract: 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: Statistical diagnosis using electrical resistance changes is performed to detect a delamination crack in a CFRP beam. This method enables to reduce data required for damage identification. First, a new measuring method of multiple electrical resistance changes is proposed to perform statistical diagnosis. The proposed method measures electrical resistance changes of multiple segments in a CFRP beam although electrical interference must be considered when multiple voltages are charged at once. Next, statistical diagnosis is performed on loading to the CFRP beam. A delamination crack is detected by the change of relative relationship between multiple electrical resistance changes due to damage occurring. As a result, the monitored states of the CFRP beam are diagnosed exactly by the proposed damage detection system.

Abstract: The structure with the sensor and the actuator is called Smart Structure. Installation of the sensor and the actuator to the structure enables the automatic damage detection. In this paper, a novel damage detection method called “SI-F method” is applied to the Smart Structure to detect the delamination in the CFRP beam. The artificial delamination in the CFRP is introduced by the out-of-plane bending. CFRP beam will be exited by the PZT actuator, and the strain of the surface will be measured by the strain gauge. As the result, the delamination is successfully detected by varying the condition of the excitation.

Abstract: The present paper proposes a new diagnostic tool for the structural health monitoring that employs a Kriging Interpolation. Structural health monitoring is a noticeable technology for aged civil structures. Most of the structural health monitoring systems adopts parametric method based on modeling or non-parametric method such as artificial neural networks or response surfaces. The conventional methods require FEM modeling of structure or a regression model. This modeling needs judgment of human, and it requires much costs. The present method does not require the process of modeling, in order to identify the damage level using the discriminant analysis. This suggest us, this technique is applicable to the health monitoring system, which identifies the damage of the structure, easily. In the present paper, we developed the damage diagnostic methods using Kriging method for identifying delamination from data. Kriging method is a interpolation technique which shown in geostatistic. We applied this method to identifications of delamination crack of CFRP structure. Delamination cracks are invisible and cause decrease of compression strength of laminated composites. Therefore, health-monitoring system is required for CFRP laminates. The present study adopts an electric potential method for health monitoring of graphite/epoxy laminated composites. The electric potential method does not cause strength reduction and can be applied existing structures by low cost. As a result, it was shown that this method is effective for identification of damages.

Abstract: Recently, applications of integrated large composite structures have been attempted to many structures of vehicles. In order to improve the cost performance and reliability, it is necessary to judge the structural integrity of composite structures. Fracture simulation techniques using FEM have been developed for the purpose. Since a number of iterations of finite element analysis are required in the fracture simulation, the simulation techniques consume many memory resources and much calculation time. In this study, a personal computer cluster (PC cluster) and the domain decomposition method were incorporated into a fracture simulation system. Calculations using a Windows PC cluster were carried out to confirm the efficiency of the proposed simulation system. As a result, it is concluded that adopting the domain decomposition method and the computer cluster is remarkably efficient to reduce calculation time.

Abstract: Statistical diagnosis using electrical resistance changes is performed to detect a delamination crack in a CFRP beam. This method enables to reduce data required for damage identification. First, a new measuring method of multiple electrical resistance changes is proposed to perform statistical diagnosis. The proposed method measures electrical resistance changes of multiple segments in a CFRP beam although electrical interference must be considered when multiple voltages are charged at once. Next, statistical diagnosis is performed on loading to the CFRP beam. A delamination crack is detected by the change of relative relationship between multiple electrical resistance changes due to damage occurring. As a result, the monitored states of the CFRP beam are diagnosed exactly by the proposed damage detection system.

Abstract: Electrical resistance change method has been applied to monitor a delamination crack of a thin CFRP laminate. For a thick CFRP laminate, multiple delamination cracks are made with many matrix cracks, and the electric current in the thick CFRP laminate may not flow in the thickness direction due to the strong orthotropic electrical conductivity. The present study employs an electric impedance change method for the identification of damage location and dimension of the damaged area; applicability of the method is investigated experimentally using thick beam-type specimens fabricated from cross-ply laminates of 36 plies. After making the damage, electrical impedance was decreased. A residual stress relief model was proposed to explain the decrease. From the measured electrical impedance changes, the relationships between the electrical impedance changes and damages are obtained by means of response surfaces. The response surfaces estimated the damage location and dimension of the damaged area exactly even for the thick CFRP laminates. The electrical impedance change method can be used as an appropriate sensor for measurement of residual stress relief due to damages of thick CFRP laminates.

Abstract: CFRP laminate can be used itself as a sensor for detecting own damage. The authors have introduced electric resistance change method and asymmetrical dual charge electric potential change method to detect a delamination in the CFRP laminate. Although the methods showed good performance of estimation analytically and experimentally, those are sensitive to electromagnetic noise. Experimental error caused by such noise in the actual use may affect the accuracy of estimation. In this paper, influence of the noise on accuracy of estimation was investigated quantitatively using finite element analysis. It was clarified that the methods have different characteristics for experimental noise.

Abstract: 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.

Abstract: 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.