Faculty Profiles - OKAYASU Mitsuhiro

Papers - OKAYASU Mitsuhiro

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Effects of Electrode Plating on Mechanical Properties of PZT Ceramics

M. Okayasu, M. Mizuno, T. Shiraishi

International Journal of Materials Engineering and Technology 8 ( 2 ) 75 - 83 2012

Microstructural characteristics in tough pitch copper for revealing the work hardening region

M. Okayasu, T. Taki, S. Takasu, S. Takeuchi, T. Shiraishi

Advances in Materials Research 1 ( 4 ) 347 - 357 2012

Material properties of long term naturally aged die cast Al-Si-Cu alloys

M. Okayasu, K. Sakai, S. Takasu

International Journal of Cast Metals Research 24 ( 5 ) 286 - 298 2011.10

Language：English Publishing type：Research paper (scientific journal) Publisher：MANEY PUBLISHING

Die cast aluminium alloys, including automotive parts, are generally understood to suffer some degradation of their mechanical properties over time due to corrosion, specifically when exposed to water and sea breeze (salt air). However, in this study, it is shown that in the absence of a corrosive environment, the mechanical properties of die cast aluminium alloys actually increase over a 10 year period due to natural aging. The increment in the mechanical properties is mainly attributed to precipitation hardening brought about by h0 phase particles. It also appears that any significant change in microstructure affects the mechanical properties, e.g. the change of the needle-like eutectic Si to a spherical form and the joining of discrete a-Al phases to make larger grains. The driving force for the microstructural changes upon natural aging is related to the residual stress in the die cast sample accumulated by high casting pressure and rapid cooling rate during injection process. © 2011 W. S. Maney & Son Ltd.

DOI： 10.1179/1743133611Y.0000000001

Etching technique for revealing work hardening regions in Mg-9Al-1Zn alloy

M. Okayasu, K. Sato, M. Mizuno

Materials Science and Technology 27 ( 1 ) 76 - 80 2011.1

Language：English Publishing type：Research paper (scientific journal) Publisher：MANEY PUBLISHING

A method for revealing the localised work hardened region (plastic deformation zone) in Mg-9Al-1Zn alloy (AZ91) has been developed using a new etching technique. With this technique, etching with the solution (9 g picric acid, 30 mL acetic acid, 150 mL ethyl alcohol and 20 mL H O) was executed on the samples after plastic deformation and then heating to 300°C for 3 h. Because of the high strain energy in the plastic deformation zone, the microstructural characteristic in the deformation zone changed substantially after the heating process, where severe precipitation of Mg Al phase occurred in the Mg rich α phase. With the change of microstructural characteristics, the deformation region can be revealed. The plastic zone was revealed by the different degrees of etching response in the deformation and undeformed regions; the plastic zone was slightly etched, whereas the other region was deeply etched. From the different surface height level, the deformation zone was found to be observable even at low magnification. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute. 2 17 12

DOI： 10.1179/174328409X425245

Domain switching characteristics of lead zirconate titanate piezoelectric ceramics during mechanical compressive loading

Mitsuhiro Okayasu, Kazuto Sato, Yoko Kusaba

Journal of the European Ceramic Society 31 ( 1-2 ) 129 - 140 2011.1

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

To better understand the domain switching characteristics of lead zirconate titanate (PZT) ceramics, the orientations of domains have been directly investigated during loading and unloading using various experimental techniques Upon loading, linear and non-linear fracture mechanics of the PZT ceramics are observed The slope of the stress-strain response is attributed mainly to lattice strain and domain switching strain During the loading process, electrical activity also occurs several times in the PZT ceramics This activity is related to a lightning-like phenomenon and consists of a bright flash with a click This electrogenerative event is caused by severe domain switching The characteristics of domain switching and reverse switching are detected during the loading and unloading processes The amount of domain switching depends on the grain due to different stress levels In addition, two patterns of 90 degrees domain switching systems are characterized, namely (i) 90 degrees turn about the tetragonal c-axis and (ii) 90 degrees rotation of the tetragonal a-axis (C) 2010 Elsevier Ltd All rights reserved

DOI： 10.1016/j.jeurceramsoc.2010.09.001

Influence of domain orientation on the mechanical properties of lead zirconate titanate piezoelectric ceramics

Mitsuhiro Okayasu, Kazuto Sato, Mamoru Mizuno

Journal of the European Ceramic Society 31 ( 1-2 ) 141 - 150 2011.1

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

The influence of domain orientation on the mechanical properties of lead zirconate titanate (PZT) piezoelectric ceramics has been investigated using un-poled and poled PZT ceramics High mechanical properties, e g, high elastic modulus and compressive strength, were obtained for the polarized PZT ceramics due to strain hardening caused by more severe domain switching during the loading process while low mechanical properties for the un-poled ceramics Fracture mechanics of the ceramics were influenced by the direction of the tetragonal lattice structure since cracks propagate along the long axis of the tetragonal structure (c-axis) Using X-ray diffraction and electron back scatter diffraction analysis the domain switching characteristics could be clarified (C) 2010 Elsevier Ltd All rights reserved

DOI： 10.1016/j.jeurceramsoc.2010.09.003

Piezoelectric Properties of Lead-Free Piezoelectric Ceramics under Cyclic Loading

M. Okayasu, Y. Sato, M. Mizuno, K. Tanoue, K. Kato

International Journal of Materials Engineering and Technology 6 ( 1 ) 1 - 10 2011

Fatigue and Tensile Properties of Titanum based Bulk Glassy Alloys

M. Okayasu, S. Takasu, T. Bitoh, D. Shin, M. Makabe

International Journal of Materials Engineering and Technology 5 ( 2 ) 77 - 90 2011

New ceramic injection systems for hot chamber die casting of aluminium alloys

M. Okayasu, M. Hitomi, H. Yamazaki

International Journal of Cast Metals Research 23 ( 4 ) 246 - 249 2010.8

Publishing type：Research paper (scientific journal)

The quality of hot chamber die cast aluminium alloys can vary, due to differences of casting conditions, in particular the casting pressure. This is caused by the plunger tip galling during the casting process. To solve this problem, a new injection system is proposed using a flexible movable plunger tip and an insert type shot sleeve. With this system, the plunger tip can be turned around and slid into the piston holder, which facilitates smooth insertion of the plunger into the shot sleeve. The proposed insert type sleeve can be made with a strong structure. Moreover, even if the sleeve is partially fractured, it can easily be replaced. With the proposed injection system, high quality hot chamber die cast components can be created. In addition, the maintenance cost of the injection system is dramatically reduced. © 2010 W. S. Maney & Son Ltd.

DOI： 10.1179/136404610X12682097603768

Microstructure and material properties of an Al-Cu alloy provided by the Ohno continuous casting technique

Mitsuhiro Okayasu, Satoshi Takasu, Shigeki Yoshie

Journal of Materials Processing Technology 210 ( 11 ) 1529 - 1535 2010.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

The specific objective in this work is an exploration of the material properties produced by a recently developed continuous casting technique. The study focuses on an Al-Cu alloy provided by the Ohno continuous casting process. The experimental approaches give rise to unusual microstructural characteristic in the cast samples, namely a fine lamellar eutectic structure with unidirectional growth along its axial direction together with a regularly oriented lattice structure. Such microstructural characteristics significantly increase the tensile and fatigue properties. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmatprotec.2010.04.012

Newly developed walking apparatus for identification of obstructions by visually impaired people

Mitsuhiro Okayasu

Journal of Mechanical Science and Technology 24 ( 6 ) 1261 - 1264 2010.6

Language：English Publishing type：Research paper (scientific journal) Publisher：KOREAN SOC MECHANICAL ENGINEERS

In this work, two visual systems to help visually impaired people walk were developed: the first is a white cane with a non-contact detection system, and the second is a three-dimensional (3D) visual system. In place of the sense of sight, the sense of touch, either via vibration or of pins, can relay information on objects and obstacles around people. In a white cane non-contact detection system, two sets of ultrasonic sensors and vibrators, respectively, are employed to indicate the positions of both low- and high-level obstructions in front of the visually impaired person. When objects are detected by the sensor, the vibrator is strongly activated. In this system, the range of obstruction detection can be adjusted between 0.5 m and 5.5 m. By comparison, the 3D visual system uses two different components, an infrared camera sensor to detect obstructions and a tooling apparatus, incorporating a number of 1 mm diameter pins, by which their 3D shapes are derived. The pins are arranged in a 10x10 matrix and move longitudinally between the retracted and extended positions based on the depth data between the infrared sensor and the obstruction. Pins are elongated individually, so that each pin tip represents a specific area of the outer surface of the object. This tactile 3D image can provide effective object information.

DOI： 10.1007/s12206-010-0332-5

Mechanical properties of Al-Si-13-Ni-1.4-Mg-1.4-Cu-1 alloys produced by the Ohno continuous casting process

Mitsuhiro Okayasu, Shigeki Yoshie

Materials Science and Engineering A 527 ( 13-14 ) 3120 - 3126 2010.5

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Examination of the mechanical properties of cast aluminum alloys (AC8A: Al-Si-13-Ni-1.4-Mg-1.4-Cu-1), created by the Ohno continuous casting (OCC) process, was conducted. The material strength and ductility of the OCC samples are twice as high as the same aluminum alloy produced by conventional gravity casting. The increase of material strength of the OCC sample is attributed to (i) tiny spherical alpha-Al grains, (ii) fewer cast defects and (iii) solid-solution strengthening by silicon. The high material ductility of the OCC sample is related to the regularly oriented crystal structure, where the crystal direction (1 1 1) in the cast sample is perpendicular to the casting direction, and (1 1 0) and (1 0 0) directions can be detected in the longitudinal direction of the sample. The yield stress of the OCC sample can be estimated by two different parameters: solid-solution strengthening, k root c, and grain size, k(y)/root d(Hall-Petch relation). (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2010.01.071

Damage Evaluation of Piezoelectric Ceramics from the Variation of the Elastic Coefficient under Static Compressive Stress

Mamoru Mizuno, Mitsuhiro Okayasu, Nozomi Odagiri

International Journal of Damage Mechanics 19 ( 3 ) 375 - 390 2010.4

Language：English Publishing type：Research paper (scientific journal) Publisher：SAGE PUBLICATIONS LTD

In the present article, columnar specimens of lead zirconate titanate (PZT) were subjected to static compressive stress, and the characteristics of fracture under compression were clarified. The fracture tests were interrupted at certain intervals, and resonance and anti-resonance frequencies and electrostatic capacity were measured by means of an impedance analyzer with the compressive stress unloaded. The interruption and measurement were repeated with the maximum stress increased up to the fracture. The material properties of the specimens such as the electromechanical coupling coefficient, the dielectric constant, the elastic coefficient, and the piezoelectric constant were evaluated based on the resonant properties, and the variation of the material properties in the process of the fracture were clarified experimentally. An elastic coefficient was also evaluated from stress-strain relations during the compression fracture tests, and the difference in the elastic coefficient depending on the method of evaluation was discussed. Furthermore, internal damage developed in the specimens during the compression fracture tests was evaluated based on the variation of the elastic coefficients indirectly as a scalar damage variable on the basis of the continuum damage mechanics. Features of the damage development and the dependence of the quantitative value of the damage variables on the evaluation method of the elastic coefficient were discussed. SEM micrographs of the fracture surface were observed and the causes of the damage were investigated.

DOI： 10.1177/1056789509103644

In situ measurement of material properties of lead zirconate titanate piezoelectric ceramics during cyclic mechanical loading

Mitsuhiro Okayasu, Eriko Sugiyama, Mamoru Mizuno

Journal of the European Ceramic Society 30 ( 6 ) 1445 - 1452 2010.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

To better understand the material properties of lead zirconate titanate (PZT) ceramics, in situ mechanical and electrical properties have been investigated continuously during cyclic mechanical loading. The material properties are demonstrated to change as a function of applied maximum stress, with the effective elastic constant increasing with increase of the stress level. The increase of effective elastic constant is attributed to the domain structure of the PZT. 90 degrees domain switching can occur anywhere in the sample, which makes the strain accumulate and leads to high values of the effective elastic constant. The domain switching characteristics are clearly revealed by electron back scattered diffraction analysis. The changes of the electrical properties (electromechanical coupling coefficient, piezoelectric constant and permittivity) are in the opposite sense because of the material strain (or material damage), caused by the change of domain orientation; the electrical properties are degraded with increasing cycle number and applied stress. Based upon the variation of the material properties, details of the damage characteristics in the PZT ceramics are discussed. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jeurceramsoc.2009.11.004

Observation of Failure and Domain Switching in Lead Zirconate Titanate Ceramics Reviewed

Mitsuhiro OKAYASU, Eriko SUGIYAMA, Kazuto SATO, Mamoru MIZUNO

Journal of Solid Mechanics and Materials Engineering 4 ( 3 ) 426 - 434 2010.4

Language：English Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Mechanical Engineers

The mechanical and electrical properties (electromechanical coupling coefficient, piezoelectric constant and dielectric constant) of lead zirconate titanate (PZT) ceramics are investigated during mechanical static and cyclic loading. There are several failure characteristics which can alter the material properties of PZT ceramics. The elastic constant increases and electrical properties decrease with increasing the applied load. This is due to the internal strain arising from the domain switching. In this case, 90° domain switching occurs anywhere in the samples as the sample is loaded. It is also apparent that electrogenesis occurs several times during cyclic loading to the final fracture. This occurrence is related to the domain switching. The elastic constant and electrical properties can decrease because of crack generation in the PZT ceramics. Moreover, the elastic constant increases with increase of the mechanical load and decreases with decrease of the load. On the contrary, the opposite sense of change of the electrical properties is observed.

DOI： 10.1299/jmmp.4.426

An etching characterization method for revealing the plastic deformation zone in a SUS303 stainless steel

Mitsuhiro Okayasu, Kazuto Sato, Satoshi Takasu

Journal of Materials Science 45 ( 5 ) 1220 - 1226 2010.3

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

Etching characteristics used to reveal localized plastic deformation zones in a SUS303 stainless steel have been examined. The etching was conducted on a sample using an etchant consisting of 5-g ferric chloride, 50-mL hydrochloric acid, and 100-mL water. The sample was deformed severely and heated to various temperatures before the etching process. With this etching technique, the plastically deformed area is clearly observed even at low magnification. This is due to a change of the microstructural characteristics in the plastic deformation zone. There are different microstructure patterns that reveal the plastic zone in the sample with and without heating, e.g., plastic zone in the sample with heating to 800 A degrees C is observed clearly due to randomly oriented crystals and recrystallized small grains with precipitated nano-size particles. Details of the etching characteristics that reveal the plastic deformation zone are further discussed.

DOI： 10.1007/s10853-009-4068-5

Fatigue failure characteristics of lead zirconate titanate piezoelectric ceramics

Mitsuhiro Okayasu, Go Ozeki, Mamoru Mizuno

Journal of the European Ceramic Society 30 ( 3 ) 713 - 725 2010.2

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

Mechanical properties and fatigue failure characteristics of lead zirconate titanate piezoelectric ceramic (PZT) have been investigated. Bending and fatigue strengths of PZT ceramic are directly attributed to the electrode status. Material hardening occurs in the PZT ceramic during the cyclic loading, which is influenced by domain switching occurring anywhere in the grains. The domain structure is clearly detected by electron back scatter diffraction analysis and etching techniques. It also appears that the poling direction causes the change of failure characteristics due to different domain and domain wall orientation. The domain orientation changes alternately from domain to domain by 90 degrees. Moreover, the domain wall orientation is well regulated in the grains perpendicular to the poling direction. An acceleration of fatigue crack growth occurs as the crack propagates along the domain wall. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jeurceramsoc.2009.09.014

Comparison of mechanical properties of die cast aluminium alloys: Cold v. Hot chamber die casting and high v. Low speed filling die casting

M. Okayasu, S. Yoshifuji, M. Mizuno, M. Hitomi, H. Yamazaki

International Journal of Cast Metals Research 22 ( 5 ) 374 - 381 2009.10

Publishing type：Research paper (scientific journal)

In this paper, the mechanical properties of die cast aluminium alloys made by various die casting technologies were examined. To create high quality aluminium alloy die castings, two die casting processing technologies were employed. These were (a) ultra slow speed filling cold chamber die casting and (b) high speed hot chamber die casting. Significant improvements of the fatigue and mechanical properties were obtained for both die casting systems compared to the normal high speed cold chamber die casting technique. By comparing ultra slow die casting with hot chamber die casting, it was found that the fatigue and mechanical strengths from hot chamber die casting were higher than those for ultra slow filling die casting. The differences in material strength were attributed directly to the material properties, e.g. microstructural morphology and internal defects. Spherical fine dendritic cells in the hot chamber die casting sample gave rise to high fatigue crack growth resistance; the low crack growth resistance for cold chamber die cast aluminium is mostly due to the growth of aluminium rich α phase and the presence of eutectic silicon fibres. The fatigue strength was also related to the number of internal defects, e.g. the lower the defect rate on the fracture surface, the higher the fatigue resistance and mechanical strength. The characteristics of the principal internal defect were different depending on the die casting technology: this showed fine porosity for hot chamber die casting but solidification shrinkage and the scattered chill structure for slow and high speed cold chamber die castings. The reasons for the change of material strength were therefore influenced by the die casting process. © 2009 W. S. Maney & Son Ltd.

DOI： 10.1179/174313309X380413

The development of a visual system for the detection of obstructions for visually impaired people

Mitsuhiro Okayasu

Journal of Mechanical Science and Technology 23 ( 10 ) 2776 - 2779 2009.10

Language：English Publishing type：Research paper (scientific journal) Publisher：KOREAN SOC MECHANICAL ENGINEERS

In this paper, the author presents a new visual system that can aid visually impaired people in walking. The system provides object information (that is, shape and location) through the sense of touch. This visual system depends on three different components: (i) an infrared camera sensor that detects the obstruction, (ii) a control system that measures the distance between the obstruction and the sensor, and (iii) a tooling apparatus with small pins (I center dot 1 mm) used in forming a three-dimensional shape of the obstruction. The pins, arranged on a 6x6 matrix, move longitudinally between the retracted and extended positions based on the distance data. The pin extends individually, while the pin tip reflects the object's outer surface. The length of the pin from the base surface is proportional to the distance of the sensor from the obstruction. An ultrasonic actuator, controlled at a 15Hz frame rate, is the driving force for the pin movement. The tactile image of the 3D shape can provide information about the obstruction.

DOI： 10.1007/s12206-009-0729-1

Mechanical and fatigue strengths of silicon nitride ceramics in liquid aluminum alloys

Mitsuhiro Okayasu, Masuo Hitomi, Hirotsugu Yamazaki

Journal of the European Ceramic Society 29 ( 11 ) 2369 - 2378 2009.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

Although the silicon nitride ceramic (Si(3)N(4)) has a high mechanical strength even at high air temperature, much reduction in the material strength occurs in liquid aluminum alloys. The bending strength of the ceramic in molten At alloys is about 20% lower than that obtained in an air temperature of 750 degrees C. Moreover, a significant reduction of the fatigue strength occurs in the At alloy melt. The change of mechanical properties of Si(3)N(4) ceramic in the melt also depends on the amount of iron in the molten aluminum alloy; a large amount of iron makes the fatigue strength low. The reduction in the material strength is attributed to the change of material properties caused by the chemical reaction between iron and silicon nitride. Details of the chemical reaction in Si(3)N(4) ceramic are discussed in the present work. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jeurceramsoc.2009.01.013

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