Papers by Author: Masahito Ueda

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: Resin transfer molding (RTM) process is getting popular for fabrications of complicated commercial products made from Glass Fiber Reinforced Polymer (GFRP) at low cost. Cure monitoring is indispensable for RTM process. In the present study, polyester resin is adopted for matrix resin of the GFRP components. The polyester is usually adopted as matrix resin of the low-cost GFRP products. Existing methods for the cure monitoring are, however, expensive for the GFRP products. The polyester resin usually changes its optical property during curing. This enables us to monitor the degree of cure by means of measurements of luminance change of the transmitted light. Since the electrical circuit for measuring luminance change is not expensive, this system utilizes the luminance change for monitoring cure is not expensive system. In the present study, the sensing system employs a LED as a light source and plastic optical fibers as light paths. A photodiode is adopted as a light power sensor. This low-cost cure-monitoring system is applied to monitoring of degree of cure of polyester resin. Degree of cure is measured by means of commercially available dielectric sensors, and results are compared with the results of luminance change. The effectiveness of the method is confirmed experimentally here.

Abstract: CFRP laminate is sensitive to impacts. Even a low impact creates delamination, bringing deterioration of the structural reliability. Monitoring for delamination is, therefore, indispensable to maintain the reliability of a CFRP structure. In this study, asymmetrical dual charge electric potential change method was introduced to estimate a delamination in the CFRP laminate. Delaminations were estimated using response surfaces as solver of the inverse problem. Learning data of response surfaces were calculated by FEM analyses. Actual delaminations in the CFRP laminate were successfully identified.

Abstract: Bolted joints are widely used for composite structures. As is well known, excessive bearing load gives rise to bearing failure at hole boundaries. Detecting bearing failure is important for assuring integrity of composite structures. Since conventional nondestructive inspection methods are expensive, cumbersome, time-consuming, and not suitable for health monitoring, a simple, low-cost inspection method for bearing failure must be developed. Authors have demonstrated the feasibility of detecting bearing failure by using an electric resistance change method. In this study, more detailed analyses were carried out to investigate the detectability in terms of the damage size and the distance between damage and electrodes. The results show that bearing failure of less than 10mm square causes the electric resistance change of a few hundred ppm and thus can be easily detected, and that the electrodes can be mounted more than 10 mm far from a bolt hole.

Abstract: For a cryogenic fuel tank of a next generation rocket, a Carbon Fiber Reinforced Plastic (CFRP) laminated composite tank is one of the key technologies. For the fuel tank made from the laminated composites, matrix cracks are significant problems that cause leak of the fuel. In the present paper, electrical resistance change method is adopted to monitor the matrix cracking of the CFRP laminate. Previous studies show that tension load in fiber direction causes electrical resistance increase due to the piezoresistivity of the carbon fibers, and fiber breakages also cause the electrical resistance increase of the CFRP laminates. In order to distinguish the electrical resistance changes due to matrix cracking from those due to the piezoresistivity and the fiber breakages, residual electrical resistance change under the complete unloading condition is employed in the present study. Experimental investigations were performed using cross-ply laminates in cryogenic temperature. As a result, it can be revealed that the residual electrical resistance change is a useful indicator for matrix crack monitoring of the cross-ply CFRP laminates.