Papers by Author: Nobuo Takeda

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Authors: Takeaki Nadabe, Nobuo Takeda
Abstract: This study investigates the computer simulation of fracture process in compressive failure of cross-ply laminate of composite materials. The purpose of this study is to establish the numerical analysis method to understand the fracture mechanism and predict the mechanical response of composite materials through the computer simulation. The stacking sequence of the simulated laminate is [0/90] s. The reinforcement fibers in 0-degree plies are modeled by circle cross-section beam elements to represent the three-dimensional effect in bending of fibers. Cohesive elements are inserted in the connection of beam elements to simulate the bending breaking of fibers. For the purpose of parallel computing, the domain decomposition method is applied, and for pre-conditioned conjugate gradient algorithm, incomplete Cholesky conjugate gradient method is applied. The simulated results show that in the initial state of the loading, the stress concentration occurs around the initial misalignment of fiber in 0-degree plies, and it also occurs around the area where fibers come close in 90-degree plies. At average applied strain 1.20 %, the fiber breaking damage initiates in 0-degree ply, and after this point the damage develops in the material. The simulated damage is close to the microscope picture of the actual composite materials obtained in the experiment. The current simulation is considered to correspond with the actual material deformation.
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Authors: Nobuo Takeda, Y. Okabe, J. Kuwahara, Toshimichi Ogisu, Seiji Kojima
Abstract: The authors developed a damage detection system that generates ultrasonic waves with a piezo-ceramic actuator and receives them by a fiber Bragg grating (FBG) sensor. In this research, this system was applied to evaluate debonding progress in CFRP skin/stringer bonded structures. FBG sensors were bonded on the stringer or embedded in the adhesive layer. Then, ultrasonic wave at 300kHz was propagated through the debonded region, and the wavelet transform was applied to the received waveform. After that, a new damage indexand a correlation coefficient were calculated from the distribution of the wavelet transform coefficient. As a result, the damage index increased and the correlation coefficient decreased with an increase in the debonded area. Hence the length of the debonding between the skin and the stringer could be well evaluated.
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Authors: Tadahito Mizutani, Takafumi Nishi, Nobuo Takeda
Abstract: Although demand for composite structures rapidly increase due to the advantages in weight, there are few effective assessment techniques to enable the quality control and guarantee the durability. In particular, an invisible microscopic damage detection technology is highly required because damages such as transverse cracks, debondings, or delaminations can lead to the critical failure of the structures. Among many non-destructive evaluation (NDE) methods for composite structures, fiber optic sensors are especially attractive due to the high sensitivity, the lightweight, and the small size. In the current trend of the structural health monitoring technology, fiber Bragg gratings (FBG) sensors are frequently used as strain or temperature sensors, and Brillouin scattering sensors are also often used for a long distance distributed measurement. The Brillouin distributed sensors can measure strain over a distance of 10km while a spatial resolution was limited to 1m. Some novel sensing method is proposed to improve the spatial resolution. The pulse-prepump Brillouin optical time domain analysis (PPP-BOTDA) is one of the latest distributed sensing applications with a cm-order high spatial resolution. The PPP-BOTDA commercial product has the spatial resolution of 10cm, and can measure the strain with a precision of ±25og. This precision, however, can be achieved by using conventional single-mode optical fibers. In our research, small-diameter optical fibers with a cladding diameter of 40om were embedded in the CFRP laminate to avoid the deterioration of the CFRP mechanical properties. Thus, in order to verify the performance of PPP-BOTDA, the distributed strain measurement was conducted with the small-diameter optical fibers embedded in the CFRP laminate.
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Authors: T. Okabe, M Nishikawa, Nobuo Takeda, Hideki Sekine
Abstract: This paper examines the stress distribution around a fiber break in alumina-fiber reinforced aluminum matrix (Al2O3/Al) composites using finite element analysis and predicts the tensile strength using tensile failure simulations. In particular, we discuss the effect of the matrix hardening on the tensile failure of the Al2O3/Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic-perfect plastic matrix and an elastic-plastic hardening matrix using finite element analysis. Second, the procedure for simulating fiber damage evolution in the Al2O3/Al composites is presented. The simulation incorporates the analytical solution for the axial fiber stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al2O3/Al composites, and discuss the effect of matrix hardening on the tensile strength of the Al2O3/Al composites. Coupled with size-scaling analysis, the simulated results express the size effect on the strength of the composites, which is seen in experimental results.
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Authors: Nobuo Takeda, Shu Minakuchi, Takahide Umehara, Ito Yusaku
Abstract: This study demonstrated fiber-optic-based life cycle monitoring of an L-shaped carbon fiber reinforced plastic (CFRP) part. Fiber Bragg grating (FBG) sensors were embedded in the corner of the L-shaped specimen during the laminate lay-up process, and was then utilized to monitor the local strain change during the cure process, the demolding, the assembly, and a subsequent bending test. FBG spectral changes induced were comprehensively presented and discussed from the viewpoint of the mechanical and thermal deformation of the specimen. Internal state of the L-shaped part was successfully monitored throughout its life, confirming effectiveness of life cycle monitoring by embedded fiber-optic-based sensors for developing highly-reliable composite structures.
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Authors: Hayato Nakatani, Talieh Hajzargarbashi, Kaita Ito, Tribikram Kundu, Nobuo Takeda
Abstract: A beamforming array technique with four sensors is applied to a cylindrical geometry for detecting point of impact. A linear array of acoustic sensors attached to the plate record the waveforms of Lamb waves generated at the impact point with individual time delay. A beamforming technique in conjunction with an optimization scheme that incorporates the direction dependent guided Lamb wave speed in cylindrical plates is developed. The optimization is carried out using the experimentally obtained wave speed as a function of propagation direction. The maximum value in the beamforming plot corresponds to the predicted point of impact. The proposed technique is experimentally verified by comparing the predicted points with the exact points of impact on a cylindrical aluminum plate and a cylindrical composite shell. For randomly chosen points of impact the beamforming technique successfully predicts the location of the acoustic source.
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Authors: Colin Haynes, Takeaki Nadabe, Nobuo Takeda, Michael D. Todd
Abstract: Structural health monitoring refers to the process of making an assessment, based on nondestructive, in-situ, autonomous measurements, about the ability of a structure to perform its intended function. This paper presents work done on a bolted connection in carbon-fiber reinforced polymer composite materials. A composite specimen is bolted in a double lap joint configuration to a test apparatus that applies an increasing tensile load. Ultimately, the load results in bearing failure of the material around the bolt hole. To monitor the progression of damage, macro fiber composite sensors are bonded in a circular array around the bolt hole. These sensors are then used to generate ultrasonic guided waves, a popular technique in nondestructive evaluation because of the favorable combination of propagation distance and sensitivity to damage. As the specimen is subjected to increasing load levels, measurements are taken repeatedly and compared with one another. Because damage will change the local mechanical properties of the material, the ultrasonic waves passing through the damaged region will be scattered differently in each direction, resulting in a different waveform arriving at the other surrounding sensors. By applying appropriate signal processing techniques, these changes may be interpreted as indicating the extent of damage that has occurred in the specimen. Preliminary analysis is presented demonstrating the correlation between changes in received strain signals and increasing damage levels.
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