Faculty Profiles - OKAYASU Mitsuhiro

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 2009.8

Language：English Publishing type：Research paper (scientific journal) Publisher：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 2009.1

Publishing type：Research paper (scientific journal)

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

Language：English Publishing type：Research paper (international conference proceedings)

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 2008.12

Publishing type：Research paper (scientific journal)

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 2008.10

Publishing type：Research paper (scientific journal)

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 2008.8

Publishing type：Research paper (scientific journal)

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 2008.6

Language：English Publishing type：Research paper (scientific journal) Publisher：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 2008.5

Publishing type：Research paper (scientific journal)

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 2008.4

Language：English Publishing type：Research paper (scientific journal) Publisher：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 2008.2

Language：English Publishing type：Research paper (scientific journal) Publisher：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 2007.5

Language：English Publishing type：Research paper (scientific journal) Publisher：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 2006.6

Publishing type：Research paper (scientific journal)

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 2005.5

Publishing type：Research paper (scientific journal)

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