Papers by Author: Yoshinobu Shimamura

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Authors: Tomoyuki Fujii, Keiichiro Tohgo, Yu Itoh, Daisuke Kato, Yoshinobu Shimamura
Abstract: This paper deals with an analysis of a crack-tip field of particulate-reinforced composites which can describe the evolution of debonding damage, matrix plasticity and particle size effect on deformation and damage. Numerical analyses were carried out on a crack-tip field in elastic-plastic matrix composites reinforced with elastic particles by using a finite element method developed based on an incremental damage theory. The particle size effect on damage is described by a critical energy criterion for particle-matrix interfacial debonding. The effect of debonding damage on a crack-tip field is discussed based on numerical results. The debonding damage initiates and progresses ahead of a crack-tip. The stress distribution shifts downward in the debonding damage area. It is concluded that a crack-tip field is strongly affected by debonding damage.
Authors: Tatsuo Sakai, Koushu Hanaki, Akiyoshi Sakaida, Kenji Okada, Yuki Nakamura, Kazutaka Mukoyama, Noriyasu Oguma, Takashi Matsumura, Yoshinobu Shimamura, Akira Ueno
Abstract: As a joint project of Committees on Fatigue and Reliability Engineering in the Society of Materials Science, Japan (JSMS), an electronic database on fatigue strength of metallic materials fabricated in Japan had been constructed and published in 1996. Book style of the same data compilation had been published at the same time by the JSMS and Elsevier. About twenty years have passed since the above publications of database and databook. Thus, a lot of new fatigue test data have been obtained during such a long period including many data on the very high cycle fatigue such as gigacycle regime. Based on such a circumstance, the JSMS has organized a new project to construct an electronic database on very high cycle fatigue. A lot of numerical data obtained by fatigue tests would be compiled together with many photographs of fracture surfaces. In this paper, fundamental view and compilation concept of the database are briefly introduced by showing a typical example of proto-type database constructed from domestic data in Japan.
Authors: Yoshinobu Shimamura, Kyohei Kageyama, Keiichiro Tohgo, Tomoyuki Fujii
Abstract: Carbon nanofiber (CNF) has good electrical conductivity. Addition of a few percentages of carbon nanofiber to polymer yields electrical conductivity but hardly affects the mechanical properties of polymer. This conductive polymer may be useful for sensing applications such as strain sensors and chem-resist sensors. Many researchers have reported on the electrical conductivity, but the electrical resistance change under strain of the carbon nanofiller composites is not fully investigated. In this study, the electrical resistance change under strain of CNF/flexible-epoxy composites was investigated experimentally. More than 100% of quasi-static strain can be measured by using CNF/flexible-epoxy composite with Young’s modulus of less than 1MPa. Cyclic and unloading behaviors were also measured and discussed. It was found that the cyclic behavior was strongly affected by viscoelasticity and damage.
Authors: Akira Todoroki, Masahito Ueda, Yoshinobu Shimamura
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.
Authors: Keiichiro Tohgo, Yuki Tochigi, Hiroyasu Araki, Yoshinobu Shimamura
Abstract: This study deals with deformation behavior of a shape-control plate which consists of an aluminum alloy plate and a pre-strained NiTi shape memory alloy (SMA) wire. The shape-control plate exhibits reciprocating bending deformation by heating and cooling. Deformation behavior of the plate is examined by electric heating and natural cooling of the SMA wire. Experimental results exhibit that the bending deformation of the plate is considerably stable over more than two thousand heating-cooling cycles and can be well controlled by electric current. Furthermore, the deformation behavior of the plate is analyzed by a simple beam theory for the aluminum alloy plate and Brinson’s one-dimensional constitutive model for the SMA wire taking account of not only martensitic transformation but also rhombohedral-phase transformation. Numerical results describe well the deformation behavior of the shape-control plate observed in the experiments.
Authors: Tomoyuki Fujii, Keiichiro Tohgo, Hiroyasu Araki, Katsunori Wakazono, Masaki Ishikura, Yoshinobu Shimamura
Abstract: This paper deals with fabrication and strength evaluation of biocompatible composites consisting of partially stabilized zirconia (PSZ) and pure titanium (Ti). The biocompatible composites of PSZ-Ti were fabricated by a hot pressing method of powder metallurgy. A volume ratio of PSZ and Ti was changed. Four-point bending tests and Vickers hardness tests of the PSZ-Ti composite were performed to determine the Young's modulus, bending strength, Vickers hardness and fracture toughness. These properties were characterized as a function of Ti volume fraction. The Young's modulus and Vickers hardness were higher than the prediction of the rule of mixture. The bending strength and fracture toughness were decreased with increasing Ti content. To discuss these results from a viewpoint of reaction products, the components of raw powders and sintered composites were investigated by X-ray diffraction analysis. It is concluded that oxide of titanium and other reaction products were created after sintering and they affected the mechanical performances of the composites.
Authors: Keiichiro Tohgo, Hiroyasu Araki, Yoshinobu Shimamura
Abstract: This paper deals with evaluation of fracture toughness in functionally graded materials (FGMs) consisting of partially stabilized zirconia (PSZ) and austenitic stainless steel SUS 304. FGMs and non-graded composites (non-FGMs) with fine and coarse microstructures are fabricated by powder metallurgy using PSZ and two kinds of SUS 304 powders. The fracture toughness is determined by conventional tests for several non-FGMs with each material composition and by a method utilizing stable crack growth for FGMs. Based on the experimental results, fracture mechanism, influences of microstructure on fracture toughness, and difference in fracture toughness between the FGMs and non-FGMs has been discussed.
Authors: Yoshinobu Shimamura, Keiichiro Tohgo, Hiroyasu Araki, Yosuke Mizuno, Shoji Kawaguchi, Masaru Hashimto, Tokuichi Inoue
Abstract: Metal free reeds are used for musical instruments like harmonica. Free reeds are small, thin cantilevers, and oscillate by blowing air. It is reported that free reeds break due to fatigue during play. In order to elongate the life of free reeds, the fatigue properties should be investigated and a motion analysis method should be developed. The experimental and analytical research on metal free reed, however, has been rarely reported. In this study, two types of fatigue testing machines were developed to obtain basic fatigue characteristics. The fatigue testing machines are designed for bending fatigue of actual free reeds whose thickness is less than 400 μm. An S-N diagram is successfully obtained up to 107 cycles by using the developed fatigue testing machines. The fracture surfaces of fatigued specimens are in good agreement with those of free reeds failed in use. Then, an analytical method for the self-excited oscillation of free reeds was developed based on a mass-damper-spring model. The proposed method can take account for the shape of free reed. The self-excited oscillation of free reeds with different shape are analyzed and in good agreement with experimental results.
Authors: Yoshinobu Shimamura, Akito Kokubo, Hitoshi Ishii, Keiichiro Tohgo, Tomoyuki Fujii, Tooru Yagasaki, Masamichi Harada
Abstract: Recently, high-strength alloy steels have been developed and used for various products. It is well known that fretting fatigue does not show fatigue limit. In other words, fretting fatigue failure may occur in very high cycle regime more than 107 cycles. However, it is difficult to investigate fretting fatigue property in very high cycle regime by using conventional fatigue testing machines because it is time-consuming. In this study, a fretting fatigue testing method for carburized alloy steels in very high cycle regime is explored by using an ultrasonic torsional fatigue testing machine. Carburized SCM420H was used for investigation. The experimental results showed that it is possible to conduct fretting fatigue testing of carburized alloy steels by using an ultrasonic torsional fatigue testing machine.
Authors: Yoshinobu Shimamura, Yosuke Yamamura, Akira Todoroki
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.
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