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Featured researches published by Keisuke Okita.


Materials Science Forum | 2005

Axial Compressive Behavior of Single-Stage Bellows of TiNi Shape Memory Alloy for Seismic Applications

Hiromasa Semba; Nagatoshi Okabe; Toru Yamaji; Keisuke Okita; Kiyoshi Yamauchi

The bellows formed of TiNi shape memory alloy (SMA) is proposed as a new type of seismic protection device. The bellows structure is known to have lower rigidity along the axial direction through effect of its shape. TiNi is known to be one of the most typical SMAs, which have high damping characteristics for dynamics engaged in its twin formation under martensite state and have the ability to recover completely from the large strain after unloaded and or heated. In this study, fundamental compressive behavior of TiNi bellows was investigated and discussed. Several shapes of TiNi single-stage bellows produced by rubber bulge method were prepared. They were heat-treated for some heat treatments and then examined on compression tests. Based on the results, the relationships among the bellows shapes and the stiffness, energy-absorbing capacity and so on were clarified. Finally, it was found from these results that single-stage bellows of TiNi SMA could be used as one of seismic protection devices.


Materials Science Forum | 2005

Processing of Single-Stage Bellows of TiNi Shape Memory Alloy Using Rubber Bulge Method

Hiromasa Semba; Nagatoshi Okabe; Toru Yamaji; Keisuke Okita; Kiyoshi Yamauchi

The bellows formed of TiNi shape memory alloy is proposed as a new type of seismic protection device. The bellows structure is known to have lower rigidity along the longitudinal direction through effect of its shape. On the other hand, TiNi is known to be one of the most typical shape memory alloys, which have high damping characteristics for dynamics engaged in its twin formation under martensite state and have the ability to recover completely from the large strain (even such as 8%) after unloaded and or heated. This paper describes a processing method of a single-stage bellows of TiNi shape memory alloy using rubber bulge method. Thin-walled TiNi tubes subjected to cold working were prepared. Several annealing conditions for the process were examined and the appropriate one was discussed. Then the rubber bulge method of displacement control was introduced. Finally, the procedure of the process including heat treatment was clarified.


Materials Science Forum | 2005

Influence of Cold Working on Deformation Behavior and Shape Memory Effect of Ti-Ni-Nb

Keisuke Okita; Nagatoshi Okabe; Toshio Sakuma; Hiromasa Semba; Yuji Mihara

Thermo-mechanical characteristic of shape memory alloy depends on cold working and heat treatment, because these processes can control the amount of defects including dislocation and precipitation [1]. In this study, the influence of cold working on the deformation behavior and the transformation characteristics was investigated on the Ti-Ni-Nb shape memory alloy (SMA). Both the tensile test and the shape recovery test were performed for the wire specimens of 1mm in the diameter with some different rates of cold working. The shape recovery tests were performed for the wire specimens of different cold working rates until the various levels of maximum applied strain, and the reverse-transformation characteristics on the process of heating after unloading were studied. It is clarified that the higher cold-working rate improves the shape memory properties of the alloy.


International Journal of Modern Physics B | 2006

DYNAMIC BEHAVIOR OF SINGLE-STAGE BELLOWS OF TITANIUM-NICKEL SHAPE MEMORY ALLOY UNDER CYCLIC LOADING

Hiromasa Semba; Nagatoshi Okabe; Toru Yamaji; Keisuke Okita; Kiyoshi Yamauchi

The dynamic behavior of TiNi shape memory alloy (SMA) bellows is examined in light of its potential use as elements in seismic protection devices. Dynamic property results obtained from cyclic tests under tension-compression loading of TiNi SMA single-stage bellows, with different shapes and with different heat treatments, are reported as a function of displacement amplitude and frequency. It was found that the displacement–force loops were almost symmetric with respect to the central point for almost all specimens. The normalized secant stiffness diminishes significantly with increasing bulge height as well as displacement amplitude. From hysteretic cycles, an equivalent damping of about 15% was recognized for longtime-aged bellows with relatively high bulge height. Frequencies, in the range of interest for seismic applications, had a small influence on damping values. Under the conditions studied in this research, the bellows had better damping performance in a martensite phase than in a rhombohedral phase. SMA bellows in martensite phase subjected to the longtime-ageing have great potential as an element in seismic devices.


International Journal of Modern Physics B | 2006

ANALYSIS MODEL FOR SHAPE MEMORY EFFECT IN Ti-Ni SYSTEM ALLOY OF RING CONFIGURATION

Keisuke Okita; Nagatoshi Okabe; Tomoyuki Satoh; Kazunari Uchida

Shape memory alloys are well known to be used as a ring element for connect coupling two pipes in various fields. In the case of structural and functional designs for a coupling ring of shape memory alloy, we have considered the shape memory effects in multi-axial stress and strain using volumetric strain in each element, so as to make FEM analysis more effective for optimizing the shape and the amount of pre-deformation. Using some uni-axial fundamental test results, as property parameters for FEM, the modeling method simulating the deformation behavior of a coupling ring has been eveloped in order to optimize the configuration and size of a coupling ring. The validity of this FEM modeling method was confirmed by verification experiments.


International Journal of Modern Physics B | 2006

RECOVERY CHARACTERISTIC FOR SHAPE MEMORY EFFECT UNDER THE CONDITION OF HEATING WITH CONSTRAINT STRAIN

Keisuke Okita; Nagatoshi Okabe; Tomoyuki Satoh; Toshio Sakuma

A new constitutive equation is proposed based on the series combined model, considering the generation of slip deformation, due to the recovery stress in heating under constraint strain, as well as external stress in loading. The martensite volume fractions in the constitutive equation were distinguished into three kinds: un-reoriented martensite, reoriented martensite and plastic damaged martensite. And the critical stress for the slip deformation, generated through the reverse transformation, in a constraint strain-heating process, was clarified experimentally and introduced into the constitutive equation. Moreover, the validity of our constitutive equation has been confirmed by experimentation conducted under various constraint strain-heating conditions in the recovery process.


Materials Science Forum | 2005

Effects of Pre-Strain and Nb Content on Transformation Temperatures in Ti-Ni-Nb Shape Memory Alloy

Toshio Sakuma; Yuji Mihara; Yasuo Ochi; M. Ozawa; Keisuke Okita; Nagatoshi Okabe

Ti-Ni-Nb shape memory alloy has a wide transformation hysteresis, and has been used as coupling devices and so on. However systematic researches of the influence of the amount of Nb addition on transformation temperatures are few. The purpose of this work is to clarify the influences of pre-strain and Nb content on transformation temperatures in Ti-Ni-Nb shape memory alloys. The specimens were Ti-Ni-Nb alloys (Ni/Ti=1.0, Nb=0-15at%), annealed at 1223 K for 0.3 ks. The variation of transformation temperatures with pre-strain and Nb content were investigated experimentally. The variation of As, Ms and transformation temperature hysteresis with pre-strain and Nb content will be discussed in relation to the elastic strain energy and the volume fraction of slip-deformed martensite.


Materials Transactions | 2002

Effect of Copper Content on Superelasticity Characteristics in Ti-Ni and Ti-Ni-Cu Alloy Wires

Toshio Sakuma; Maho Hosogi; Nagatoshi Okabe; Uichi Iwata; Keisuke Okita


Materials Transactions | 2002

Effect of Cyclic Deformation on Thermo-Mechanical Characteristics in Ti-Ni-Cu Alloy Wires with Various Copper Contents

Toshio Sakuma; Maho Hosogi; Nagatoshi Okabe; Uichi Iwata; Keisuke Okita


Materials Transactions | 2002

A New Constitutive Equation for Superelastic Deformation and Prediction of Martensite Volume Fraction in Titanium-Nickel-Cupper Shape Memory Alloy

Maho Hosogi; Nagatoshi Okabe; Toshio Sakuma; Keisuke Okita

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Toshio Sakuma

Central Research Institute of Electric Power Industry

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Uichi Iwata

Central Research Institute of Electric Power Industry

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Yuji Mihara

University of Electro-Communications

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Yasuo Ochi

University of Electro-Communications

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