研究者詳細 - 岡安　光博

論文 - 岡安　光博

分割表示 >> ／全件表示 134 件中 1 - 134 件目

Mechanical Properties of Epoxy- and Dicyclopentadiene-Based Carbon-Fiber-Reinforced Plastics at Low and Room Temperatures

Takashi Fujiwara, Masaki Takeuchi, Che Daoyuan, Yao Liang, Naoki Nishioka, Mitsuhiro Okayasu

Applied Composite Materials 2025年

掲載種別：研究論文（学術雑誌）

In this study, the material properties of carbon-fiber-reinforced polymers (CFRPs) were investigated at room temperature (20 °C) and cryogenic temperature (–196 °C). Two types of CFRPs incorporating thermoset resins, namely epoxy (E) and dicyclopentadiene (D), were employed to evaluate their mechanical properties and failure behaviors. For low-temperature testing, an originally designed apparatus was utilized, in which mechanical properties were evaluated using a polystyrene container filled with liquid nitrogen (LN). The LN level was maintained automatically via a capacitance sensor-based monitoring system that regulated its replenishment as necessary. The results revealed that the bending and fatigue strengths of both CFRP types increased by approximately 30% at –196 °C compared to those at 20 °C. This enhancement may be attributed to various reasons, including increased internal strain induced by the disparity in thermal expansion coefficients between the carbon fiber and the resin. CFRP exhibited relatively higher creep resistance at –196 °C, compared to that at 20 °C. Under static bending load, E-CFRP displayed a sudden stress drop at approximately 2% bending strain, likely due to delamination between the epoxy resin and carbon fibers. In contrast, D-CFRP demonstrated enhanced strain tolerance, sustaining strains exceeding 5% without severe delamination, which can be attributed to the superior wettability of dicyclopentadiene with carbon. Details of the wettability and the above material properties of both CFRPs were analyzed in this paper.

DOI： 10.1007/s10443-024-10302-4

Hydrogen Embrittlement Characteristics of a Die-Casting Die after a High-Pressure Die-Casting Operation

Mitsuhiro Okayasu, Katsunori Shigematsu

International Journal of Metalcasting 18 ( 1 ) 810 - 820 2024年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER INT PUBL AG

In this work, the failure characteristics of a die-casting die manufactured from conventional hot-worked steel (SKD61) were investigated experimentally. A die with heat-checking failure on its cavity due to high-pressure die casting was employed. Apparently, the microstructural characteristics of the die around the regions exhibiting heat checking were unaltered. However, the hardness values decreased slightly because of the die heating via the injection of molten aluminum alloy, which is the tempering-like effect. Hydrogen was detected in the die around the regions with heat checking, which can slightly reduce the tensile fracture strain, i.e., hydrogen embrittlement (HE). The extent of HE was sensitive to the loading speed, particularly to a low loading speed. When heated to 450 degrees C, hydrogen gas was detected in a commercial die lubricant, which may lead to hydrogen infiltration in the die. This paper discusses the details pertaining to hydrogen penetration and die failure mechanisms.

DOI： 10.1007/s40962-023-01081-y

Effects of cutting conditions on materials properties of austenitic stainless steels

Ryutaro Kondo, Mitsuhiro Okayasu, Hao Wen, Jin Kawasaki

Journal of Engineering Research 2024年

掲載種別：研究論文（学術雑誌）

Austenitic stainless steel SUS304 is widely used in various engineering applications, including semiconductor manufacturing equipment and food processing machinery, owing to its exceptional material characteristics such as non-magnetic properties, high corrosion resistance, strength, and ductility. However, concerns have arisen in recent years regarding potential alterations in the material properties of austenitic stainless steel after machining processes. As the cutting process was conducted widely to make various engineering applications with different machining conditions, an examination of the effect of machining conditions on the material properties is important. In the present study, the influence of cutting conditions on the microstructure and mechanical of SUS304 was experimentally and numerically investigated, where three types of ball-end mills with rake angles (- 20°, − 3°, and 5°) and different cutting speeds (50 m･min−1 and 100 m･min−1) were conducted with plane and side machining to investigate the cutting properties. Material properties of SUS304 can be changed by the cutting resistance of the above cutting process, in particular the low rake angles under plane machining, where the high cutting resistance makes high dislocation density and strain-induced martensite formation. This occurrence made the low corrosion resistance as well as low ductility. On the contrary, no significant effect of the cutting speeds on the cutting resistance was detected compared to the condition of the rake angle on the plane machining. The reasons behind this will be discussed in detail systematically in this paper. From this approach, it could be clarified to maintain the high material properties of SUS304 even after the cutting process, which could be useful to make design of the engineering applications.

DOI： 10.1016/j.jer.2024.04.014

Hydrogen Embrittlement of Austenitic Stainless Steels with Different Surface Treatments

Mitsuhiro Okayasu, Hao Wen, Ryutaro Kondo

International Journal of Steel Structures 24 ( 3 ) 477 - 487 2024年

掲載種別：研究論文（学術雑誌）

This study evaluated the effect of surface conditions on the hydrogen embrittlement (HE) characteristics of SUS304 austenitic stainless steel, as SUS304 is one of the candidate structural materials for hydrogen energy systems. Various machining processes, including milling (ML), shot peening (SP), and cold rolling (CR), were employed to modify the surface roughness, internal strain, and microstructural characteristics of the test sample. Namely, this approach was conducted as the surface-absorbed hydrogen is related to effective hydrogen: a high internal strain was obtained in the entire CR area along with a rough surface, while SP and ML samples displayed high strain levels near the surface. The strain value was reflected in the hardness level due to their work hardening and strain-induced martensite formation. Concerning this, the hardness values of CR and SP samples were higher than 6 and 2 times the as-received samples. The hydrogen content charged to the samples was contingent upon the strain level: higher strain corresponded to elevated hydrogen content, particularly in CR samples. Despite the notable high hydrogen content in CR samples, HE was not detected in the tensile test. Conversely, even with a low hydrogen content, severe HE was observed in all samples during the fatigue test. The susceptibility of stainless steel to HE proved sensitive to cyclic loading, wherein surface-absorbed hydrogen migrated to the crack tip during cyclic loading. Detailed discussions on the reasons for these observations are provided based on the experimental results.

DOI： 10.1007/s13296-024-00829-4

Formability Improvement for Bending Magnesium Alloy AZ31 Sheet via Microstructural Control

Mitsuhiro Okayasu, Takumi Ogawara, Minoru Matsushima

Journal of Materials Engineering and Performance 2024年

掲載種別：研究論文（学術雑誌）

To investigate the formability of the AZ31 magnesium alloy, U-bent samples were fabricated through the cold- and hot-bending processes using the 1-mm thick AZ31 sheet. The hot-bending (HB) process was conducted at various temperatures (200-400 °C). On the other hand, the cold-bending (CB) process was carried out using the AZ31 sheet with and without tensile pre-strain (0, 5, and 10%) before heating to various temperatures: 200-450 °C. The AZ31 sheet made via the HB process at above 300 °C exhibited a higher formability with a lower springback of ϕ = 5° (0%-300 °C-HB) compared with that for the CB-AZ31 (0%-25 °C-CB), with ϕ = 36°. To improve the CB formability of the AZ31 alloy, the microstructure was controlled via the pre-strain and heating processes. In this instance, the grain growth occurred significantly in the AZ31 alloy with 5% pre-strain before heating to more than 250 °C (5%-T > 250 °C-CB). The large grains led to low yield stress, low hardness, and severe plastic deformation (twin deformation and grain rotation), resulting in high CB formability. The mechanical properties and deformation characteristics of the CB- and HB-AZ31 samples were investigated via several experimental approaches to interpret the change in the formability of the AZ31 alloy.

DOI： 10.1007/s11665-024-09822-8

Hydrogen Embrittlement Characteristics of Low Carbon Steels After Heat and Surface Treatments

Mitsuhiro Okayasu, Takehiro Kokado

Transactions of the Indian Institute of Metals 77 ( 8 ) 2127 - 2137 2024年

掲載種別：研究論文（学術雑誌）

The hydrogen embrittlement (HE) characteristics of the low carbon steels: Fe-xC-1.5Mn-0.03Si (0.05C, 0.1C, and 0.15C) with ultimate tensile strength (UTS) of ~ 1200 MPa, were examined experimentally after heating and surface treatment processes. For this investigation, the low carbon steels with different microstructural characteristics, including phase structures, precipitates, and internal strain, were employed. The high internal strain was seen in the Fe-0.15C steels after cold rolling (CR), water quenching (WQ), and bake hardening (BH) process, while the low strain was obtained in the steel samples after the annealing process. The high strain for CR and WQ (and BH) is caused by severe plastic deformation and the phase transformation of the martensite structure, respectively. Carbide particles were precipitated in the WQ and BH samples, and the amount of carbide was correlated with their carbon content. HE occurred only in the CR-0.15C and WQ-0.15C samples although the low UTS of ~ 1200 MPa was used, where the fracture strain decreased significantly. HE was attributed to the high internal strain and high carbon content which led to the hydrogen trapping site. A multiple regression analysis was also carried out to estimate the extent of HE of the low carbon steels.

DOI： 10.1007/s12666-024-03288-x

Hydrogen embrittlement properties of several stainless steels

Mitsuhiro Okayasu, Hisanobu Matsuura

International Journal of Fracture 248 ( 1 ) 201 - 220 2024年

掲載種別：研究論文（学術雑誌）

The hydrogen embrittlement (HE) characteristics of various stainless steels were investigated. In this study, as-received, heated (1100 °C, 15 h), and cold-rolled (30% strain) γ-austenite (AS), α-ferrite (FS), α′-martensite (MS), and γ–α duplex (DS) stainless steels were employed. For as-received stainless steels, severe HE occurred for DS and MS with static tensile loading, while no clear and weak HE was observed for AS and FS, respectively. This could be attributed to the different extent of hydrogen diffusivity in the stainless steel. A large amount of hydrogen penetrated to (i) lattice vacancy with low atomic density for body-centered cubic FS, DS, and MS, compared to that for face-centered cubic (AS); (ii) the phase boundary between γ-austenite and α-ferrite for DS; and (iii) the boundary between the Cr base precipitate and the martensite matrix for MS. HE also occurred strongly for heated-DS owing to the grain growth, i.e., a high hydrogen concentration in grain and phase boundaries. Although no clear HE was detected in as-received AS with static loading, HE occurred in cold-rolled AS, where hydrogen penetrated lattice vacancies and α′-martensite formed through strain-induced martensite. Owing to strain-induced martensite created during cyclic loading, HE was detected even for as-received AS, which is dissimilar to the result of the tensile test. Details of HE characteristics of the strainless steels were examined using the four stainless steels with different microstructures, diferent strain level and oxide layer. Moreover, those were investigated under different loading conditions, such as constant, static, and cyclic loading.

DOI： 10.1007/s10704-024-00809-z

Microstructural Control and Alloy Design for Improving the Resistance to Delayed Fracture of Ultrahigh-Strength Automotive Steel Sheets

Takehide Senuma, Mitsuhiro Okayasu, Hardy Mohrbacher

Metals 13 ( 8 ) 2023年8月

掲載種別：研究論文（学術雑誌）

The demand for higher-strength automotive steel sheets has increased significantly for lightweight and safe body concepts. However, the increment of the steel strength is often limited by the potential occurrence of delayed fracture. This paper discusses proper microstructure control and alloy design to improve the resistance against the delayed fracture of ultrahigh-strength automotive steel sheets in order to increase the usable upper limit of their strength and provides basic data serving as a practical guide for solving the problem of delayed fracture in ultrahigh-strength automotive steel sheets. It is confirmed that grain refinement, the appropriate dual-phase structure of martensite with ferrite or retained austenite, and surface decarburization, increase the resistance to delayed fracture. In terms of alloy design, the effects of Nb, Mo, and B on the delayed fracture resistance of hot-stamped steels have been investigated. The results suggest that there are other reasons for Nb to improve delayed fracture resistance in addition to grain refinement and the ability to trap hydrogen by its precipitates, as has been conventionally believed. Regarding Mo, it was clearly demonstrated that the segregation of this element at the grain boundary plays a main role in improving the delayed fracture resistance.

DOI： 10.3390/met13081368

An Edge Device Framework in SEMAR IoT Application Server Platform

Yohanes Yohanie Fridelin Panduman, Nobuo Funabiki, Sho Ito, Radhiatul Husna, Minoru Kuribayashi, Mitsuhiro Okayasu, Junya Shimazu, Sritrusta Sukaridhoto

Information (Switzerland) 14 ( 6 ) 2023年6月

掲載種別：研究論文（学術雑誌）

Nowadays, the Internet of Things (IoT) has become widely used at various places and for various applications. To facilitate this trend, we have developed the IoT application server platform called SEMAR (Smart Environmental Monitoring and Analytical in Real-Time), which offers standard features for collecting, displaying, and analyzing sensor data. An edge device is usually installed to connect sensors with the server, where the interface configuration, the data processing, the communication protocol, and the transmission interval need to be defined by the user. In this paper, we proposed an edge device framework for SEMAR to remotely optimize the edge device utilization with three phases. In the initialization phase, it automatically downloads the configuration file to the device through HTTP communications. In the service phase, it converts data from various sensors into the standard data format and sends it to the server periodically. In the update phase, it remotely updates the configuration through MQTT communications. For evaluations, we applied the proposal to the fingerprint-based indoor localization system (FILS15.4) and the data logging system. The results confirm the effectiveness in utilizing SEMAR to develop IoT application systems.

DOI： 10.3390/info14060312

Material properties of Japanese and western wooden nails

Mitsuhiro Okayasu, Kohei Uotani, Toshiro Tanaka

MATERIALIA 28 2023年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The corrosion characteristics of Japanese (J) and Western (W) nails with and without coating the Fe3O4 layer on the nail surface were examined experimentally to understand their mechanical properties and corrodibility. In this study, (i) one of the historical J-nails made by the forging process and (ii) commonly employed commercial W-nails created by extruding were used. Although the grain size and strain value differed depending on the nail and its location(sharpened tip, body, and flattened head), the microstructure of both nails consisted of ferrite and cementite structures. A high internal strain was created in the W-nail due to the nail creation by the extruding process. In particular, a high strain was obtained in the sharpened tip and flattened head areas of the W-nail compared to its body area, caused by severe plastic deformation. A high strain was also obtained in the sharpened tip area of the J-nail owing to the severe forging process. Such a high strain was related to the high hardness. The high hardness of the sharpened tip area of the J-nail (220 HV) was slightly lower than that of the W-nail's tip area. The hardness of the J -nail decreased non-linearly to its head area, which was affected by a low strain and largely grown grains. The corrosion occurred on the J- and W-nail surfaces, where lepidocrocite (gamma-FeOOH) was detected due to the weak-acidity oxidation, i.e., acid rain. The corrosion extent was different depending on the nail. Although the corrosion was detected for the J-nail after exposure for 3 months, the corrosion occurred on W-nail after exposure to the atmosphere for a week. Different corrosion extents between J- and W-nails were affected by their microstructural characteristics and strain levels, i.e., different surface energy. The corrosion resistance of both nails was improved by coating the Fe3O4 layer on the nail surface, where the corrosion extent for both J- and W-nails was similar due to the corrosion protection by the Fe3O4 layer. The corrosion characteristics directly affected their mechanical properties: tensile strength and ductility. This work systematically analyzed details of the mechanical properties of the nails after corrosion.

DOI： 10.1016/j.mtla.2023.101717

Hydrogen embrittlement characteristics of the hot- and cold-stamped 22MnB5 steel

Mitsuhiro Okayasu, Yuki Tanaka

International Journal of Fracture 240 ( 2 ) 243 - 255 2023年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

The hydrogen embrittlement (HE) characteristics of hot-stamped (HS) 22MnB5 steels were investigated experimentally with and without low-temperature heating. The HS process was carried out on a 22MnB5 plate that was heated to the temperature of its austenite region above the A(3) line before die quenching. Because of the varying degrees of work hardening and quenchability, the material properties of the HS samples differed depending on the area. Due to the variety of hydrogen-trapping sites, HE occurred significantly more with different systems. HE occurred with intergranular failure in the prior austenite grain and lath boundaries of the martensite phase in the HS samples without low-temperature heating. Moreover, the resistance of HE increased because of the creation of the epsilon-carbide trapping site in the HS samples after low-temperature heating. However, Ti-base precipitates were created during the heating process acceleration of HE. Specifically, the trapping sites obtained in this study play different HE roles: epsilon-carbide improves HE resistance, while the grain boundary of prior austenite and Ti-base precipitates led to HE. Furthermore, this study explored the HE mechanisms in detail.

DOI： 10.1007/s10704-022-00684-6

Effects of Artificial Agingon Material Properties of Die-Cast Al-Si-Cu-Mg-xNa Alloy

Naoki Sahara, Reiya Ikeda, Shaohua Wu, Mitsuhiro Okayasu

INTERNATIONAL JOURNAL OF METALCASTING 17 ( 1 ) 515 - 525 2023年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER INT PUBL AG

The influence of sodium on the microstructural characteristics and mechanical properties of die-cast Al-Si-Cu-Mg (ADC12) alloy has been examined experimentally. Mechanical properties of the die-cast Al-Si-Cu-Mg alloy have increased with the addition of 0.03% Na, because of modification of the shape of the Si eutectic particles from acicular to fine and fibrous appearing. However, the mechanical properties decreased when 0.04% Na or more was added, due to the formation of shrinkage porosity. The fine and fibrous appearing eutectic Si has been created by the interruption of the nucleation and growth of Si eutectic by Na. The mechanical properties of Na containing die-cast ADC12 alloy were further enhanced by a T5 treating which is a low-temperature aging, giving rise to significant precipitation of fine CuAl2 and Mg2Si. In contrast, the mechanical properties of die-cast ADC12 T6-treated samples were lower due to the formation of blister defects. Thus, the solid solution process in the T6 treatment may not make severe precipitation hardening of die-cast ADC12 due to overaging. Moreover, severe precipitation hardening occurred by the artificial aging after microstructural refinement by Na.

DOI： 10.1007/s40962-022-00793-x

Effect of Artificial Aging on the Mechanical and Fatigue Properties of the Hot-Stamped 6022 Aluminum Alloy

Mitsuhiro Okayasu, Masakazu Okawa

Journal of Materials Engineering and Performance 31 ( 8 ) 6386 - 6394 2022年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

The mechanical and fatigue properties of the 6022 aluminum alloy were investigated experimentally using U-bend samples fabricated through the hot-stamping process. The hot-stamping process was carried out at 550 degrees C before conducting artificial aging under several conditions. The alloy obtained via the hot-stamping process exhibited a lower degree of internal strain and a higher formability with a lower spring back compared with that obtained via the cold-stamping process. As the hot-stamping process was conducted at a temperature close to the solution temperature of the 6022 alloy, precipitation hardening occurred after artificial aging. The hardness of the hot-stamped 6022 alloy increased with increasing the aging temperature, and the high hardness was detected at 170 degrees C for 4 and 12 h due to the different strengthening mechanisms, namely dislocations and Mg2Si base precipitates. Like the results of the hardness, a high ultimate tensile strength and a high fatigue strength were obtained for the hot-stamped 6022 alloy after artificial aging at 170 degrees C for 4 h. The tensile strength is more than 35% higher than that of the cold-stamped sample. Although a high tensile strength was obtained after the aging process, the resistance to crack growth was not very high. This was attributed to the high crack driving force caused by the weak crack closure and low strain energy of the aged 6022 alloy. The failure characteristics of the hot-stamped 6022 samples were investigated via several experimental approaches to understand in detail the material properties of the aged 6022 alloy.

DOI： 10.1007/s11665-022-06716-5

Effect of the microstructural characteristics of die-cast ADC12 alloy controlled by Na and Cu on the mechanical properties of the alloy

Mitsuhiro Okayasu, Naoki Sahara, Norihito Mayama

Materials Science and Engineering A 831 142120 2022年1月

掲載種別：研究論文（学術雑誌）

The mechanical properties of a die-cast Al–Si–Cu alloy (DC ADC12) were improved using new processing techniques that induce grain refinement and precipitation hardening. Several gravity-cast ADC12 (GC ADC12) samples with different microstructural sizes were also evaluated to better understand the material properties of DC ADC12. The material properties of DC ADC12 indicate that the alloy has both advantages (fine grains) and disadvantages (defects). The mechanical properties of ADC12 can be improved by adding Na before the T5 process. The grain refinement of ADC12 was achieved by introducing Na using benign and inexpensive NaHCO3 rather than metallic Na. The Na elements including sodium oxide retained in the melt cause fine spherical eutectic structures to form in the spherical α-Al matrix by interrupting the Si phase growth. The phenomenon can be detected via a three-dimensional atom probe tomography analysis. Precipitation hardening of the die-cast Al alloy was achieved by subjecting it to T5 treatment and a large amount of Cu, replacing the T6 treatment, for 3 h at 175 °C, because the die casting process can be replaced with the solid solution process in T6 owing to its high cooling rate. Precipitation hardening occurred in the grain-refined DC ADC12-Na sample during the T5 process, which was verified via in situ observation of its microstructure using laser microscopy during the heating and cooling processes. Al2MgCu, CuAl2, and fine Si particles, which were different from the precipitates obtained using the T6 process, were produced in the spherical α-Al phase. The demonstrated techniques resulted in remarkable improvements in the ultimate tensile strength and fracture strain of DC ADC12.

DOI： 10.1016/j.msea.2021.142120

Effect of carbon nanotubes on the mechanical properties of cast Al–Si–Cu alloys

Mitsuhiro Okayasu, Shauha Wu, Indah Uswatun Hasanah

Materials Science and Technology (United Kingdom) 38 ( 4 ) 237 - 245 2022年

掲載種別：研究論文（学術雑誌）

To enhance the mechanical properties of cast Al–Si–Cu alloys (ADC12), 0.3% of carbon nanotubes (CNTs) was added. The cast ADC12–0.3%CNT samples were synthesised via heated mould continuous casting for the creation of fine grains, and the artificial aging process was conducted at 173°C for 13 h to obtain precipitation hardening. The reinforcement and refinement of eutectic Si and precipitates (Al2Cu and Mg2Si) were achieved by adding CNTs, which leads to a high tensile strength of about 450 MPa and a relatively high ductility of about 10%. The obtained tensile strength and 0.2% proof stress are more than twice as high as those of the conventional cast ADC12 alloy. The refinement of the microstructures could be due to the interruption of their growth caused by the CNTs.

DOI： 10.1080/02670836.2022.2037058

Effect of cooling rate on the precipitation characteristics of cast Al–Si–Cu alloy

M. Okayasu, N. Sahara, M. Touda

Archives of Foundry Engineering 21 ( 4 ) 55 - 60 2021年12月

The effects of the microstructural characteristics of Fe–0.33C–1.2Mn–xNb–xMo steels on hydrogen embrittlement fracture

Mitsuhiro Okayasu, Ryo Arai, Takehide Senuma

International Journal of Fracture 231 ( 2 ) 257 - 274 2021年10月

掲載種別：研究論文（学術雑誌）

The hydrogen embrittlement (HE) characteristics of Fe–0.33C–1.2Mn–xNb–xMo steels were investigated experimentally using various samples with differing microstructural characteristics. HE in steels was affected by hydrogen trapping sites: ε-carbide-based, Nb-based, and Mo-based precipitates, which were effective at enhancing HE resistance. In contrast, the prior austenite (γ) grain boundary within steel could act as hydrogen trapping sites and accelerate HE. In addition, hydrogen trapping occurred around the crack, leading to an acceleration of crack growth rate. There are various trapping sites in the steels with negative and positive effects on HE. The extent of the HE was clarified via tensile strength and resistance of delayed failure. Furthermore, the HE characteristics were analyzed using the samples with different quantity of hydrogen charged with two different methods. Based upon the above work, high HE resistance of the steel was proposed as Fe–0.33C–1.2Mn–0.05Nb–0.5Mo steels after a bake-hardening process at 170 °C for 20 min.

DOI： 10.1007/s10704-021-00586-z

Hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel

Mitsuhiro Okayasu, Takafumi Fujiwara

International Journal of Hydrogen Energy 46 ( 37 ) 19657 - 19669 2021年5月

掲載種別：研究論文（学術雑誌）

Hydrogen embrittlement (HE) characteristics of 22MnB5 steel (U-bent specimen) manufactured using hot-stamping process at various temperatures were experimentally and numerically investigated. Steel resistance to HE was examined through delayed failure tests under static and cyclic loading during hydrogen charging. First, the low cyclic loading caused severe HE, in which a clear difference in the extent of HE was obtained depending on the hot-stamped sample, which directly affected the microstructural characteristics and stress–strain distribution. The hot-stamped samples with large martensite phase showed low resistance to HE compared with those with small martensite phase because of the high concentration of hydrogen trapped in the phase boundaries. Moreover, the dual phase (ferrite and martensite) of the hot-stamped samples reduced their resistance to HE, which is caused by the hydrogen trapped in the laminar-shaped pearlite phase. The resistance to HE was improved by low-temperature heating at 200 °C for 1 h because of the generation of ε-carbides as trap sites as they render the hydrogen non-diffusible. Furthermore, the internal strain in the U-bent sample could accelerate HE because of the high concentration of hydrogen. These results were verified by experimental and numerical analyses. Thus, the hydrogen trapping mechanism was proposed as a valid mechanism for HE in 22MnB5.

DOI： 10.1016/j.ijhydene.2021.03.092

Effects of microstructural characteristics on the hydrogen embrittlement characteristics of austenitic, ferritic, and γ–α duplex stainless steels

Mitsuhiro Okayasu, Takafumi Fujiwara

Materials Science and Engineering A 807 2021年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

Hydrogen embrittlement (HE) characteristics of γ (AS), α (FS), and γ–α duplex (DS) stainless steels were examined experimentally and numerically. Severe HE occurred in the DS sample, whereas weak HE was detected in the AS and FS samples. This was attributed to the high hydrogen concentrations at the DS-trapping sites. Hydrogen trapping occurred in the low atomic density zones in the boundaries between α and γ phases in DS sample. The chemical bonding between atomic-scale phase boundaries was weakened by hydrogen penetration. This resulted in a crack growth along the DS α/γ phase boundaries. The ductility of DS decreased as the hydrogen content increased, especially when it exceeded 15 ppm. In contrast, the weak HE observed among AS and FS samples was attributed to the small hydrogen levels that infiltrated both samples. Finally, HE mechanism was proposed on the basis of these experimental results.

DOI： 10.1016/j.msea.2021.140851

A study of the mechanical properties of a Mg-Al-Zn alloy (AZ91) produced via Thixomolding

Mitsuhiro Okayasu, Takahiro Fukui

International Journal of Material Forming 14 ( 2 ) 271 - 280 2021年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER FRANCE

In this work, the mechanical properties and failure characteristics of a Mg-9.0Al-0.8Zn alloy (AZ91) fabricated via the Thixomolding process (TC) were investigated and compared with those of alloys formed via other casting processes, including cold-chamber die-casting (CD) and hot-chamber die-casting (HD). The microstructure of the three cast samples mainly consisted of the α-Mg phase and eutectic Mg17Al12. Fine, uniformly organized spherical α-Mg grains formed in the TC sample, and this morphology can be related to the TC process. A relatively small microstructure, comparable to the TC sample, was also obtained with CD because of the high solidification rate. However, large grains and cast defects were observed in the HD sample. Shrinkage porosity cast defects were dominant with HD, and they were caused by the low casting pressure. The ultimate tensile strength and 0.2% proof stress of the TC sample were 279 MPa and 192 MPa, respectively, which was ~10% higher than for the CD sample. The cast defects and large grains for the HD sample caused a significant reduction in its tensile properties, although its 0.2% proof stress was relatively similar to the CD sample. High fatigue strength and high crack growth resistance were realized with TC. Failure analysis was also carried out to probe the excellent mechanical properties of the TC sample.

DOI： 10.1007/s12289-020-01589-2

Piezoelectric properties of lead zirconate titanate ceramics at low and high temperatures

Mitsuhiro Okayasu, Masakazu Okawa

Advances in Applied Ceramics 120 ( 3 ) 127 - 133 2021年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：TAYLOR & FRANCIS LTD

The material properties and damage characteristics of lead zirconate titanate(PZT) ceramics were investigated at various temperatures. A positive voltage was obtained when the sample was cooled from 20°C to −190°C, while a negative voltage was obtained when the sample was heated from −190°C to 180°C. The difference between the positive and negative values depended on the thermal stress. Compressive stress generated a more positive voltage in the cooling process, while tensile stress led to a more negative voltage in the heating process. The voltage values also depended on the cooling (or heating) rate of the sample, e.g. the greater the cooling (or heating) rate, the greater the voltage. When cyclic loading was conducted mechanically at −190°C, the voltage reduced, but it was recovered after heating to 20°C. Damage of the PZT ceramic (90° domain switching) was detected when the sample was cooled to −190°C due to the high thermal stress.

DOI： 10.1080/17436753.2021.1904765

Microstructural Characteristics of Unidirectionally Solidified Cast Al–Si–Cu Alloy

Shaohua Wu, Mitsuhiro Okayasu, Tatsuya Kuwada

International Journal of Metalcasting 15 ( 3 ) 1073 - 1083 2021年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER INTERNATIONAL PUBLISHING AG

The properties of a conventional Al–Si–Cu alloy (ADC12) material, fabricated using a unidirectional casting process and slowly solidified in the range 0.02–0.14 °C s−1, were determined. The microstructural characteristics of the cast sample were dependent on sample area and resulted from changes in the amount of Si present during solidification. In the lower and middle regions, where the α-Al phase formed relatively organized crystal structures of different patterns, e.g., 〈101〉, columnar grain growth of α-Al dendrites with a low eutectic Si content was observed. Although columnar grain growth was also found in the upper region, it was randomly formed and the area was narrower. Random crystal orientation (i.e., weak control of unidirectional solidification) was created by interrupting columnar α-Al dendrite growth, which resulted from changes in the dynamics of the alloyed Si atoms. Eutectic Si is considered the only Si precipitate in ADC12; however, primary Si was also formed in the middle and upper regions, which was attributed to high Si concentrations resulting from Si migration to the upper region. Fine and coarse microstructures were observed in the lower and upper regions, respectively, with the middle region acting as a transition zone in which the amount of Si rapidly increased following transport between the lower and upper regions. A high amount of hard Si precipitate in the upper region of the sample resulted in high hardness values. In contrast, due to its fine microstructure with unidirectional crystal formation, the lower region exhibited high tensile strength and high ductility.

DOI： 10.1007/s40962-020-00542-y

Examination of Hydrogen Diffusivity in Carbon Steels Using a Newly Developed Hydrogen Permeation System

M. Okayasu, M. Sato

Experimental Mechanics 2021年

掲載種別：研究論文（学術雑誌）

Background: Hydrogen energy has received increased attention because of environmental needs and because it is an attractive replacement for fossil fuels. However, the presence of hydrogen in steels is known to be prejudicial to their global performance due to reduced ductility and unpredictable failures. Objective: To understand the diffusivity of hydrogen in carbon steels, a new hydrogen permeation system was developed to reveal hydrogen diffusion characteristics in carbon steels. Methods: Hydrogen gas was applied directly to one of the chambers to determine if hydrogen would permeate through the steel plate and into the other chamber. The hydrogen gas charged into the chamber at a pressure of 0.7 MPa after air removal using a vacuum pump, and fresh air was charged into the other chamber at atmospheric pressure before the measurement of the penetrated hydrogen. Results: Hydrogen did not diffuse substantially through a steel plate heated to less than 50 °C, but it did diffuse effectively through a carbon steel plate heated to more than 100 °C. The amount of hydrogen that diffused through the steel plate increased nonlinearly with increasing plate temperature and charging time. However, the diffusion amount saturated at 100 °C even after more than 2 h of charging, and hydrogen penetration through the steel plate stopped. In particular, the hydrogen atoms trapped in the steel plate interrupted the penetration of newly charged hydrogen atoms. Conclusion: By using newly proposed system, it is now possible to accurately quantify the diffusion amount of the hydrogen in the carbon steels, and hydrogen charging of the carbon steels was found to be irreversible. Based upon the results, hydrogen trapping system and hydrogen embrittlement characteristics were proposed.

DOI： 10.1007/s11340-021-00753-2

Decarburizing behavior and its effect on mechanical properties of ultrahigh strength steel sheets

Kuwata Kentaro, Takemoto Yoshito, Okayasu Mitsuhiro, Bian Jian, Senuma Takehide

ISIJ International 61 ( 4 ) 1300 - 1308 2021年

掲載種別：研究論文（学術雑誌）

In this study, the decarburization behavior, and its effect on mechanical properties of 2 000 MPa class martensitic steel were investigated with the purpose of collecting the basic data to realize the further strengthening of structural members for automobiles. Concerning the decarburization behavior, the maximum of the decarburization rate of 2 000 MPa class martensitic steel with a basic chemical composition of 0.35%C-1.2%Mn was found around 750°C and the thickness of the decarburized zone grew proportional to the square root of time. The addition of Nb reduced the decarburization rate. Concerning the effect of decarburization on mechanical properties, the decarburization significantly improved bendability even though the thickness of the decarburized layer is relatively thin. An observation of the crack behavior revealed that the initiation and propagation of the crack were suppressed by the decarburization layer. The resistance to delayed fracture was improved by decarburization. This improvement is presumed to be based on a similar mechanism for the improvement of bendability.

DOI： 10.2355/isijinternational.ISIJINT-2020-416

Failure Characteristics of PZT Ceramic During Cyclic Loading

Mitsuhiro Okayasu, Tsukasa Ogawa

Journal of Electronic Materials 49 ( 9 ) 5534 - 5541 2020年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

Failure characteristics of PbZrTiO3 (PZT) ceramic plates are investigated under cyclic loading with rods of different diameters, i.e., different contact areas (0-20 mm). The voltage generated under loading by the rod with the smallest diameter (contact area) is higher than those for the larger contact areas. This is due to the high strain induced in the PZT ceramic. However, the opposite trend is seen when the loading exceeds 60 N, i.e., the voltage obtained for the smallest contact area is lower. This is caused by failure of the PZT ceramic. The voltage generated under cyclic loading by the 5-mm, 10-mm, 15-mm, and 20-mm rods drops by about 10% in the early cyclic loading stage, but then remains constant until 10,000 cycles. The reduction in voltage is influenced mainly by 90 degrees domain switching. In this case, many grains (about 15% of the total) are switched: a random domain orientation is switched to the < 100 > direction perpendicular to the ceramic plate, i.e., a crystalline texture is formed. In contrast, there is significant reduction in voltage under loading by the 0-mm rod (point contact). As the extent of domain switching for the 0-mm rod is similar to that for the other rods, the reduction in electrical generation can be attributed to crack generation resulting from the high deformation.

DOI： 10.1007/s11664-020-08303-7

The effect of precipitations (NbC and carbide) in Fe-C-Mn-xNb steels on hydrogen embrittlement characteristics

Mitsuhiro Okayasu, Masaya Sato, Daiki Ishida, Takehide Senuma

Materials Science and Engineering A 791 2020年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

Hydrogen embrittlement (HE) characteristics in Fe-C-Mn-xNb steels were examined via various analyses, including electron backscatter diffraction analysis, scanning transmission electron microscopy and three-dimensional atom-probe tomography. For the investigation, the steel samples were prepared with varying Nb contents and heat treatment processes. The material properties of steel samples that were subjected to: (i) water quenching and (ii) quenching and tempering at 170 degrees C for 20 min, were determined to be nearly similar, although different degrees of HE were detected. After the tempering process, epsilon-carbide precipitated clearly in the matrix, which could act as a trapping site for hydrogen atoms and lead to improved HE resistance. Moreover, with addition of Nb, niobium base precipitates (e.g., NbC) with a diameter of a few nanometers were obtained in the martensite matrix, which could also function as hydrogen trapping sites. There was slight improvement in the HE resistance with NbC. Hydrogen-assisted failure mechanisms under both static and cyclic loading were observed with intergranular brittle cracking for the water quenched sample, even though the brittle and ductile mix failure mode was detected for the sample after the quenching and tempering process.

DOI： 10.1016/j.msea.2020.139598

Microstructure-dependent hydrogen diffusion and trapping in high-tensile steel

Mitsuhiro Okayasu, Jun Motojima

Materials Science and Engineering A 790 2020年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

In this work, the hydrogen embrittlement (HE) characteristics of high-tensile steel sheets with different microstructural characteristics were investigated. The sheets were fabricated via cold rolling (CR), water quenching (WQ), baking hardening (BH), and low-temperature annealing (LT), and their HE characteristics were clarified by examining the relationships between the microstructural characteristics and the severity of HE. Severe HE occurred in the WQ sample with hydrogen trapping at the boundaries of the retained austenite phases, resulting in intergranular and cleavage-like brittle failure. A reduction in HE was realized after the BH and LT processes. In these cases, hydrogen trapping was divided between the epsilon-carbide in the lattice spacings and at the boundaries of retained austenite, resulting in a mixed ductile/brittle failure mode. The extent of HE in the CR sample was similar to those in the BH and LT samples. However, the trapping sites were different; hydrogen trapping in the CR sample occurred in the slip band and around dislocations, resulting in delamination-like brittle failure on the slip planes. The extent of HE was also affected by the strain rate. More severe HE occurred in both the WQ and BH samples loaded slowly at 0.01 mm min(-1) compared to the samples loaded 1.0 mm min(-1) (i.e., intergranular failure). In this case, HE was affected by the large amount of hydrogen atoms trapped at the boundaries of the retained austenite phases. The hydrogen atoms in the lattice structure and epsilon-carbide migrated to the boundaries via dislocation movement. The extent of deterioration in tensile strength was two times higher in the samples loaded at the higher speed of 1.0 mm min(-1) compared to those loaded at 0.01 mm min(-1). Finally, the hydrogen trapping and failure mechanisms on the nano and atomic scales were discussed based on the results of the microstructural analyses.

DOI： 10.1016/j.msea.2020.139418

Hot-stamping technology for carbon fiber reinforced thermoplastic plates based on electrical resistance heating

Mitsuhiro Okayasu, Masaya Sato

Journal of Composite Materials 54 ( 10 ) 1353 - 1361 2020年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SAGE PUBLICATIONS LTD

In the present work, a hot-stamping system for carbon fiber reinforced thermoplastic (CFRTP) plates based on electrical resistance heating was developed, where CFRTP consisted of polyphenylene and polyacrylonitrile. With the hot-stamping process, a simple hat-shaped sample was made. The heating rate and maximum sample temperature varied depending on the electrical resistance of the CFRTP plate. Moreover, the contact conditions between the electrodes and the CFRTP plate also affected the sample temperature owing to their influence on the electrical resistance, which was determined by the amount of exposed carbon fiber (CF) on the sample surface. Temperature measurements performed using samples with various amounts of exposed CF (20%-95% CF) revealed that approximately 65% CF afforded the highest sample temperature and fastest heating rate. The CFRTP plate underwent non-uniform heating, especially during the early stages, e.g. less than 10 s. Sample heating to 150celcius resulted in permanent deformation of the hat-shaped CFRTP samples with less springback, whereas heating to higher temperatures above the melting point led to meandering of the samples. In contrast, CFRTP samples subjected to hot-stamping at lower temperatures, such as 110celcius, exhibited rough surfaces. In addition to the sample temperature, the formability of CFRTP during hot-stamping was affected by the holding time. When hot-stamping was performed without a holding time, even at high temperatures of 150celcius and above, low-quality samples with dented surfaces and irregular sample thickness were obtained. The results of this study indicate that a temperature of 150celcius and a holding time of 10 s are optimal for fabricating high-quality hot-stamped CFRTP with smooth surfaces and uniform thickness.

DOI： 10.1177/0021998319877559

An Electrical Resistance Joining Technology for Carbon Fiber-Reinforced Polyphenylene Sulfide Composites

Mitsuhiro Okayasu, Tomohumi Kubota

Journal of Materials Engineering and Performance 29 ( 4 ) 2134 - 2143 2020年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

This paper proposes a fusion bonding technology with electrical resistance heating for carbon fiber-reinforced plastic using polyphenylene sulfide thermoplastic resin with 60% carbon fiber (CFRTP), and suitable bonding conditions are investigated experimentally. The concept of the fusion bonding system is that the thermoplastic resin in the CFRTP sheet is heated to near its melting point of 280 degrees C via electric charging after clamping two CFRTP sheets at high pressures, 310 kPa, using electrodes made from aluminum rods. The heating temperature was controlled using the electrical resistances (5-20 O) between the two CFRTP sheets which were controlled using the exposed carbon fiber (CF) on the CFRTP surface: a lower exposure rate gives a higher electrical resistance, e.g., 15 O for CF20% and 5 O for CF80%. In addition, the sample temperature was dependent on the electric current and the electric charging time. Two CFRTP sheets were successfully bonded using the fusion bonding system with the maximum strength of more than 30 MPa, where an electric current of 4.0 A was applied for 10 s. The bonding strength was attributed to changes in the material properties, especially the bonding resin. The hardened bonding resin was created from heat processing at 250 degrees C, which led to a high bonding strength.

DOI： 10.1007/s11665-020-04764-3

Effects of loading contact on electric-power generation of lead zirconate titanate piezoelectric ceramic plate

Mitsuhiro Okayasu, Tsukasa Ogawa

Journal of Advanced Ceramics 8 ( 4 ) 509 - 518 2019年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGEROPEN

To better understand the generation of electric power for piezoelectric PbZrTiO3 (PZT) ceramic plate (phi 25 mm), an attempt was made to investigate experimentally and numerically electric-power generation characteristics during cyclic bending under various loading fixtures (phi 0-phi 20 mm), i.e., different contact areas. Increasing the load-contact area on the PZT ceramic leads to a nonlinear decrease in the generated voltage. Decreasing contact area basically enhances the generated voltage, although the voltage saturates during loading when the contact area is less than phi 5 mm. A similar voltage is generated for phi 0 and phi 5 mm, which is attributed to strain status (ratio of compressive and tensile strain) and material failure due to different stress distribution in the PZT ceramic. On the basis of the obtained electric generation voltage, suitable loading conditions are clarified by loading with the phi 5 mm fixture, which generates a higher voltage and a longer lifetime of the PZT ceramic. From this approach, it is appeared that the area contact with the area ratio of 0.04 (phi 5 mm/phi 20 mm) is suitable to obtain the high efficiency of the electric voltage.

DOI： 10.1007/s40145-019-0331-7

Mechanical and fatigue properties of long carbon fiber reinforced plastics at low temperature

Mitsuhiro Okayasu, Yuki Tsuchiya

Journal of Science-Advanced Materials and Devices 4 ( 4 ) 577 - 583 2019年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：VIETNAM NATL UNIV

The mechanical properties of long unidirectional (UD) and crossply (CR) carbon fiber reinforced plastics (CFRPs) were investigated at a low temperature (-196 degrees C). The CFRPs were fabricated from 60 vol.% carbon fiber and epoxy resin. The bending strength of the UD-CFRP was approximately twice that of the CR-CFRP. The high strength of the UD-CFRP was directly attributed to the amount of carbon fiber oriented along the loading direction: 60% for UD-CFRP compared with 30% for CR-CFRP. The low-temperature (-196 degrees C) tensile and fatigue strengths of the UD-CFRP were over 1.5 times greater than those at room temperature (20 degrees C). This was attributed to the increased epoxy strength at low temperatures along with the internal compressive stress arising from the different thermal expansion coefficients of the carbon fiber and epoxy. Both the epoxy strength and internal compressive strength were employed as factors in a compound law to numerically estimate the low-temperature tensile strength. This work presents a systematic analysis for changes in the CFRP material properties at low temperatures. (C) 2019 Publishing services by Elsevier B.V. on behalf of Vietnam National University. Hanoi.

DOI： 10.1016/j.jsamd.2019.10.002

Effect of Microstructural Characteristics on Mechanical Properties of Ferritic Stainless Steel

Mitsuhiro Okayasu, Tomoki Shigeoka

Journal of Materials Engineering and Performance 28 ( 11 ) 6771 - 6778 2019年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

To improve the mechanical properties of hot-rolled ferritic stainless steel (SUS430), the microstructural characteristics of SUS430 were changed using a heating process under various conditions. The hardness of SUS430 decreased upon the increase in the heating temperature to 900 degrees C, and the hardness increased when the sample was heated to temperatures greater than 900 degrees C. The high hardness of the sample heated at 1000 degrees C (H-1000 degrees C) is attributed to the heating time: A high hardness was obtained for a H-1000 degrees C sample that was heated for 1 h (H1000 degrees C-1h), but this decreased when the heating time was increased to more than 1 h. The high hardness of H1000 degrees C-1h is caused by the fine Cr23C6 precipitates that are distributed in the sample around the grain boundaries. On the other hand, the large precipitates of Cr23C6 in H1000 degrees C-12h decrease the hardness. The hardness value of SUS430 is directly attributed to the mechanical properties and the ultimate tensile strength. The tensile strength of H1000 degrees C-1h was found to be about 200% and 20% higher than the as-received and H1000 degrees C-12h samples, respectively. Despite the increase in the tensile strength of the H1000 degrees C-1h sample, the ductility was not found to decrease significantly, for example, the fracture strain was approximately 25%. This occurrence is affected by a severe slip in the ferrite base grain, and the high strength of H1000 degrees C-1h is influenced by the interruption of the slip by the Cr23C6 precipitates. Unlike the tensile strength, similar fatigue properties were observed for both H1000 degrees C-1h and H1000 degrees C-12h, which is associated with the low crack driving force of H1000 degrees C-12h, caused by the roughness-induced crack closure arising from the large Cr23C6 precipitates.

DOI： 10.1007/s11665-019-04426-z

Fatigue Failure Characteristics of Single-Crystal Fe-3%Al Alloy

Mitsuhiro Okayasu, Keisuke Noda

Journal of Materials Engineering and Performance 28 ( 10 ) 5978 - 5987 2019年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

To obtain a better understanding of the mechanical properties and failure characteristics of the single-crystal Fe-3%Al alloy (SC), tensile and fatigue properties of the SC and polycrystalline Fe-3%Al alloy (PC) have been investigated. The tensile strength and fracture strain for the SC samples were different depending on the loading direction, i.e., < 111 >, < 110 > and < 100 >, which could be explained by the Schmid factor. Fatigue strength for the SC samples was basically correlated with their tensile strength. Different fatigue properties were obtained for the PC sample, e.g., the high fatigue strength was obtained in the early fatigue stage, and significant reduction in the fatigue strength was appeared in the later fatigue stage, e.g., low endurance limit at 10(7) cycles. The reason for this was caused by the material brittleness due to the accumulated dislocation at the grain boundaries. Because the high ductile SC samples made severe plastic deformation ahead of the crack tip, the crack growth rate did not enhance even if the high stress intensity factor range was applied in Region II (da/dN versus Delta K). The crack growth rate dropped just before the final fracture. Effective stress intensity factor range was estimated using the crack closure model, but the crack growth rate is found to be affected strongly by the slip system of SC sample.

DOI： 10.1007/s11665-019-04331-5

Structural Health Monitoring System for Remote Inspection of Material Failure

Mitsuhiro Okayasu, Toshiki Yamasaki

Journal of Nondestructive Evaluation 38 ( 2 ) 2019年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER/PLENUM PUBLISHERS

A new structural health monitoring (SHM) system is proposed for inspecting the failure characteristics of engineering materials, which are monitored remotely. This system is controlled using electrical devices with wireless networking system. A PZT ceramic transducer, an accelerometer and a strain gauge are used to understand the failure characteristics. These sensors are attached to the test sample and reveal its static and dynamic strain behaviour, i.e., information on the material failure (crack generation) and related strains. It is also designed that a dummy plate with a PZT material is attached to the test sample. The dummy plate is designed to fracture earlier than the test sample during the loading process, whereupon the SHM system is triggered via the piezoelectric effect. A compact prototype SHM system is created to evaluate the effectiveness of our system. The failure characteristics are monitored well using the prototype SHM system, which might be able to predict the lifetime of the test sample, i.e., the SHM system reveals the critical strain value and crack growth rate just before the final failure of the test samples.

DOI： 10.1007/s10921-019-0592-7

Effect of Microstructural Characteristics on Mechanical Properties of Austenitic, Ferritic, and - Duplex Stainless Steels

Mitsuhiro Okayasu, Daiki Ishida

Metallurgical and Materials Transactions A 50A ( 3 ) 1380 - 1388 2019年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

Materials properties of (austenite), (ferrite), and - duplex stainless steels were experimentally examined using samples with different grain sizes (8 to 1000 mu m) and different ratios of the to phase ( proportion: 35 to 78pct). The mechanical properties (hardness and tensile strength) of the duplex stainless steel were about 1.5 times higher than those of the austenitic and ferritic stainless steels. Two main reasons for the high strength of duplex stainless steel were identified as follows: (i) severe interruption of slip deformation in the phase on the phase; (ii) a high misorientation angle around phase boundaries between the and phases, caused by bonding of the different lattice structures: -fcc and -bcc. The ultimate tensile strength of duplex stainless steel increased with increasing proportion of the phase to 50pct, but decreased with a further increase in the amount of phase. The mechanical properties improved with decreasing grain size of the stainless steels, which follows the Hall-Petch relationship; however, the reverse relationship was obtained for ferritic stainless steel, especially with large grain sizes (100 to 1000 mu m), in which the size of hard Cr23C6 precipitates increased with increasing grain size.

DOI： 10.1007/s11661-018-5083-4

Influence of microstructure on the mechanical properties and hydrogen embrittlement characteristics of 1800MPa grade hot-stamped 22MnB5 steel

Mitsuhiro Okayasu, Lele Yang

Journal of Materials Science 54 ( 6 ) 5061 - 5073 2019年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

The influence of the microstructural characteristics on the mechanical and hydrogen embrittlement properties of 1800MPa grade hot-stamped 22MnB5 steel was experimentally investigated using samples processed under various hot-stamping conditions, i.e., different heating temperatures and strain levels. The tensile strength increased with increasing hot-stamping temperature up to approximately 920 degrees C and subsequently decreased owing to the increasing sizes of the lath martensite and prior austenite phases. Some degree of internal strain was introduced into the 22MnB5 steel specimen during hot stamping at 920 degrees C, which led to a slightly higher hardness although no clear microstructural change was observed. The severity of hydrogen embrittlement of the hot-stamped 22MnB5 steel samples was investigated after immersion in a NH4SCN solution, and the degree of hydrogen embrittlement was found to be directly associated with the amount of hydrogen that penetrated into the grain boundary and lath martensite boundary. The high-strength 22MnB5 steel with a very small lath martensite phase exhibited severe hydrogen embrittlement due to the large amount of hydrogen in the sample, and the high internal strain (or high dislocation density) could lead to accelerated hydrogen embrittlement. Severe hydrogen embrittlement occurred upon charging with more than approximately 0.8ppm hydrogen. Based on the obtained results, models are proposed for the hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel.

DOI： 10.1007/s10853-018-3175-6

Fatigue properties of a nanocrystalline titanium based bulk metallic glassy alloy

Mitsuhiro Okayasu, Tomoki Shigeoka

Journal of Science-Advanced Materials and Devices 3 ( 4 ) 478 - 484 2018年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：VIETNAM NATL UNIV

To obtain a better understanding of the fatigue properties and crack growth characteristics of a nanocrystalline titanium based bulk metal glasses (Ti-BMG) made by vacuumed casting process, the fatigue failure mechanisms of Ti-BMG have been investigated via S - N and da/dN - Delta K tests. For comparison, the crystalline Ti alloy Ti-Al6V4 was also employed. The fatigue strength in the early fatigue stage was high for Ti-BMG due to the high tensile strength. However, the fatigue strength decreased significantly in the late fatigue stage. The higher slope of S - N relation was detected for Ti-BMG, which crossed that for the Ti-Al6V4 sample around 5 x 10(3) cycles. In the higher Region II, the fatigue crack growth rate was of similar level for both Ti-BMG and Ti-Al6V4 due to their similar strain energy. In the lower Region II, however, the lower crack growth resistance was obtained for Ti-BMG, as compared to Ti-Al6V4. This was attributed to the high crack driving force for Ti-BMG, caused by the weak roughness-induced crack closure. Such crack closing characteristics of Ti-BMG were systematically investigated by various experimental techniques. (c) 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.

DOI： 10.1016/j.jsamd.2018.10.001

Method for reducing aluminum alloy heat loss in high pressure die casting shot sleeves

M. Okayasu, S. Wu, T. Hirayama

International Journal of Cast Metals Research 31 ( 5 ) 308 - 316 2018年9月

掲載種別：研究論文（学術雑誌）

To reduce the amount of casting defects in diecast components, a new shot-sleeve is proposed. Cold flakes are generated by solidification of the casting material in the shot-sleeve before the injection process. To solve this problem, a thermal insulation system is introduced on the inner shot-sleeve surface, with tiny grooves being machined to create an air gap between the melt and the sleeve surface. Various casting materials with different material properties are used to examine the insulating effect of the shot-sleeve. The degree of thermal insulation is found to be influenced by the material properties, with the surface tension being significant factors. This thermal insulation effect allows the production of high-quality samples and therefore with good mechanical properties. When casting materials with low surface tension are used, the melt is able to penetrate deeply into the grooves of the shot-sleeve, lessening the insulation effect and leading to the defect.

DOI： 10.1080/13640461.2018.1470370

Tensile Properties of Unsaturated Polyester and Epoxy Resin Reinforced with Recycled Carbon-Fiber-Reinforced Plastic

Mitsuhiro Okayasu, Yuta Kondo

Applied Composite Materials 25 ( 3 ) 561 - 568 2018年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

To better understand the mechanical properties of recycled carbon-fiber-reinforced plastic (rCFRP), CFRP crushed into small pieces was mixed randomly in different proportions (0-30 wt%) with two different resins: unsaturated polyester and epoxy resin. Two different sizes of crushed CFRP were used: 0.1 mm x 0.007 mm (milled CFRP) and 30 mm x 2 mm (chopped CFRP). The tensile strength of rCFRP was found to depend on both the proportion and the size of the CFRP pieces. It increased with increasing proportion of chopped CFRP, but decreased with increasing proportion of milled CFRP. There was no clear dependence of the tensile strength on the resin that was used. A low fracture strain was found for rCFRP samples made with chopped CFRP, in contrast to those made with milled CFRP. The fracture strain was found to increase with increasing content of milled CFRP up to 20 wt%, at which point, coalescence of existing microvoids occurred. However, there was a reduction in fracture strain for rCFRP with 30 wt% of milled CFRP, owing to the formation of defects (blow holes). Overall, the fracture strain was higher for rCFRPs based on epoxy resin than for those based on unsaturated polyester with the same CFRP content, because of the high ductility of the epoxy resin. The different tensile properties reflected different failure characteristics, with the use of chopped CFRP leading to a complicated rough fracture surface and with milled CFRP causing ductile failure through the presence of tiny dimple-like fractures. However, for a high content of milled CFRP (30 wt%), large blow holes were observed, leading to low ductility.

DOI： 10.1007/s10443-017-9635-3

MECHANICAL PROPERTIES OF CAST Al-Mg-5 ALLOY PRODUCED BY HEATED MOLD CONTINUOUS CASTING

Mitsuhiro Okayasu, Shuhei Takeuchi

International Journal of Metalcasting 12 ( 2 ) 298 - 306 2018年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER INTERNATIONAL PUBLISHING AG

To make the high strength and high ductility of the cast aluminum alloys, the Al alloy samples are created by the heated mold continuous casting (HMC) process. Because of the rapid and unidirectional solidification in the HMC process, tiny grains with uniformly orientated crystals are created. In this case, influence of the solidification speed of HMC on the mechanical properties of the Al-Mg-based alloy is studied. The microstructures are altered with the casting speed because of the different solidification rates. The mean secondary dendrite arm spacing (SDAS) for the HMC-Al-Mg alloy made at the high casting speed of 2.7 mm/s is about one-fifth of that for the conventional gravity cast Al-Mg alloy. Nonlinear relationship between the tensile properties (strength and ductility) and the cooling rate are obtained with sigma(UTS) and sigma(0.2) increasing and epsilon(f) decreasing with increasing the cooling rate. The high strength of the HMC samples at the high cooling rate results from the existence of tiny microstructures. The high ductility of the HMC sample at the low cooling rate is affected by the uniform crystal orientation, where shear slips are facilitated by the uniform crystal orientation.

DOI： 10.1007/s40962-017-0163-6

Domain switching behavior in lead zirconate titanate piezoelectric ceramics

Mitsuhiro Okayasu, Kohei Bamba

Scripta Materialia 146 272 - 275 2018年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD

The domain switching behaviors of lead zirconate titanate ceramics were monitored via direct, in-situ lattice observation using transmission electron microscopy. The lattice structures were monitored via video as a sample was heated from room temperature to 300 degrees C. The lattices vibrated in the [110] direction several times over a short period, e.g., 0.1 s. Moreover, lattice sliding occurred in the [100] direction for about 15 A. Those lattice motions were detected at around 83-84 degrees C. Lattice orientations were analyzed using the domain switching model to substantiate whether these movements can be attributed to domain switching. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.scriptamat.2017.12.003

Mechanical properties of the riveted connecting forming of CFRP plates

Mitsuhiro Okayasu, Ryo Naito

International Journal of Material Forming 11 ( 2 ) 225 - 232 2018年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER FRANCE

Mechanical properties of the unidirectional CFRP sheets connected by various rivet formations are experimentally investigated to propose the suitable riveted connection of the CFRP sheets. In this approach, the rivet connection is conducted with different position and different number. The connecting strength is dependent on the fracture mode. In basic, two different fracture modes are obtained: rivet- and CFRP-based fracture, where the high and the low strength are obtained for the sample fractured mainly by rivet and CFRP, respectively. However, in addition to the two fracture modes, rivet failures with micro-crack in CFRP are detected. The micro-cracks make change of the stress-strain curve although, like the rivet fractured sample, the high strength is still obtained. The failure in CFRP is attributed to the fiber/matrix interfacial debonding. The failure characteristics of CFRP can be monitored during the shear loading test with variation of the electric current in the CFRP plates. The concept of this approach is to examine the change of the electric resistance due to the material failure. In this case, the decrement of the current value is related to the failure in the rivet and CFRP. With the rivet formation, the fracture mode is altered; and appropriate rivet positions are determined to make the high connecting strength of the CFRP plates.

DOI： 10.1007/s12289-017-1344-9

Effects of machined circular holes on the mechanical properties of unidirectional carbon fiber-reinforced plastics

Mitsuhiro Okayasu, Ryo Naito, Daisuke Fukuyama, Keiji Ogi

Journal of Composite Materials 52 ( 4 ) 465 - 474 2018年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SAGE PUBLICATIONS LTD

The effects of machined circular holes on the mechanical properties and failure characteristics of a unidirectional CFRP were investigated. Our approach was to change the location and the number of holes: (i) a two-set hole was machined with different modes: (A) parallel, (B) 45 degrees and (C) a direction perpendicular to the loading direction; and (ii) multiple holes from n=0 to 5 were made in the sample parallel to the loading direction. The higher tensile and higher fatigue strengths were obtained for the CFRP sample produced by mode A, compared to modes B and C. This was attributed to the different extend of the maximum stress and stress distribution, caused by the geometrical effects on the sample. The ultimate tensile strength (sigma(UTS)) of the sample was well predicted by the geometrical criterion. If the number of holes was increased from n=0 to n=2, the tensile strength decreased dramatically. However, the tensile strength did not strongly decrease further for samples with multiple holes from n=2 to 5. The tensile strength is correlated with the maximum stress adjacent to the hole(s). Those stress values were verified by a 2D digital image correlation and a finite element analysis. Material failure of the CFRP during tensile loading was revealed by nondestructive testing using a piezoelectric ceramic, and debonding of the fibers occurred even at a low applied stress of approximately 35% sigma(UTS).

DOI： 10.1177/0021998317709331

圧電セラミックスの90°ドメインスイッチング特性に及ぼす熱応力の影響

馬場康平, 岡安光博

日本機械学会論文集 84 ( 863 ) 2018年

Corrosion characteristics of a tungsten alloy die-casting mould material in molten aluminium alloy

Mitsuhiro Okayasu, Lele Yang

International Journal of Cast Metals Research 31 ( 3 ) 162 - 168 2018年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：TAYLOR & FRANCIS LTD

To understand the corrosion characteristics of W90 tungsten alloy and compare them with those of conventional hot-worked steel (SKD61), immersion tests were carried out in molten aluminium alloy (ADC12). Severe corrosion occurred in SKD61, whereas the corrosion resistance of W90 was about 40 times greater, with little occurring even after 300 h immersion. However, significant microstructural changes occurred in W90 resulting from penetration of aluminium and silicon through grain boundaries to create WAl5 and WSi2 phases, detectable even after a few hours' immersion. Relatively soft (hardness 1.5 GPa) WAl5 formed between the W based grains, and hard (10 GPa) WSi2 outside the W based grains. Tensile strength decreased slowly with increasing immersion time, while a significant reduction in fracture strain occurred. These changes in tensile properties were caused by the microstructural changes, e.g. soft and brittle intermetallic compounds. Crack propagated along the WAl5 phases (intergranular fracture), whereas transgranular fracture was dominant for uncorroded or less corroded W90.

DOI： 10.1080/13640461.2017.1405528

Experimentally and analyzed property of carbon fiber reinforced thermoplastic and thermoset plates

M. Okayasu, Y. Tsuchiya, H. Arai

Journal of Materials Science Research 7 ( 3 ) 12 - 21 2018年

In situ TEM observations of microstructural characteristics of lead zirconate titanate piezoelectric ceramic during heating to 1000 °C

M. Okayasu, T. Ogawa, Y. Sasaki

Ceramics International 43 ( 18 ) 16306 - 16312 2017年12月

High strength of Mg–9%Al–1%Zn alloys achieved by severe working

M. Okayasu, T. Muranaga

Journal of Materials Engineering and Performance 26 ( 10 ) 4977 - 4989 2017年10月

Effects of 90 degrees domain switching on electric generation properties of PZT ceramic

Mitsuhiro Okayasu, Toshiki Yamasaki

Ceramics International 43 ( 4 ) 3590 - 3600 2017年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The influence of domain switching on the electric generation properties of lead zirconate tinatate (PZT) ceramic has been investigated after static and cyclic loadings under various conditions. A PZT ceramic of size phi 8.0 mmx0.17 mm, consisting of a tetragonal lattice structure, i.e., c/a not equal 1.014, was used. Domain switching occurred as a result of the applied stress, where three different switching modes were employed: (I) simple 90 switching; (II) 90 switching with 90 rotation; and (III) 90 switching with 180 rotation. The rates of the switching mode were different: the simple switching mode (Mode I) was 20%; and both complicated switching modes (Mode II and III) were 40%. The extent of 90 domain switching was different depending on the grain and where the direction of the tetragonal structure (c-axis direction) was affected, e.g., the closer the parallel between the c-axis and the stress direction, the stronger the domain switching. The electric generation voltage increased with increasing applied cyclic stress; however, that voltage dropped suddenly as the stress value was close to its elastic limit. This is due to the 90 domain switching. Such domain switching (reduction of the electric voltage) occurred in the early cyclic stage.

DOI： 10.1016/j.ceramint.2016.11.196

Analysis of microstructural effects on mechanical properties of copper alloys

Mitsuhiro Okayasu, Takuya Muranaga, Ayana Endo

Journal of Science-Advanced Materials and Devices 2 ( 1 ) 128 - 139 2017年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：VIETNAM NATL UNIV

With the aim of obtaining copper alloys with favorable mechanical properties (high strength and high ductility) for various engineering applications, the microstructural characteristics of two conventional copper alloys - an aluminum bronze (AlBC; Cu-Al-9.3-Fe-3.8-Ni-2-Mn-0.8) and a brass (HB: Cu-Al-4-Zn-25-Fe-3-Mn-3.8) - and a recently developed aluminum bronze (CADZ: Cu-Al-10.5-Fe-3.1-Ni-3.5-Mn-1.1-Sn-3.7), were controlled by subjecting the alloys to two different processes (rolling and casting) under various conditions. For the rolling process, the rolling rate and temperature were varied, whereas for the casting process, the solidification rate was varied. Microstructural characteristics, as examined by electron backscatter diffraction analysis, were found to differ among the alloys. Complicated microstructures formed in CADZ led to high hardness and high tensile strength (sigma(UTS)), but low ductility (epsilon(f)). For CADZ, casting at a high solidification rate allowed an increase in ductility to be obtained as a result of fine-grained structure and low internal stress. In contrast, high ductility (with a fracture strain of more than 30%) was found for both cast AlBC and cast HB; moreover, both of these alloys possessed high tensile strength when produced by warm rolling at 473 K. For CADZ, on the other hand, no clear effect of rolling on tensile strength could be found, owing to the many microcracks caused by its brittleness. The results of this study indicate that copper alloys with excellent mechanical properties can be produced. This is especially the case for the conventional alloys, with a high tensile strength sigma(UTS) = 900 MPa and a high fracture strain epsilon(f) = 10% being obtained for warm-rolled brass. (C) 2016 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license.

DOI： 10.1016/j.jsamd.2016.12.003

Effects of Oxide Scale on the Corrosion and Mechanical Properties for a High Speed Steel and a Graphite Cast Iron

Mitsuhiro Okayasu, Shaohua Wu

Oxidation of Metals 87 ( 1-2 ) 159 - 178 2017年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER/PLENUM PUBLISHERS

The mechanical properties and the oxidation characteristics of the high speed steel (HSS) for hot-rolling process, produced by centrifugal casting process, were investigated experimentally. Because of the complicated microstructure with tiny V-, Cr-, and W-based precipitations in the matrix, high hardness of the HSS samples was obtained. Moreover, high compressive strength and high compressive strain were detected, while low tensile properties were obtained. In spite of the high hardness of HSS, the wear resistance was a low level due to severe oxidation on the HSS surface and generated cracks, but the wear resistance of HSS was still better than conventional graphite cast iron. Although the corrosion occurred by the wear test, no clear microstructural change was detected. Wear and corrosion characteristics were examined at high temperatures and high humidity using an originally proposed testing machine. With this testing machine, failure characteristics of HSS were clarified, in which the HSS sample was worn away easily due to the brittle oxidation scale during the rolling process, leading to the rough HSS surface. Furthermore, the low tensile strength of HSS made an acceleration of the failure especially crack growth. Such rough surface has made reduction in the quality of the rolled materials.

DOI： 10.1007/s11085-016-9663-7

Phase transformation system of austenitic stainless steels obtained by permanent compressive strain

Mitsuhiro Okayasu, Sai Tomida

Materials Science and Engineering A 684 712 - 725 2017年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

In order to understand more completely the formation of strain-induced martensite, phase structures were investigated both before and after plastic deformation, using austenitic stainless steels of various chemical compositions (carbon C=0.007-0.04 mass% and molybdenum Mo=0-2.10 mass%) and varying pre-strain levels (0-30%). Although the stainless steels consisted mainly of gamma austenite, two martensite structures were generated following plastic deformation, comprising epsilon and alpha' martensite. The martensitic structures were obtained in the twin deformation and slip bands. The severity of martensite formation (epsilon and alpha') increased with increasing C content. It was found that alpha' martensite was formed mainly in austenitic stainless steel lacking Mo, whereas a high Mo content led to a strong epsilon martensite structure, i.e. a weak alpha' martensite. The formation of alpha' martensite occurred from gamma austenite via e martensite, and was related to the slip deformation. Molybdenum in austenitic stainless steel had high slip resistance (or weak stress-induced martensite transformation), because of the stacking fault energy of the stainless steel affecting the austenite stability. This resulted in the creation of weak alpha' martensite. Models of the martensitic transformations gamma (fcc)-> epsilon (hcp)-> alpha' (bcc) were proposed on both the microscopic and nanoscopic scales. The alpha' martensite content of austenitic stainless steel led to high tensile strength; conversely, epsilon martensite had a weak effect on the mechanical strength. The influence of martensitic formation on the mechanical properties was evaluated quantitatively by statistical analysis.

DOI： 10.1016/j.msea.2016.12.101

Domain switching characteristics of lead zirconate titanate piezoelectric ceramics on a nanoscopic scale

Mitsuhiro Okayasu, Kohei Bamba

Journal of the European Ceramic Society 37 ( 1 ) 145 - 159 2017年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

Domain switching characteristics of lead zirconate titanate ceramics with and without poling under compressive loading are investigated using electron backscatter diffraction. For loading in the poling direction, the switching strain is stronger than that for loading perpendicular to the poling direction. There is strong domain switching when the domain (c-axis of the tetragonal structure) is orientated close to the loading direction. A large number of domains are switched between 85.4 degrees and 90.0 degrees, with many crossing the loading axis. Each grain consists of domains with three different patterns; i.e., with c-axis orientated in three directions in each grain. The patterns remain unchanged even with domain switching and strong deformation. However, the ratios among the patterns depend on compressive stress. Under stress, one or two specific domain modes are switched to about 90, although others are not switched as much. These domain switching characteristics are related to the poling and loading directions. 90 domain switching model is proposed on the basis of twin deformation model. Due to the aspect ratio of c/a = 1.014 (tetragonal structure), the angle of the switching is less than 90 degrees (89.2 degrees). This angle is corresponding to the switching angle obtained by an electron backscatter diffraction analysis (Ave. 88.9 degrees). (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jeurceramsoc.2016.07.034

A New Connecting Technology of Carbon Fiber Reinforced Plastic Plates

I.U. Hasanah, S. Baba, M. Okayasu

International Journal on Smart Material and Mechatronics 4 ( 1 ) 257 - 260 2017年

Mechanical properties of Al-Si-Cu alloys produced by the continuous casting process with heating process

Mitsuhiro Okayasu, Shuhei Takeuchi, Toshihiro Ochi

International Journal of Cast Metals Research 30 ( 4 ) 217 - 225 2017年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：TAYLOR & FRANCIS LTD

The mechanical properties of aluminium alloys produced by the continuous cast process and heating process (heat-cast-sample) were investigated, where the aluminium alloys are heated continuously to high temperatures for 1h immediately following heated mould continuous casting (HMC) and sand gravity casting (SGC). The material strength and ductility of the aluminium alloys were irregularly altered depending on the heating temperature. The mechanical properties decreased when the heating temperature increased to 400 degrees C and were then recovered when the temperature increased to 520 degrees C. However, these properties decreased again when heated to more than 540 degrees C. The mechanical properties of the HMC-heat-cast-sample showed overall higher than those for the SGC-sample. In addition to high tensile strength, high ductility was obtained for the HMC-520 degrees C samples compared with those for the as-cast-sample. Such changes were found to be directly attributable to the different severity of precipitate; moreover the crystal orientation was unchanged even after the heating process.

DOI： 10.1080/13640461.2017.1286556

Mechanical Properties of Magnesium Alloys Produced by the Heated Mold Continuous Casting Process

M. Okayasu, S. Wu, T. Tanimoto, S. Takeuchi

Archives of Foundry Engineering 16 ( 4 ) 208 - 216 2016年12月

掲載種別：研究論文（学術雑誌）

Investigation of the tensile and fatigue properties of cast magnesium alloys, created by the heated mold continuous casting process (HMC), was conducted. The mechanical properties of the Mg-HMC alloys were overall higher than those for the Mg alloys, made by the conventional gravity casting process (GC), and especially excellent mechanical properties were obtained for the Mg Y Zn -HMC alloy. This was because of the fine-grained structure composed of the α-Mg phases with the interdendritic LPSO phase. Such mechanical properties were similar levels to those for conventional cast aluminum alloy (Al Si Cu Fe Zn alloys: ADC12), made by the GC process. Moreover, the tensile properties (σ and ϵ ) and fatigue properties of the Mg Y Zn -HMC alloy were about 1.5 times higher than that for the commercial Mg Al Zn -GC alloy (AZ91). The high correlation rate between tensile properties and fatigue strength (endurance limit: σ ) was obtained. With newly proposed etching technique, the residual stress in the Mg Y Zn alloy could be revealed, and it appeared that the high internal stress was severely accumulated in and around the long-period stacking-order phases (LPSO). This was made during the solidification process due to the different shrinkage rate between α-Mg and LPSO. In this etching technique, micro-cracks were observed on the sample surface, and amount of micro-cracks (density) could be a parameter to determine the severity of the internal stress, i.e., a large amount to micro-cracks is caused by the high internal stress. 97 2 1 84.7 10.5 2.5 1.3 1 UTS f 97 2 1 90 9 1 l 97 2 1

DOI： 10.1515/afe-2016-0111

Mechanical properties of austenitic stainless steel with high niobium contents

M. Okayasu, S. Wu, K. Noda, D. Y. Lin, S. M. Yang

Materials Science and Technology (United Kingdom) 32 ( 13 ) 1382 - 1394 2016年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：TAYLOR & FRANCIS LTD

To better understand the influence of the microstructural characteristics on the mechanical properties of Fe-Cr -Ni -Nb austenitic stainless steel (ASS-Nb ), the mechanical properties were investigated. Nb addition was conducted with four different amounts: 0, 0.29, 0.58 and 0.86%. With the increasing Nb content, the mean grain size for the Fe-Cr-Ni-based alloys decreased, while the size of Nb precipitate increased. Owing to the different microstructural characteristics, their mechanical properties were altered. The highest tensile strength was obtained for ASS-Nb alloy. However, with the increasing Nb content in ASS-Nb alloys, the tensile strength decreases despite the grain refinement. The mechanical properties of the ASS-Nb alloys were influenced by the Nb precipitation hardening and the grain boundary cohesive strength, arising from the size of Nb precipitations. 22 25 x x 0.29 0.29–0.86

DOI： 10.1080/02670836.2015.1126431

A study of the electric power generation properties of a lead zirconate titanate piezoelectric ceramic

Mitsuhiro Okayasu, Keisuke Watanabe

Ceramics International 42 ( 12 ) 14049 - 14060 2016年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

Electric power generation characteristics of lead zirconate titanate (PZT) piezoelectric ceramic have been investigated experimentally and numerically. A thin PZT ceramic plate attached to a thin brass plate was used to examine the electric voltage generated during cyclic loading. On increasing the number of PZT ceramic plates combined together in the longitudinal direction, the electric voltage increases with the highest electric voltage being obtained for four PZT ceramic plates; and the maximum electric voltage becomes almost constant even if the number of PZT ceramic plates combined together increases more than four. This is attributed to the low strain level and the mixed strain (compressive and tensile strain). The effect of strain characteristic on the electric voltage value was analyzed numerically using our strain definition, and a clear correlation between the extent of compressive strain and generated electric voltage is clarified. A different electric generation characteristic was further observed depending on the stress conditions: generation of positive and negative electric voltage occurs when the PZT ceramic is subjected to mainly compressive and tensile stress, respectively. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

DOI： 10.1016/j.ceramint.2016.06.012

Precise analysis of compressive strain effects on electric power generation properties of a lead zirconate titanate piezoelectric ceramic

Mitsuhiro Okayasu, Keisuke Watanabe

Journal of Advanced Ceramics 5 ( 1 ) 35 - 39 2016年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGEROPEN

Electric power generation characteristics of lead zirconate titanate (PZT) piezoelectric ceramic have been examined during cyclic loading with different compressive strain value. A thin PZT ceramic plate attached to a thin brass plate was used. With an increasing compressive strain value in the PZT ceramic plate, the electric voltage increased, and the highest electric voltage was obtained for the sample with the high strain level (0.4%). The electric voltage decreased for the sample with more than 0.45% strain because of the failure in the PZT ceramic. The electric voltage was different depending on the strain condition, where the higher strain and the wider strain range (Delta epsilon) made the high electric voltage. Moreover, the electric voltage was attributed to the strain rate, where the higher strain rate made higher electric voltage due to high kinetic energy. The electric generation characteristics could be estimated with an operation of the strain value and strain ratio (epsilon(min)/epsilon(max)) especially for the sample loaded cyclically under the slow loading speed. This was affected by the linear correlation between the electric voltage and the strain value. Based upon the experimental work, suitable loading condition to make high electric voltage could be proposed.

DOI： 10.1007/s40145-015-0169-6

Effects of Sb, Sr, and Bi on the material properties of cast Al-Si-Cu alloys produced through heated mold continuous casting

Mitsuhiro Okayasu, Shuhei Takeuchi, Shaohua Wu, Toshihiro Ochi

Journal of Mechanical Science and Technology 30 ( 3 ) 1139 - 1147 2016年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：KOREAN SOC MECHANICAL ENGINEERS

Rare earth metals can create a fine eutectic Si structure in cast Al-Si-10.6-Cu-2.5 (ADC12) alloys produced through heated mold continuous casting. Fine and spherical eutectic Si phases are created in the ADC12 alloys through the addition of Sr0.04, and fine lamellar eutectic Si phases are created through Sb and Bi addition. Crystal orientation on the face perpendicular to the casting direction is formed by [110]; however, this uniform formation is collapsed in the ADC12 alloy with an increasing amount of Sr addition, such as Sr > 0.04%. The shape of the eutectic Si is statically analyzed, and the effects of the eutectic Si characteristics on the mechanical properties are examined experimentally. On the one hand, the mechanical properties of the ADC12-Sr alloy increase with increasing Sr content because of the fine eutectic Si, the randomly orientated crystal formation, and so on. On the other hand, the material ductility increases in the ADC12 alloy with increasing addition of Sb and Bi elements. A high fracture strain of approximately 14% is obtained for the ADC12-Bi1.5 alloy.

DOI： 10.1007/s12206-016-0218-2

Mechanical properties and failure characteristics of cast and extruded Mg97Y2Zn1 alloys with LPSO phase

Mitsuhiro Okayasu, Shuhei Takeuchi, Masafumi Matsushita, Naoya Tada, Michiaki Yamasaki, Yoshihito Kawamura

Materials Science and Engineering A 652 ( Jan. ) 14 - 29 2016年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

Mechanical properties and failure characteristics of Mg97Y2Zn1 (at%) alloys, prepared by gravity casting and extrusion processes, are examined experimentally. High tensile and fatigue strengths are obtained for extruded Mg97Y2Zn1 alloys, with the values being approximately twice as high as those for cast Mg97Y2Zn1 alloys. Such high strengths are attributed to the high internal stress arising from the complicated microstructure and severe lattice strain in the alpha-Mg and long-period stacking order phases. On the other hand, a high fracture strain is obtained for cast Mg97Y2Zn1 alloy, which is affected not only by the lower internal stress, but also severe kink deformation. The failure characteristics, e.g., crack growth characteristics are examined via direct observation during tensile and fatigue tests at micro- and nano-scales. Meandering fatigue crack growth is obtained for the cast Mg97Y2Zn1 alloys. The mean roughness of the fracture surface for cast Mg97Y2Zn1 alloys is more than twice as high as that for commercially available AM60 and AZ91 cast alloys. A coarse fracture surface that was observed in the cast Mg97Y2Zn1 alloys would make severe roughness-induced crack closure, leading to high fatigue strength. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2015.11.069

Material properties of Al-Si-Cu aluminium alloy produced by the rotational cast technology

M. Syahid, M. Okayasu, S. Wu

International Journal on Smart Material and Mechatronics 3 ( 1 ) 145 - 151 2016年

The electric power generation characteristics of a lead zirconate titanate piezoelectric ceramic under various cyclic loading conditions

Mitsuhiro Okayasu, Keisuke Watanabe

Ceramics International 41 ( 10 ) 15097 - 15102 2015年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The electric power generation properties of lead zirconate titanate (PZT) ceramic have been studied under various cyclic loading conditions. The maximum electric voltage increases with an increase in the frequency under the triangular and sinusoidal wave modes, whereas the voltage decreases with an increase in the frequency for the square wave mode. The maximum electric voltage for all wave modes is saturated at around 15 Hz, namely, the electric voltage for the three wave modes is almost constant even if the frequency increases by more than 15 Hz. With an increase in the loading frequency (50 Hz) under the square and triangular wave modes, the electric voltage generates a sinusoidal-like curve formation, which is not reflected by the loading wave mode. The electric voltage increases with an increase of the frequency for the triangular and sinusoidal wave modes, because of the high kinetic energy. For the square wave formation especially at high frequency, a low electric voltage is obtained, which is caused by the switching back of the electric generation arising from the rapid loading speed. The electric voltage increases non-linearly with increasing loading value, and the voltage values become almost constant when loaded with more than 10 N because of the limitation of the amount of electrons in the PZT ceramic. In the early loading stage, the electric voltage values do not increase with high efficiency, in which case the weak fitting of the PZT ceramic plate to the test fixture is affected. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

DOI： 10.1016/j.ceramint.2015.08.076

The Effects of Defects on Tensile Properties of Cast ADC12 Aluminum Alloy

Mitsuhiro Okayasu, Hikoyuki Sakai

Metallurgical and Materials Transactions A 46A ( 11 ) 5418 - 5430 2015年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

To better understand the effects of cast defects on mechanical properties, cast aluminum alloys with various porosities were used. Porosity in the cast samples was created during the casting process, and to clearly identify the porosity effects on the mechanical properties, artificial defects (porosity-like tiny holes) were created mechanically. The tensile properties for the cast aluminum alloys appear to be attributed to the area fraction of the porosity on the fracture surface (namely, the defect rate, DR), although there were different trends because of the different stress concentrations: the ultimate tensile strength and 0.2 pct proof strength were linearly related to DR, while a non-linear correlation was detected for fracture strain. Even in Al alloys with small amounts of defects, significant reductions in the fracture strain were observed. These results were verified using tensile tests on specimens containing artificial defects. The effects of artificial defects on the tensile properties were further investigated using numerous tiny holes, created in several formations. The artificial defects (several small holes), lined up at perpendicular (90 deg) and 45 deg directions against the loading direction, made significant reductions in the tensile properties, even though only weak defect effects were observed for the 90 deg loading direction. No severe defect effects were obvious for the specimen with a tiny defect of I center dot 0.1 mm, because of the lower stress concentration, compared to the microstructural effects in the cast Al alloys: the grain boundaries and the second phases. Such phenomena were clarified using tensile tests on cast samples with differently sized microstructures. There were no clear defect effects on the yield strength as the defect amount was less than 10 pct, and microstructural effects were not detected either in this case. Failure characteristics during tensile loading were revealed directly by in-situ strain observations using high-speed cameras.(C) The Minerals, Metals & Materials Society and ASM International 2015

DOI： 10.1007/s11661-015-3134-7

Material Properties of Various Cast Aluminum Alloys Made Using a Heated Mold Continuous Casting Technique with and without Ultrasonic Vibration

Mitsuhiro Okayasu, Yuta Miyamoto, Kazuma Morinaka

Metals 5 ( 3 ) 1440 - 1453 2015年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：MDPI AG

This work was carried out to develop high-quality cast aluminum alloys using a new casting technology. For this purpose, commercial Al alloys were created by heated mold continuous casting (HMC) with ultrasonic vibration (UV). With the HMC process, the grain size and the crystal orientation of the Al alloys were controlled, i.e., fine grains with a uniformly organized lattice formation. In addition, an attempt was made to modify the microstructural formation by cavitation. These microstructural characteristics made excellent mechanical properties. Using UV in the continuous casting process, more fine and spherical grains were slightly disordered, which was detected using electron backscattered diffraction. The mechanical properties of the UV HMC Al alloys were slightly higher than those for the related cast Al alloys without UV. Moreover, the severe vibration caused higher mechanical properties. The lattice and dislocation characteristics of the cast samples made with and without UV processes were analyzed systematically using electron backscattered diffraction.

DOI： 10.3390/met5031440

Effects of cyclic loading on fatigue properties of austenite stainless steel

Mitsuhiro Okayasu, Hironobu Fukui

Journal of Mechanical Science and Technology 29 ( 9 ) 3663 - 3668 2015年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：KOREAN SOC MECHANICAL ENGINEERS

Fatigue properties of stainless steels have been investigated by various researchers. The present work examined fatigue properties under various cyclic loading conditions to determine the significant factors of loading condition to fatigue strength (sigma(e)). From this approach, no clear frequency effects on the S-N (f) relation were detected, but clear R-ratio effects were identified on fatigue strength. A higher R-ratio yielded higher sigma(e), and a clear correlation was observed between R-ratio and endurance limit. A different trend was observed for the S-N (f) curve when the stress value on the ordinate of the S-N f relation was plotted using the maximum tensile stress (sigma(max)). However, the S-N (f) curves obtained overlaid each other when using stress amplitude (sigma(a)) instead of sigma(max). Fatigue properties were analyzed further using several relations proposed previously, such as modified Goodman relation. In this case, the fatigue data obtained were clearly approximated by an elliptical relation. Severe microstructural damage arising from plastic deformation was observed in the sample fractured under the R = -1 condition, where striation was a main failure mode. In contrast, no clear microstructural damages were observed in the samples fractured at R = 0.1 even when a higher maximum tensile stress was applied (e.g., sigma(max) = 60% sigma(UTS)), where ripple-like failure was the dominant feature.

DOI： 10.1007/s12206-015-0810-x

Precise analysis of effects of aging on mechanical properties of cast ADC12 aluminum alloy

Mitsuhiro Okayasu, Shoka Go

Materials Science and Engineering A 638 208 - 218 2015年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

To better understand the effects of aging on mechanical properties of cast Al-Si-Cu-based aluminum alloy (ADC12), artificial aging was carried out under a range of conditions after water quenching. Two casting methods were employed: gravity casting (GC) and heated-mold continuous casting (HMC). The Vickers hardness of GC samples was higher overall than that of HMC samples both before and after aging. This was due to the high solubility of the alloy elements in the alpha-Al matrix and to the presence of complicated dislocation walls in the GC samples. The hardness of GC and HMC samples aged at 433, 448 and 463 K increased similarly with increasing aging time, and an aging peak occurred in the age-hardening curve between 10 and 15 h. In contrast to the hardness results, the ultimate tensile strength (UTS) for the as-cast GC sample was about 30% lower than that for the as-cast HMC sample, because of large brittle eutectic Si and Fe structures. The highest UTS was obtained for the HMC sample aged at 448 K for 13 h, and was about 20% and 10% higher, respectively, than those of the as-cast HMC sample and the GC sample aged at 448 K for 13 h. The high UTS for the HMC sample was due to precipitation of theta' (Al2Cu) metastable phases as well as the formation of tiny microstructures. With over-aging at 463 and 493 K for 100 h, the tensile strength decreased for both GC and HMC samples, although the fracture strain increased only for HMC samples (by more than 30%). Such high ductility in ADC12 alloy has not been seen in previous studies. In contrast, no clear improvement in material ductility was detected in GC samples. The tensile strength was directly related to fatigue properties, although no significant effect of ductility on fatigue strength was found. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2015.04.072

Crystallization characteristics of cast aluminum alloys during a unidirectional solidification process

Mitsuhiro Okayasu, Shuhei Takeuchi

Materials Science and Engineering A 633 ( May ) 112 - 120 2015年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

The crystal orientation characteristics of cast Al-Si, Al-Cu and Al-Mg alloys produced by a unidirectional solidification process are examined. Two distinct crystal orientation patterns are observed: uniform and random formation. A uniform crystal orientation is created by columnar growth of alpha-Al dendrites in the alloys with low proportions of alloying element, e.g., the Al-Si alloy (with Si <12.6%) and the Al-Cu and Al-Mg alloys (with Cu and Mg < 2%). A uniformly organized crystal orientation with [100] direction is created by columnar growth of alpha-Al dendrites. With increasing proportion of alloying element (>2% Cu or Mg), the uniform crystal orientations collapse in the Al-Cu and Al-Mg alloys, owing to interruption of the columnar alpha-Al dendrite growth as a result of different dynamics of the alloying atoms and the creation of a core for the eutectic phases. For the hypo-eutectic Al-Si alloys, a uniform crystal orientation is obtained. In contrast, a random orientation can be detected in the hyper-eutectic Al-Si alloy (15% Si), which results from interruption of the growth of the alpha-Al dendrites due to precipitation of primary Si particles. There is no clear effect of crystal formation on ultimate tensile strength (UTS), whereas crystal orientation does influence the material ductility, with the alloys with a uniform crystal orientation being elongated beyond their UTS points and with necking occurring in the test specimens. In contrast, the alloys with a nonuniform crystal orientation are not elongated beyond their UTS points. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2015.03.001

Precise Analysis of Microstructural Effects on Mechanical Properties of Cast ADC12 Aluminum Alloy

Mitsuhiro Okayasu, Shuhei Takeuchi, Masaki Yamamoto, Hiroaki Ohfuji, Toshihiro Ochi

Metallurgical and Materials Transactions A 46A ( 4 ) 1597 - 1609 2015年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

The effects of microstructural characteristics (secondary dendrite arm spacing, SDAS) and Si- and Fe-based eutectic structures on the mechanical properties and failure behavior of an Al-Si-Cu alloy are investigated. Cast Al alloy samples are produced using a special continuous-casting technique with which it is easy to control both the sizes of microstructures and the direction of crystal orientation. Dendrite cells appear to grow in the casting direction. There are linear correlations between SDAS and tensile properties (ultimate tensile strength sigma (UTS), 0.2 pct proof strength sigma (0.2), and fracture strain epsilon (f)). These linear correlations, however, break down, especially for sigma (UTS) vs SDAS and epsilon (f) vs SDAS, as the eutectic structures become more than 3 mu m in diameter, when the strength and ductility (sigma (UTS) and epsilon (f)) decrease significantly. For eutectic structures larger than 3 mu m, failure is dominated by the brittle eutectic phases, for which SDAS is no longer strongly correlated with sigma (UTS) and epsilon (f). In contrast, a linear correlation is obtained between sigma (0.2) and SDAS, even for eutectic structures larger than 3 mu m, and the eutectic structure does not have a strong effect on yield behavior. This is because failure in the eutectic phases occurs just before final fracture. In situ failure observation during tensile testing is performed using microstructural and lattice characteristics. From the experimental results obtained, models of failure during tensile loading are proposed. (C) The Minerals, Metals & Materials Society and ASM International 2015

DOI： 10.1007/s11661-015-2747-1

Influence of solidification rate on the material properties of cast aluminum alloys based on Al–Si–Cu and Al–Si–Mg

M. Okayasu, S. Yoshida

International Journal of Cast Metals Research 28 ( 2 ) 105 - 116 2015年4月

Mechanical Properties of Samurai Swords (Carbon Steel) Made Using a Traditional Steelmaking Technology (tatara)

M.Okayasu, H.Sakai, T.Tanaka

Journal of Material Sciences & Engineering 4 ( 2 ) 1 - 6 2015年

Effects of Stress Concentration on the Mechanical Properties of Carbon Fiber Reinforced Plastic

R. Naito, M. Okayasu, D. Fukuyama

International Journal on Smart Material and Mechatronics 2 ( 2 ) 136 - 139 2015年

Mechanical strength and failure characteristics of cast Mg–9%Al–1%Zn alloys produced by a heated-mold continuous casting process: tensile properties

M. Okayasu, S. Takeuchi, H. Ohfuji

Metallurgical and Materials Transactions A 45A ( 12 ) 5767 - 5776 2014年11月

In Situ Observation of Material Failure in Cast Aluminum Alloy During Monotonic Loading Observed by a High-Speed Camera

M. Okayasu, S. Takeuchi, K. Aizawa

Experimental Mechanics 54 ( 8 ) 1479 - 1489 2014年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

In situ observation of the failure characteristics of a cast aluminum alloy has been conducted using a testing system with a high-speed camera. The failure process of the cast Al alloy was captured clearly with an high image resolution (1,024 × 1,024 pixels) at a high frame rate (20,000/s), where the specimen surface for observation was dyed dark using a black oil-based ink. A dark curtain was set behind the test specimen as background. Strong lighting of about 10 klx was used, which was applied to the dark specimen surface for clarification of material failure. The aim of this approach was to detect the failure characteristics or failure objects with bright zone. Using this system, both debris particles flying from the fracture sample and dislocation-like movements were detected. These were observable as tiny bright dots. The flying debris particles of about 35 μm in diameter consisted mainly of Si- and Fe-based eutectic structures. The flying speed of the debris particles was about 1,800 mm/s and their flight distance from the specimen was about 100 mm. The velocity of the dislocation-like movements was found to be less than 1,000 mm/s, and this motion was seen repeatedly before and after sample failure occurred.

DOI： 10.1007/s11340-014-9913-z

Characteristics of electrical power generation from lead zirconate titanatepiezoelectric ceramics under cyclic loading

Mitsuhiro Okayasu, Mamoru Mizuno, Tetsuro Shiraishi

Ceramics International 40 ( 5 ) 6589 - 6595 2014年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The influence of oscillation condition on the electrical power generation properties of lead zirconate titanate (PZT) piezoelectric ceramics is investigated. It is found that the power generation increases with an increase of both applied load and load frequency. At lower loading frequencies, e.g., 0.1 and 0.5 Hz, the voltage rises instantly to the maximum level under square-wave mode, although the voltage increases gradually under triangular-wave mode. After this initial increase, there is a rapid fall to zero, followed by generation of increasingly negative voltage as the applied load is removed for both wave modes. Variation of the voltage is reflected by the cyclic loading at higher loading frequencies, 15 and 30 Hz. On the basis of the obtained experimental results for the two wave modes, the electrical power generation characteristics of PZT ceramics are proposed, and the voltages generated during loading and unloading are accurately estimated by numerical analysis. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

DOI： 10.1016/j.ceramint.2013.11.114

Mechanical Strength and Failure Characteristics of Cast Mg–9%Al–1%Zn Alloys Produced by a Heated-Mold Continuous Casting Process

M. Okayasu, S. Takeuchi

Materials Science and Engineering A 600 211 - 220 2014年4月

Strain induced martensite formation characteristics of austenite stainless steel during various loading conditions

M. Okayasu, H. Fukui, H. Ohfuji, T. Shiraishi

Materials Science and Technology (United Kingdom) 30 ( 3 ) 301 - 308 2014年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：MANEY PUBLISHING

The strain induced martensitic transformation (SMT) of the austenite stainless steel (SUS 304) under cyclic loading and static loading was investigated directly using electron backscattered diffraction. Two different SMT characteristics are observed, which are attributed to the differences of plastic and twinning deformation. The maximum cyclic stress has a strong influence on the SMT. The total area fraction of the Fe-α' phase increases significantly when the maximum cyclic load is >80% σ . In other words, the SMT is apparently absent when the samples are loaded with less than σ =70% σ , although such samples are fractured completely. Moreover, there is a clear R ratio effect on the SMT. For example, the loading condition R=-1 gives rise to a strong SMT compared to R=0·1 due to the more severe strain caused by the compressive stress. In contrast, no clear frequency effect (1 versus 30 Hz) on the SMT is detected, which may be attributed to the same maximum cyclic stress. Like the SMT characteristics under cyclic loading, the proportion of Fe-α' phase shows no clear increase until the sample is loaded statically to a tensile stress <70% σ . © 2014 Institute of Materials, Minerals and Mining. UTS max UTS UTS

DOI： 10.1179/1743284713Y.0000000353

Influence of microstructural characteristics on mechanical properties of ADC12 aluminum alloy

M. Okayasu, K. Ota, S. Takeuchi, H. Ohfuji, T. Shiraishi

Materials Science and Engineering A 592 189 - 200 2014年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

The effects of microstructural characteristics on the mechanical properties of an aluminum alloy (ADC12: Al-Si -Cu -Zn -Fe -Mg -Mn ) produced by various casting technologies are studied experimentally and numerically. Six different casting processes are employed: gravity casting, cold-chamber die-casting, hot-chamber die-casting, squeeze casting, twin-rolled continuous casting and heated-mold continuous casting. Microstructural characteristics, dislocation density and defect density vary depending on the casting method, owing to differences in solidification rate, casting pressure and injection speed. The material characteristics of the samples affect their mechanical properties. Multiple regression analysis is carried out to find equations to predict tensile strength using five independent factors: secondary dendrite arm spacing, microporosity rate, diameter of eutectic structures, aspect ratio of eutectic structures and dislocation density. All these factors influence the tensile properties, although to different degrees. The estimated values of tensile strength are in good agreement with experimental results. © 2013 Elsevier B.V. 11.3 1.9 0.8 0.8 0.2 0.2

DOI： 10.1016/j.msea.2013.10.098

Effects of loading conditions on the fatigue failure characteristics in a polycarbonate

M. Okayasu, K. Yano, T. Shiraishi

Advances in Materials Research 3 ( 3 ) 163 - 174 2014年

Corrosion and mechanical properties of cast aluminium alloys

M. Okayasu, S. Takeuchi, T. Shiraishi

International Journal of Cast Metals Research 26 ( 6 ) 319 - 329 2013年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

The corrosion and mechanical properties of cast aluminium alloys [ADC6(Al-Mg) and ADC12(Al-Si-Cu)] have been investigated. Specifically, cast samples with different microstructural characteristics were produced using two different casting techniques, namely, gravity casting (GC) and heated mould continuous casting (HMC). For ADC6, intergranular corrosion occurs mainly through the eutectic Mg Si and Al (Mn,Fe) phases and the precipitate free zone around the eutectic structures. On the other hand, pitting corrosion in the α-Al phase is the dominant feature for ADC12. Such corrosion characteristics can be interpreted from the material properties (polarization resistance and electrode potential) of the individual structures, such as the a-Al phase, eutectic a-Al phase and precipitate phase. It also appeared that the extent of corrosion is affected by the size of themicrostructure; the ADC6 sample with small second phases has a high corrosion resistance due to the limited area for corrosion. In addition, the higher proportion of solid solution in the α-Al phase of ADC6, arising from the high cooling rate, makes a high corrosion resistance because of the change of electrode potential. A different trend is obtained for ADC12 due to the pitting corrosion, where low corrosion resistance is obtained for the sample with tiny grain. The tensile strength and ductility for both HMC samples (ADC6 and ADC12) are higher than those for the GC equivalents, which is caused by the presence of tiny microstructures and uniform crystal orientation. However, the mechanical properties of HMC samples decrease dramatically during corrosion, although these are still high compared to those of the GC samples without corrosion. © 2013 W. S. Maney & Son Ltd. 2 6

DOI： 10.1179/1743133613Y.0000000065

Mechanical properties and failure characteristics of a recycled CFRP under tensile and cyclic loading

Mitsuhiro Okayasu, Tomohiro Yamazaki, Kohei Ota, Keiji Ogi, Tetsuro Shiraishi

International Journal of Fatigue 55 ( Oct ) 257 - 267 2013年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

An examination has been made of the mechanical and failure properties of a recycled short carbon fiber reinforced plastic (rCFRP). The rCFRP samples were fabricated by the following process: the CFRP, consisting of epoxy resin with carbon fiber, is ground before mixing with acrylonitrile butadiene styrene resin with different weight fractions of CFRP. The tensile strength (sigma(UTS)) increased with increasing CFRP content, but dropped considerably for the sample with higher fiber content. From in situ measurement of localized failure in rCFRP, it appeared that material failure occurs even if a low tensile stress of 30% sigma(UTS) is applied. The localized damage was related to the pull-out (or debonding) of the fibers from the matrix. The fatigue strength increased with increasing the content of the recycled carbon fiber even for the samples with low tensile strength. This was attributed to the low crack driving force arising from severe crack closure. Details of the crack growth behavior were discussed using various crack growth models proposed in previous studies. (C) 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2013.07.005

Strain-induced martensite formation in austenitic stainless steel

Mitsuhiro Okayasu, Hironobu Fukui, Hiroaki Ohfuji, Tetsuro Shiraishi

Journal of Materials Science 48 ( 18 ) 6157 - 6166 2013年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

In situ measurements of the strain-induced martensitic transformation (SMTs) of SUS304 stainless steel that takes place during tensile loading at room temperature were performed around the notch of a dumbbell-shaped specimen where high stress concentration occurs. Even in the low plastic strain regime, with loading to 0.2 % proof stress (sigma (0.2)), some SMTs occurred. However, the area fraction of the Fe-alpha'-martensite phase did not increase significantly even when the sample was loaded to the ultimate tensile strength (sigma (UTS)). After the sigma (UTS) point, the total fraction of the Fe-alpha' phase increased dramatically to the fracture point (sigma (f)). The phase textures of Fe-alpha' and Fe-gamma were almost equal at (sigma (UTS) - sigma (f))/2, and the Fe-alpha' phase was observed over almost the entire measurement area around the notch at the sigma (f) point. However, the area fraction of the Fe-alpha' phase at the sigma (f) point decreased far away from the fracture surface, to an extent that the total fraction of the Fe-alpha' phase was almost the same as that of the Fe-gamma phase in an area about 1.7 mm from the fracture face. Different martensite characteristics were detected in the stainless steel, depending on the applied load level. This was attributed to the severity of deformation. In particular, deformation twinning, created around sigma (UTS), and severe plastic deformation before fracture make a strong Fe-alpha' phase. Details of this phenomenon are interpreted using various approaches, including electron backscatter diffraction analysis and finite element analysis.

DOI： 10.1007/s10853-013-7412-8

Mechanical properties of AC4CH alloys produced by heated mould continuous casting proces

M. Okayasu, S. Takeuchi, Y. Ohkura, T. Shiraishi

International Journal of Cast Metals Research 26 ( 3 ) 160 - 167 2013年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Maney Publishing

This paper reports an investigation of the mechanical properties of cast aluminium AC4CH alloys produced by a unique casting method known as the heated mould continuous casting (OCC) process. The tensile and fatigue strengths and material ductility of the OCC samples are much higher than that for the same alloy produced by conventional gravity casting. The high tensile and high fatigue strengths of the OCC samples are reflected by the presence of tiny round α-Al grains, and the high ductility of OCC is a result of the regularly organized crystal orientation. © 2013 W. S. Maney & Son Ltd.

DOI： 10.1179/1743133612Y.0000000046

Material properties of bismuth layered ferroelectrics and lead zirconate titanate piezoelectric ceramics

Mitsuhiro Okayasu, Yuki Sato, Satoshi Takasu, Mamoru Mizuno, Tetsuro Shiraishi

Ceramics International 39 ( 3 ) 3301 - 3306 2013年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The material properties of bismuth layer-structured ferroelectrics (BLSFs) and lead zirconate titanate (PZT) piezoelectric ceramics have been investigated. The fatigue strength of the PZT sample was higher than that of the BLSF. The electrical properties were altered as a result of cyclic loading. The piezoelectric constant (d(33)) and electromechanical coupling coefficient (k(33)) decreased with increasing applied load for both piezoelectric ceramics. However, the rate of reduction for the BLSF sample is smaller than that for the PZT ceramic. This different rate is attributed to the different material characteristics. During cyclic loading, the strain value increased with increasing cycle number for the PZT sample. Like the PZT ceramic, the strain value increases for the BLSF but the back strain occurred after the cyclic loading was conducted for certain period of times. This different strain behavior is influenced by the different domain switching characteristics and this was clarified from the respective strain-electric field relationships. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

DOI： 10.1016/j.ceramint.2012.10.018

Crystallisation characteristics of primary silicon particles in cast hypereutectic Al-Si alloy

M. Okayasu, S. Takeuchi, T. Shiraishi

International Journal of Cast Metals Research 26 ( 2 ) 105 - 113 2013年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：MANEY PUBLISHING

The crystallisation characteristics of primary silicon particles in a cast hypereutectic Al-Si alloy were examined using various casting processes. The size of the primary silicon particles depends on the cooling rate. However, the primary silicon particles cannot be crystallised completely when the cooling rate is>20 K s , since uncrystallised Si atoms migrate to the regions at low cooling rates, producing large primary Si particles. Moreover, turbulent metal flow under conditions of high cooling rate leads to the macrosegregation of the primary Si particles, resulting in poor mechanical properties. From the crystallisation characteristics obtained, a suitable solidification criterion can be proposed. © 2013 W. S. Maney & Son Ltd. -1

DOI： 10.1179/1743133612Y.0000000040

Fatigue properties (S-N and da/dN-dδ) of cast aluminium alloys

M. Okayasu, S. Takasu, A. Niikura, S. Okano, T. Shiraishi

International Journal of Cast Metals Research 26 ( 2 ) 82 - 91 2013年4月

記述言語：英語掲載種別：研究論文（学術雑誌）

In this paper, the fatigue strengths of cast aluminium alloys have been examined using the S-N and da/dN-dδ approaches on various cast samples having different microstructural characteristics. These were made by several different casting techniques, namely gravity casting, die casting and twin rolled continuous casting. Owing to the different microstructures, the fatigue properties varied. The S-N relations, e.g. the endurance limit, of the twin-rolled casting were apparently high compared to the other cast samples. The high fatigue strength was related to the high tensile strength and high ductility caused by the tiny spherical α-Al phase and fine eutectic structure. On the other hand, the low fatigue strength for the gravity and die cast samples was caused by the high stress concentration caused by the α-Al phase and needle-like eutectic silicon. Unlike the fatigue strength in the S-N approach, the da/dN-dδ relationships for the twin rolled casting sample, i.e. the resistance to fatigue crack propagation, were low compared to the gravity casting and the die cast samples. Such a difference in the crack growth resistance was influenced by the crack growth characteristics. For example, for the gravity cast and the die cast samples, the crack growth occurs in the grain boundaries of the coarse α-Al phase and along the long eutectic silicon grains (zigzag crack path), which promotes a strong crack closure tendency, leading to high crack growth resistance. The difference in the fatigue strength determined from the S-N relation and da/dN-dδ relation was interpreted by experimental and numerical examination of the stress-strain distribution. © 2013 W. S. Maney & Son Ltd.

DOI： 10.1179/1743133612Y.0000000030

Mechanical properties of SPCC low carbon steel joints prepared by metal inert gas welding

Mitsuhiro Okayasu, Yuki Ohkura, Tatsuaki Sakamoto, Shuhei Takeuchi, Hiroaki Ohfuji, Tetsuro Shiraishi

Materials Science and Engineering A 560 ( 10 ) 643 - 652 2013年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

To better understand the fatigue and tensile properties of SPCC steels welded by metal inert gas welding, the mechanical properties of the welded component in several localized regions, e.g., weld metal, heat affected zone (HAZ) and base metal, were investigated. The tensile and fatigue properties of the weld metals were high compared to the other areas (base metal and HAZ) due to the precipitated Ti containing oxide inclusions in acicular ferrite (bainite). Two typical microstructures were mainly observed in the heat affected zones (HAZ): (i) bainite in a ferrite matrix (HAZ-B) and (ii) a ferrite phase with low internal stress (HAZ-A). The hardness of HAZ-B was higher than HAZ-A because of the partially formed bainite structure and precipitated Ti containing oxide inclusions. The mechanical properties of the weld sample were further investigated using test specimens that included all regions, i.e., weld metal, HAZ and base metal (BHW). The tensile and fatigue properties of the BHW sample were found to be lower than those in all other regions, which was influenced by the high internal stress. The mechanical properties were analyzed using microstructural and crystal characteristics, as examined by TEM and EBSD analysis. In addition, the fatigue strength can be precisely evaluated using a modified Goodman diagram. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2012.10.008

Material Properties of Cast Aluminium Alloys Produced by Various Casting Processes

Mitsuhiro Okayasu, Kohei Ota, Shuhei Takeuchi, Tetsuro Shiraishi

Light Metals Technology 765 241 - + 2013年

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）出版者・発行元：TRANS TECH PUBLICATIONS LTD

The material and mechanical properties of cast aluminium alloys, produced using various casting technologies, have been investigated experimentally. In this study, several casting processes were selected, including gravity casting (GC), cold-chamber diecasting (CD), hot-chamber diecasting (HD), squeeze diecasting (SQ), twin rolled continuous casting (TRC) and heated mould continuous casting (HMC). Although all cast samples were made of the same aluminium alloy of Al-Si-Cu (ADC12), different material properties were obtained, in particular microstructural characteristics, crystal orientation and residual stress. In this instance, the microstructures of the GC and CD samples were formed mainly of coarse alpha-Al phase and needle-shaped eutectic structures; and a microstructure of fine round grain and tiny eutectic structures were observed for the HC, TRC and HMC samples. On the other hand, spherical shaped a-Al phase with a relatively large size was detected in SQ. The different microstructural characteristics led to different mechanical properties. A uniformly orientated lattice structure was detected in the HMC and SQ samples, which gave high material ductility. High internal compressive stress high dislocation density, arising from the rolling process, led to the high tensile properties for the TRC procedure. Various cast defects, such as blow holes and shrinkage porosity, were detected in GC and CD, which gave a reduction in their tensile properties.

DOI： 10.4028/www.scientific.net/MSF.765.241

Development of New White Canes to Assist the Mobility of Visually Impaired People

M. Okayasu

Journal of The Japan Society for Welfare Engineering 15 ( 2 ) 23 - 29 2013年

Mechanical and wear properties of Cu-Al-Ni-Fe-Sn-based alloy

M. Okayasu, D. Izuka, Y. Ninomiya, Y. Manabe, T. Shiraishi

Advances in Materials Research 2 ( 4 ) 221 - 235 2013年

New Experimental Techniques for In-situ Measurement of the Damage Characteristics of Piezoelectric Ceramics under High Cycle Fatigue Testing

M. Okayasu, Y. Sato

Experimental Mechanics 52 ( 8 ) 1009 - 1020 2012年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

To better understand the failure characteristics of lead titanate zirconate (PZT) piezoelectric ceramics, in-situ measurements of displacement and crack growth rates were conducted during high cycle fatigue testing, e. g., 5 kHz. A commercial PZT ceramic (used in a buzzer) was employed as the specimen. To examine the failure characteristics, two newly proposed systems were used: (i) a high speed camera system and (ii) a condenser microphone system. The former system consisted of two high speed cameras with an analytical system, which could measure the displacement of the PZT ceramic during the cyclic loading. The maximum displacement value of the ceramic was found to be approximately 20 μm at 0.5 kHz. The three-dimensional shape of the PZT ceramic during cyclic loading could be clearly observed. With the latter system, the displacement intensity arising from the ceramic vibration was detected continuously. It was found that the crack growth rate was not correlated with the fatigue frequency due to the resonance caused by the ceramic oscillation. There is a linear relationship between the crack growth rate and sonar intensity. On the basis of the crack growth behavior, the failure characteristics of the PZT ceramic could be clearly determined. © 2011 Society for Experimental Mechanics.

DOI： 10.1007/s11340-011-9550-8

The effects of artificial asperities on crack closing behavior in quenched and tempered medium carbon steel

Mitsuhiro Okayasu, Zhirui Wang

Journal of Mechanical Science and Technology 26 ( 10 ) 3081 - 3090 2012年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：KOREAN SOC MECHANICAL ENGINEERS

To better understand the effect of an asperity on crack closure behavior, K-CMOD relations were examined using artificial asperity/wedge, inserted into the fatigue crack in a three point bending specimen made of a hardened medium carbon steel. Experimental results revealed that the unloading phase of the K vs. CMOD curve exhibited a concave shape if soft artificial asperity (Al alloy) was inserted, signifying acceleration in the CMOD decrease at zero applied load. This was mainly related to elastic and plastic deformation in the wedge material during the unloading process. On the contrary, the linear unloading portion of K vs. CMOD was obtained as hard asperity (high carbon steel) was employed, which specified deceleration in the CMOD decrease at zero applied load, where the only elastic deformation in the asperity was affected. From their unloading curves, the severity of crack closure or Delta K-eff value was found to be related to the strength of the asperity material. The values of Delta K-eff were examined in two different ways, e.g., (i) the remote displacement method and (ii) the adjusted compliance ratio method (ACR). The Delta K-eff value, measured using both approaches, decreases with increasing wedge strength, such as hardness and yield strength. The rate of reduction in Delta K-eff was, however, changed depending on the manner of Delta K-eff examination, in which the Delta K-eff decreased at a higher rate for the compliance ratio method and at a lower rate for the remote displacement method. The reason for this is discussed in the present work.

DOI： 10.1007/s12206-012-0836-2

Effects of anisotropic microstructure of continuous cast Al-Cu eutectic alloys on their fatigue and tensile properties

Mitsuhiro Okayasu, Ryo Sato, Satoshi Takasu

International Journal of Fatigue 42 ( Sept ) 45 - 56 2012年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The objective of this work is to explore the effects of anisotropic microstructure on the material properties of a continuous cast Al-33%Cu eutectic alloy, produced by the Ohno continuous casting technique (OCC). A clear anisotropic microstructure was obtained in the OCC samples, namely a fine lamellar eutectic structure with unidirectional growth along the axial direction. The eutectic structure was formed by a primary alpha-Al phase and secondary CuAl2 phase. The hardness of CuAl2 is about 2.8 times higher than that of the alpha-Al phase. Due to the anisotropic microstructure, the mechanical properties of the OCC samples depended on the loading direction. The tensile and fatigue properties of the OCC samples in the longitudinal direction were more than 30% higher than those in the perpendicular direction. In addition, the mechanical properties were influenced directly by the fine eutectic structure in the longitudinal direction. The ultimate tensile strength of the OCC sample in the longitudinal direction could be estimated theoretically using three different parameters: solid-solution strengthening, interlamellar eutectic structure and work hardening strengthening. (C) 2011 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2011.05.015

A study of the effects of vibration on the electric power generation properties of lead zirconate titanate piezoelectric ceramic

Mitsuhiro Okayasu, Daisuke Sato, Yuki Sato, Masashi Konno, Tetsuro Shiraishi

Ceramics International 38 ( 6 ) 4445 - 4451 2012年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

To better understand how the electric power generated from PZT piezoelectric ceramics is affected by mechanical loading conditions the power generation was examined during cyclic loading under various loading conditions. The electric power generation was continuously examined using a monitoring system that we have recently developed. This system revealed that the electric power increased with increase of the applied load but then decreased when the applied load exceeded a certain level. In addition, greater electric power was generated with a simple beam configuration compared with a cantilevered beam. The change of electric power generation was directly related to the stress direction; high stress in the tetragonal structure parallel to the c-axis gave rise to high electric power generation. On the other hand, material failure, including domain switching and crack generation, caused a reduction of the electric power generated. Based upon our experimental data, suitable loading conditions to give high piezoelectric energy generation have been clarified. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

DOI： 10.1016/j.ceramint.2012.02.018

Mechanically controlled domain structure in PZT piezoelectric ceramics

Mitsuhiro Okayasu, Yuki Sato, Mamoru Mizuno, Tetsuro Shiraishi

Ceramics International 38 ( 6 ) 4579 - 4585 2012年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The influence of the mechanically applied load on the piezoelectric properties and domain switching characteristics has been investigated experimentally using a soft lead zirconate titanate (PZT) piezoceramic, which de-poled to make randomly orientated domain structures. Mechanical loads were applied to rectangular PZT ceramics under compressive and tensile stresses. With the applied stress, 90 degrees domain switching occurred in many locations in the PZT ceramics, where the higher stress caused more severe domain switching. The domain switching occurred with different directions, depending on the stress direction. For instance, the (2 0 0) peak increased and the (0 0 2) peak decreased when more severe compressive stress was applied. The opposite trend was detected as more severe tensile stress was applied; the (2 0 0) peak decreased and the (0 0 2) peak increased. With the applied loading process, the domains (long tetragonal c-axis) were orientated regularly in the PZT ceramics. Furthermore, like the electrically poled PZT ceramics, the domain orientations were more aligned when the sample surfaces were constrained during the loading process. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

DOI： 10.1016/j.ceramint.2012.02.036

A study of the mechanical properties of an Al-Si-Cu alloy (ADC12) produced by various casting processes

M. Okayasu, Y. Ohkura, S. Takeuchi, S. Takasu, H. Ohfuji, T. Shiraishi

Materials Science and Engineering A 543 ( 1 ) 185 - 192 2012年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

The mechanical properties of an Al-Si-Cu alloy (ADC12), produced using various casting technologies, have been examined experimentally. Four different casting processes were employed, including gravity casting (GC), cold-chamber die-casting (CD), twin rolled continuous casting (TRC) and the Ohno continuous casting process (OCC). Although these produced the same Al-Si-Cu aluminum alloy, different mechanical properties were obtained, in particular microstructural characteristics and dislocation density. The microstructure of GC and CD samples was formed mainly with coarse α-Al phase and needle-shaped Si and Fe based eutectic structures. In contrast, a fine round α-Al phase and tiny eutectic structures were observed for the TRC and OCC samples. Such a change of microstructure was caused by the different casting process parameters, namely injection speed, casting pressure and cooling rate. High internal stress as well as high dislocation density was detected for GC and TRC, caused by the high shrinkage force and high applied rolling force, respectively. Because of the different material properties, the tensile and fatigue strength were altered. A clear Hall-Petch relation with σ =k d +B was obtained, and the fatigue properties were evaluated with the power law dependence σ =σ N . The mechanical properties obtained were also analyzed in relation to the crystal orientation and lattice mis-orientation angle. © 2012 Elsevier B.V.. 0.2 y a f f -0.5 -b

DOI： 10.1016/j.msea.2012.02.073

Study of resonance fatigue properties of piezoelectric ceramics using new fatigue testing system

M. Okayasu, Y. Sato, M. Mizuno, T. Shiraishi

Advances in Applied Ceramics 111 ( 4 ) 181 - 186 2012年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：MANEY PUBLISHING

To better understand the material damage of lead zirconate titanate (PZT) ceramics, fatigue tests have been carried out using a newly proposed testing system. The material damage was evaluated with the change in sonar intensity from the ceramics, where a sensing control was operated using a condenser microphone system. This testing system produces cyclic loading with a wide range of fatigue frequencies: 1 Hz-72 kHz. The influence of fatigue frequency on the fatigue properties of a PZT ceramic has been investigated, and it appeared that the fatigue strength is not linearly correlated with the fatigue frequency. The reason for this is due to the resonance caused by the oscillation. Thus, the fatigue frequency can change the severity of sample vibration. The extent of sample vibration is associated with the sonar intensity from the ceramic, which is linearly related to the crack growth rate. Details of the resonance fatigue properties have been systematically examined. © 2012 Institute of Materials, Minerals and Mining.

DOI： 10.1179/1743676111Y.0000000055

Effect of domain wall characteristics on material properties of lead zirconate titanate piezoelectric ceramics

M. Okayasu, Y. Sato, M. Mizuno, T. Shiraishi

Advances in Applied Ceramics 111 ( 4 ) 187 - 195 2012年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：MANEY PUBLISHING

To better understand the material properties of lead zirconate titanate (PZT) ceramics, the influence of domain wall characteristics on the electrical and mechanical properties of PZT ceramics has been investigated. To obtain various domain wall characteristics, the poling process was carried out with different patterns with respect to the PZT ceramic. The domain walls were aligned in the PZT ceramic in the direction perpendicular to the poling direction. Such domain wall characteristic produced different crack growth behaviours. The crack growth occurred mainly along the domain walls of {110} for a (crack growth direction perpendicular to the poling direction) because of the high stress concentration between the domain walls. In contrast, the crack propagated along both domain walls and grain boundaries for a (crack growth direction parallel to the poling direction), leading to high crack growth resistance and good mechanical properties. The fracture toughness K values for the PZT ceramics in the a and a directions were about 0.5 and 2.6 MPa m respectively. It also appeared that the domain walls collapsed when a number of poling processes are conducted along different directions. These PZT ceramics had high mechanical properties due to the low stress concentrations. © 2012 Institute of Materials, Minerals and Mining. 31 33 IC 31 33 1/2

DOI： 10.1179/1743676111Y.0000000057

Mechanical properties of Al-Si-Cu alloys produced by the twin rolled continuous casting process

Mitsuhiro Okayasu, Ryo Sato, Satoshi Takasu, Akio Niikura, Tetsuro Shiraishi

Materials Science and Engineering A 534 ( 1 ) 614 - 623 2012年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

The mechanical properties of cast aluminum alloys (ADC12: Al-Si-Cu alloy), created by the twin rolled continuous casting (TRC) process are explored. The TRC-ADC12 exhibits an anisotropic microstructure formed by a fine dendritic microstructure of the columnar and equiaxial grains in the longitudinal direction. Deformation twinning occurs in the TRC sample for several reasons including a high strain rate, high rolling force and fine grains; lattice rotation occurs along {1 1 1} and {1 1 (2) over bar}. The tensile and fatigue strengths of the TRC samples are about twice those of the same aluminum alloy produced by conventional gravity casting. This increase is attributed to the fine grains and fewer casting defects for the TRC samples. The 0.2% proof stress of the TRC samples can be estimated by two different parameters, solid-solution strengthening k root c and secondary dendrite arm spacing k(y)/root d. An anisotropic microstructure of the TRC samples makes a slight difference in the mechanical properties between the longitudinal and perpendicular directions, and the tensile and fatigue strengths of the samples in the longitudinal direction are slightly higher than those in the perpendicular direction. Based upon the microstructural characteristics, further details of the mechanical and fatigue properties are discussed based on experimental and numerical data. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2011.12.018

Relevance of instrumented nano-indentation for the assessment of the mechanical properties of eutectic crystals and alpha-Al grain in cast aluminum alloys

Mitsuhiro Okayasu, Satoshi Takasu, Mamoru Mizuno

Journal of Materials Science 47 ( 1 ) 241 - 250 2012年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

To better understand the material properties of cast aluminum alloys, the mechanical properties of microscopic structures (the eutectic structure and the alpha-Al phase) have been systematically examined using a special nano-indentation hardness test machine. In this approach, a triangular indentation is applied directly to the eutectic structure or alpha-Al phase, and the mechanical properties evaluated through hardness and load-strain relationships. The hardness of the eutectic phase varied, depending on the structural characteristics of the different intermetallic compounds. High values of hardness were obtained structures of the DO(3) type, e.g., Al(5)FeSi, but low hardness for CuAl(2) and Mg(2)Si. The hardness of CuAl(2) and Mg(2)Si had almost the same values as that of the alpha-Al matrix. In addition, from the nano-indentation hardness test, the effects of alpha-Al grain characteristics on the mechanical properties were clarified. The hardness of the alpha-Al grain was linearly related to the grain size and the distance from the grain boundary, where the higher the hardness, the smaller the grain size and the closer to the grain boundary. The hardness level was attributed to the different severity of slip resistance of the atoms during the indentation loading.

DOI： 10.1007/s10853-011-5791-2

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年

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年

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年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年

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年

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年8月

掲載種別：研究論文（学術雑誌）

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年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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 査読

Mitsuhiro OKAYASU, Eriko SUGIYAMA, Kazuto SATO, Mamoru MIZUNO

Journal of Solid Mechanics and Materials Engineering 4 ( 3 ) 426 - 434 2010年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

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年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

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年10月

掲載種別：研究論文（学術雑誌）

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

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年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Temperature dependence of the fatigue and mechanical properties of lead zirconate titanate piezoelectric ceramics

Mitsuhiro Okayasu, Masashi Otake, Teruo Bitoh, Mamoru Mizuno

International Journal of Fatigue 31 ( 8-9 ) 1254 - 1261 2009年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

This paper presents research results on the temperature dependence of the fatigue and mechanical properties of piezoelectric ceramics. The material being examined is a lead zirconate titanate piezoelectric ceramic, PZT. The fatigue strength apparently increases with increasing sample temperature. The mean endurance limit at 10(5) cycles for the sample tested at 573 K is twice as high as that at 293 K. Similarly, the bending strength of the PZT increases with increasing sample temperature. A maximum bending strength is obtained at a sample temperature between 573 K and 773 K. The increment in the bending strength is attributed to the distorted lattice structure, in which a tetragonal lattice system is being changed to a cubic structure at 573 K, the Curie temperature. On the other hand, the reduction in the bending strength at temperatures greater than 773 K is caused by a reduction in the concentration of oxygen defects and high thermal energy. leading to dislocation movement. A wavy fracture surface with a mixture of transgranular and intergranular fractures is obtained in the samples fractured at temperatures between 573 K and 773 K due to the distorted lattice structure. In contrast, a flat face with transgranular fracture appears in tests at the room temperature and the high temperature (over 773 K). Details of the fracture mechanism are further discussed. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2009.01.008

Damage characteristics of lead zirconate titanate piezoelectric ceramic during cyclic loading

Mitsuhiro Okayasu, Nozomi Odagiri, Mamoru Mizuno

International Journal of Fatigue 31 ( 8-9 ) 1434 - 1441 2009年8月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The effects of the damage characteristics on the material properties of a piezoelectric ceramic are examined during cyclic loading. The material being examined is a lead zirconate titanate piezoelectric ceramic (PZT). The electrical properties, such as the electromechanical coupling coefficient (k(33)), are changed during cyclic loading. The k(33) coefficient decreases rapidly to a low level as the sample is loaded cyclically with a high applied stress, while the k(33) value decreases slowly or does not change when loaded with a low applied stress. Such a change of material degradation is influenced by the severity of the material damage in the PZT ceramic. The material damage in the PZT occurs, and this occurrence is related to a lightning-like phenomenon (a bright flash with a click). Details of the damage characteristics in PZT ceramic are discussed in the present work. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2009.04.002

Effects of atmospheric corrosion on fatigue properties of a medium carbon steel

M. Okayasu, K. Sato, K. Okada, S. Yoshifuji, M. Mizuno

Journal of Materials Science 44 ( 1 ) 306 - 315 2009年1月

掲載種別：研究論文（学術雑誌）

The effects of atmospheric corrosion on fatigue properties were examined using a medium carbon steel, corroded in various atmospheres. Three different places, having various atmospheric conditions, were selected for the corrosion tests: (i) an industrial area, (ii) near the ocean, and (iii) beside a river in a hot spring region. A water and/or air electrochemical cell corroded the carbon steel to rust that had several forms, depending on the atmosphere. The form of the corrosion was distinguished visually and by spectroscopy. Strong oxidation occurred in all samples with the formation of rust. In addition, a more severe chemical reaction with chlorine was detected near the ocean although carbon was obtained in the industrial area. On the other hand, a high level of sulfur reacted with the sample near the river. Such chemical reactions gave rise to different corrosion mechanisms leading to different corrosion surfaces. A rough corrosion face with corrosion pits was obtained in two of the samples (industrial area and near the ocean), while a smooth surface was produced for the sample near the river. The change of the surface morphology clearly affected the fatigue strength, e.g., the rougher the sample surface, the lower the fatigue strength. On the basis of the corrosion system, details of the fracture and fatigue characteristics are discussed in the present work. © 2008 Springer Science+Business Media, LLC.

DOI： 10.1007/s10853-008-3053-8

Formulation of damage development for piezoelectric ceramics under static compressive stress

M. Mizuno, T. Nishikata, M. Okayasu

12th International Conference on Fracture 2009, ICF-12 1 762 - 770 2009年

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

In order to formulate a damage evolution equation, damage development of columnar specimens under static compressive stress was evaluated from variation of elastic coefficient based on the continuum damage mechanics. The elastic coefficient was evaluated from gradient of stress-strain relations of compression fracture tests. Then the damage evolution equation was formulated by an exponential function of strain with a threshold to start the damage development. Moreover, material functions of the damage variable were also formulated for the variation of material properties. By using the damage evolution equation and material functions, stress-strain relations were predicted properly taking into account the variation of elastic coefficient and accumulation of residual strain accompanied with damage development. Then the prediction was compared with experimental results, and the validity of the formulation was verified by the good agreement.

Examination of chemical reaction for revealing plastic deformation zone in low carbon steel

M. Okayasu, A. Sato, M. Mizuno

Materials Science and Technology 24 ( 12 ) 1477 - 1483 2008年12月

掲載種別：研究論文（学術雑誌）

In order to expose a clear plastic deformation zone in a low carbon steel, an etching technique was carried out on the plastically deformed material under various conditions. In this study, the etchant solution, etching time and a polishing process were employed as control factors in the design of the etching technique. The deformation zone was revealed on the etched surface with different degrees of clarity depending on the etching conditions, and a suitable etching system for showing up the clear plastic zone was found. Moreover, a polishing process after the etching could make the plastic zone clearer. Details of the etching mechanism for exposing the plastic deformation zone are discussed on the basis of electrode potential considerations. © 2008 Institute of Materials, Minerals and Mining.

DOI： 10.1179/174328407X179692

Hot chamber diecasting with Si<inf>3</inf>N<inf>4</inf> ceramic injection system: Application to aluminium alloy components

M. Okayasu, M. Hitomi, H. Yamazaki

International Journal of Cast Metals Research 21 ( 5 ) 339 - 348 2008年10月

掲載種別：研究論文（学術雑誌）

Although hot chamber diecasting is an appropriate technology for producing high quality cast components, there are material limitations; at the process temperature, aluminium alloy erodes the immersed steel shot sleeve and plunger. Attempts have been made to employ a Si N ceramic in place of steel. High quality Al hot chamber diecast components can be manufactured in this manner, but long production runs are not possible because failures occur in the ceramic sleeve even after, for example, a few thousand shots. The fracture in the ceramic sleeve is caused by high stresses arising from the plunger galling the sleeve surface owing to penetration of the Al alloy into the gap between the sleeve and plunger. The infiltrated Al alloy has been found to react chemically with the ceramics; silicon in the ceramic reacts with iron particles present as inclusions in the Al alloy. Hence, the purity of the molten Al alloy in the crucible is a significant factor in the production of high quality diecast components. To prevent penetration of the Al alloy, the injection system is cleaned periodically using hydrochloric acid, which dissolves the infiltrated Al alloy. Since etching by HCl can reduce the strength of the ceramic parts, this cleaning process must also be reconsidered. Recommendations to improve the quality of hot chamber diecast Al components are proposed. © 2008 W. S. Maney & Son Ltd. 3 4

DOI： 10.1179/136404608X320715

Fatigue properties of ultra-fine grained dual phase ferrite/martensite low carbon steel

M. Okayasu, K. Sato, M. Mizuno, D. Y. Hwang, D. H. Shin

International Journal of Fatigue 30 ( 8 ) 1358 - 1365 2008年8月

掲載種別：研究論文（学術雑誌）

An examination has been made of the fatigue properties of ultra-fine grained two phase ferrite/martensite low carbon steel, produced by equal channel angular pressing (ECAP). The fatigue strength and fracture mechanism of the ECAP steel were compared with those of the as-received sample. The fatigue strength of the ECAP steel was twice as high as that of the as-received steel. The fatigue strength was linearly related to the tensile strength for both the ECAP and as-received steels although their slopes were different; the slope for the ECAP steel was greater. In addition, different fracture characteristics were observed; intergranular fractures were dominant in the ECAP steel whereas transgranular fractures occurred in the as-received steel. Such differences in fatigue strength and fracture characteristic are attributed to their material properties. © 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2007.10.011

Effects of silver-based metal electroplate on fatigue properties of PZT ceramics

Mitsuhiro Okayasu, Satoshi Aoki, Mamoru Mizuno

International Journal of Fatigue 30 ( 6 ) 1115 - 1124 2008年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

The effects of silver-based metal plating for electrodes on the fatigue properties of PZT ceramics were examined. The plating was created on the surface of the PZT by a firing process carried out at approximately 973 K for several hours. Due to the high-temperature of the firing process, the silver-metal infiltrated the grain boundaries of the PZT matrix. This penetration made the PZT ceramics brittle and reduced the mechanical strength of the ceramics, e.g., hardness and bending strength. The fracture characteristics of this material are also attributed to the silver-based plated electrode; fractures of transgranular-type occurred near the electrode whilst fractures were intergranular in other areas. These changes of the fracture surface were influenced by the speed of crack growth arising from the roughness of the plated surface; a rough surface gives rise to transgranular fractures due to the high growth rate of the cracks. The roughness of the electroplated Surface leads to a reduction in the mechanical and fatigue strength. (c) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2007.08.002

Experimental technique for observation of three-dimensional metallography in brass and carbon steel

M. Okayasu, T. Ishikawa, M. Mizuno, T. Ono

Materials Science and Technology 24 ( 5 ) 547 - 552 2008年5月

掲載種別：研究論文（学術雑誌）

In the present study, an experimental technique is proposed by which the three-dimensional (3D) metallography (or crystals) can be observed on a fracture surface. The observation of the 3D metallography in brass and carbon steel has enabled their microstructural characteristics to be clarified. In brass, unique 3D lead crystal shapes were detected on the fracture surface with 'butterfly' and 'grass-like' shapes. On the other hand, grain boundaries and laminar shaped cementite were revealed in 3D for medium carbon steel. To obtain such 3D metallography, the samples were cooled to -196°C during the fracture process, produced by a compressive load. The essence of this approach was to embrittle the material, especially grain and crystal boundaries, during the fracture process. This helped to reveal the true shape of a variety of phases. Details of the mechanism for revealing the 3D metallography are discussed. © 2008 Institute of Materials, Minerals and Mining.

DOI： 10.1179/174328408X284496

A new etching technique for revealing the plastic deformation zone in an Al-Cu-Mg alloy

Mitsuhiro Okayasu, Kazuto Sato, Mamoru Mizuno

Journal of Materials Science 43 ( 8 ) 2792 - 2798 2008年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：SPRINGER

A new etching technique for revealing the plastic deformation zone in an Al-Cu-Mg alloy has been developed. The etching with the proposed etching agent was conducted on the deformed sample after being heated to 673 K for 3 h. With this etching technique, the plastic deformation zone was clearly observed even under low magnification. This was due to the change of microstructural characteristics in the plastic deformation zone after the heating process, in which there is significant precipitation of Al(2)Cu and Mg(2)Si, caused by the high energy arising from the severe deformation.

DOI： 10.1007/s10853-008-2544-y

Effect of carbon content on etching response for identification of plastic deformation zones in carbon steels

Mitsuhiro Okayasu, Dong H. Shin, Mamoru Mizuno

Materials Science and Engineering A 474 ( 1-2 ) 140 - 147 2008年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCIENCE SA

A recently developed etching technique to detect localized plastic deformation on a macro scale in 0.10-0.55% carbon steels has been studied. Etching was carried out on samples plastically deformed and then heated to 550 degrees C for a certain period of time. The plastic deformation zone was clearly seen in the low carbon steel (similar to 0.15%C), whereas it was mainly opaque in the medium carbon steel (0.25-0.35%C) and could not be seen in the high carbon steel (0.55%C). In the case of low carbon steels, severe chemical reaction occurred in the undeformed ferrite grains, while there was a weak reaction in the deformed ferrite. The plastic zone was detectable from the different surface states. Such a change in etching response in the above carbon steels was found to be related to their microstructural morphology (ferrite/pearlite), and a large amount of ferrite grain in the sample, i.e. the low carbon steels, was more suitable for revealing the plastic zone. This was due to the change in microstructure: (i) many tiny cementite particles (Fe3C) are precipitated in the deformed ferrite matrix during heating process, and (ii) finer ferrite grains are produced in the plastic zone. Because the main reason for observation of the plastic zone is related to the change in microstructure in the ferrite matrix, the material having a large amount of pearlite (high carbon steel) did not show the plastic deformation zone. Based upon the change of microstructure in ferrite, details of the etching mechanism for identification of the plastic zone are discussed. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.msea.2007.04.011

Experimental investigation of the effects of artificial wedges on fatigue crack growth and crack closing behavior in annealed SAE1045 steel

Mitsuhiro Okayasu, Zhirui Wang

International Journal of Fatigue 29 ( 5 ) 962 - 976 2007年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：ELSEVIER SCI LTD

To better understand the effect of asperity on the crack closure and fatigue crack growth behavior, the load-CMOD relations and crack propagation rate were examined through the addition of artificial wedges into pre-cracks. Experimental results revealed that the unloading phase of the load vs. CMOD curve exhibited always a concave shape, signifying the acceleration in the CMOD decrease. This was related to the plastic deformation in the wedge as well as in the specimen material surrounding the wedge. With the addition of an artificial wedge, reduced fatigue crack growth rate was found. The crack growth rate was then correlated with the effective load intensity factor range, Delta P-eff = P-max - P-min,P-real, which itself was correlated also to the deformation severity in the asperity as well as in the specimen material. Furthermore, the Delta P-eff value was found to change with increasing the crack length. In a short crack range, approximately 0.1 mm crack length, the value of Delta P-eff decreased due to the asperity- and plasticity-induced crack closing behavior. As the crack length increased, the Delta P-eff value increased as well due to the reduction in the closing behavior until just prior to the final fracture. Based upon the Delta P-eff variation as a function of crack length, details of the crack closing behavior were further discussed. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2006.07.019

Experimental study of the effect of loading condition on fracture surface contact features and crack closure behavior in a carbon steel

Mitsuhiro Okayasu, Daolun Chen, Zhirui Wang

Engineering Fracture Mechanics 73 ( 9 ) 1117 - 1132 2006年6月

掲載種別：研究論文（学術雑誌）

To better understand the crack closure effect in the fatigue process, influence of fatigue stress amplitude and R ratio on the contact features of fracture surfaces in an annealed carbon steel was studied via two special experimental approaches: (i) the collection of the fracture debris fallen from the crack surfaces, and (ii) the direct observation of the contact zones on the fracture surface through an ink dyeing method. The results of this study show that the change of fatigue CMOD value as a function of a/W ratio depends strongly on the loading condition; the fatigue stress amplitude and R ratio are the major factors that determine the contacting status between the mating fracture surfaces; the severity of the fracture surface contact can also be characterized by the dropping rate of the fracture debris particles collected during the fatigue test. © 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.engfracmech.2006.01.001

Etching technique for revelation of plastic deformation zone in low carbon steel

M. Okayasu, Z. Wang, D. L. Chen

Materials Science and Technology 21 ( 5 ) 530 - 538 2005年5月

掲載種別：研究論文（学術雑誌）

In this study, an etching technique to detect the localised plastic deformation behaviour in a low carbon steel was developed. With this technique, etching with Fry solution under ultrasonic vibration was carried out on samples plastically deformed and then heated at 550°C for a certain period of time. The plastic zone was revealed by different degrees of etching in the plastically deformed and non-deformed regions; the plastic zone was found to be only slightly etched, whereas the other region was deeply etched. From the surface offset after etching, the deformation zone was found to be observable even at low magnification, such as 10 times. As the heating duration increased, the plastic zone became clearer. The mechanism for such an etching reaction is discussed on the basis of electrochemical analysis. © 2005 Institute of Materials, Minerals and Mining.

DOI： 10.1179/174328405X36511

　前のページ - 次のページ　

1