材料
Online ISSN : 1880-7488
Print ISSN : 0514-5163
57 巻 , 8 号
選択された号の論文の17件中1~17を表示しています
特集 分子動力学
論文
  • 屋代 如月, 西村 正臣, 冨田 佳宏
    2008 年 57 巻 8 号 p. 755-760
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    We have so far discussed the deformation behavior of inhomogeneous systems, e.g. nano-polycrystalline and amorphous metals, from the unique viewpoint of “local lattice stability” or the positive definiteness of atomic elastic stiffness coefficients, Bαij. In the present study, the physical meaning of the “unstable atoms”, or the atoms of det Bαij < 0, is clarified in the molecular dynamics simulations of a monatomic bulk amorphous of nickel. The criterion of det Bαij < 0 definitely points out the existence of “defects” in the homogeneous random structure. They show hydrostatic compression at the initial equilibrium under σxx = σyy = σzz = 0 ; that is, they find more surrounding atoms than the stable ones. In other words, the unstable atoms have less free volume than stable ones. The ratio of unstable atoms doesn't change in the linear stress–strain response at the early stage of uniaxial tension and compression. The initial negative stress on unstable atoms works as deformation buffer under the early stage of uniaxial tension ; the unstable atoms show higher stress increase than the stable ones. The onset of blunting and plateau region in the stress–strain curve coincides with the point where the stress difference between the stable and unstable atoms vanishes in the tensile direction, and the ratio of unstable atoms begins to increase. Here, the unstable atoms always feel compressive stress in the lateral direction while the stable ones almost zero stress in the plateau region. Thus we can deduce that the stable local configuration crushes in the lateral direction to absorb the elongation in the plateau or steady flow deformation. This mechanism corresponds to the conventional picture of the free volume. It is also true for compression that the ratio of unstable atoms begins to increase at the point where the stress–strain curve deviate from the initial linearity. In the case of compression, the local configuration crushes in the loading direction by the stable → unstable transition, since the unstable atoms always feel higher compression than the stable ones despite of the increase in the number of unstable atoms.
  • 下川 智嗣, 喜成 年泰, 新宅 救徳
    2008 年 57 巻 8 号 p. 761-767
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Mechanical properties of nanocrystalline metals of which the grainsize is below about 100nm are affected by the relationship betweenthe intergranular and intragranular deformation. Intragranulardeformation in nanocrystalline metals could be caused by the movementof perfect dislocations or Shockley's partial dislocations, hencestacking fault energy is one of the important value to control theinternal structure. Moreover, the proportion of the grain boundaryregion dramatically increases with grain size decreasing, hence it isalso important to investigate the effects of individual grain boundarystructures and the distribution of grain boundary characteristics onthe macroscopic mechanical properties of polycrystalline materials. Inthis study, we investigate deformation mechanism and mechanicalproperties of nanocrystalline Al and Cu that show the differentstacking fault energy using molecular dynamics simulations, and wealso consider the effects of grain boundary misorientationdistributions on the mechanical properties of such materials.
  • 武富 紳也, 松本 龍介, 宮崎 則幸
    2008 年 57 巻 8 号 p. 768-773
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Despite the extensive investigations and the recent remarkable progress in experimental equipments, the effects of hydrogen on materials properties have not been sufficiently clarified yet. The difficulties to reveal the essential effects of hydrogen are mainly attributed to the characteristics of hydrogen such as ppm order extremely low concentration and high diffusivity in metals. In this study, we performed molecular statics (MS) analyses of hydrogen occupation sites around a {112} ‹111› edge dislocation in alpha iron. The stable hydrogen occupation region, where the hydrogen trap energy is strong, is not consistent with the result obtained by continuum theory of elasticity that hydrogen concentrate at the high hydrostatic stress region. According to the MS calculations, hydrogen trap energy has the different characteristics depending on the occupation sites under shear stress as well as hydrostatic stress. Furthermore, this results suggest that the hydrogen concentration on the slip plane around a {112} ‹111› dislocation can be extremely high.
  • 政家 利彦, 土井 祐介, 中谷 彰宏
    2008 年 57 巻 8 号 p. 774-779
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Hydrogen diffusion process in nickel crystal is examined by using molecular dynamics method. Firstly the diffusion coeffcients of hydrogen at several temperature are evaluated. The diffusion coeffcients obtained by using mean square displacements agree well with those obtained by the jump diffusion theory. Considering the correlation factors of hydrogen diffusion, the diffusion coeffcients obtained by the jump diffusion theory is more close to that obtained by using mean square displacements. Secondly, the elementary process of hydrogen diffusion is discussed. The activation energy of hydrogen diffusion obtained by using mean square displacements and that obtained by nudged elastic band method are different, since the jump of hyrogen atom to next stable site is finished before the nickel atoms construct the stable structure with transition site including hydrogen. Considering the energy landscape between the stable site and transition site including hydrogen, the activation energy obtained by nudged elastic band method is modified to be close to that obtained by mean square displacements.
  • 國澤 康平, 山本 昌裕, 尾方 成信, 渋谷 陽二
    2008 年 57 巻 8 号 p. 780-785
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Anisotropic growth process of two kinds of steps on Al(111) substrates is performed using kinetic Monte Carlo (kMC) method. Employed kMC parameters of activation energy and attempt frequency are estimated by nudged elastic band (NEB) method and transition state theory. Obtained set of results suggest that degree of the anisotropic growth clearly depends on substrate temperature and deposition rate. We find microscopic origin of the anisotropic growth is difference of diffusion rates along {111} and {100} steps, and there is a particular growth condition in which strong anisotropy is observed. At high deposition rate and low temperature, new islands which are easily generated on terraces, hinder the growth anisotropy weaker.
論文
  • 野田 尚昭, 長尾 優樹, 飯間 智史, 高瀬 康
    2008 年 57 巻 8 号 p. 787-793
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    This paper deals with intensity of singular stress field at the corner of debonded ends of an elastic cylindrical inclusion in an infinite body under tension. The problem is formulated as a system of singular integral equations, where unknown functions are densities of body forces distributed in infinite bodies having the same elastic constants as those of matrix and inclusion. In the numerical analysis, the unknown function of the body force densities are expressed as a linear combination of two types of fundamental density function and power series, where the fundamental density functions are chosen to express the symmetric stress singularity of the from rλ1-1 and the skew-symmetrics stress singularity of the from rλ2-1. Then, generalized stress intensity factors, which control the singular stress fields at the end of the cylindrical inclusion are discussed with varying the fiber lengths and elastic modulus ratio. The effect of debonding length on the generalized stress intensity factors is also discussed in comparison with the results of a cylindrical cavity. The results are also compared with ones for a rectangular inclusion.
  • 西村 禎人, 小野 長門, 三浦 精
    2008 年 57 巻 8 号 p. 794-799
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    In order to clarify the grain size dependence on the fatigue behavior of polycrystalline metals including twins from the view point of dislocation theory, pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys having the average grain sizes ranging from 9 to 122μm were prepared by a strain-annealing method. Fatigue tests at room temperature for each specimen were performed under the constant stress amplitude over a life range of 104 to 107cycles. The fatigue properties at elevated temperatures for Cu-14.8at%Al alloys were confirmed up to 773K, and also the distribution of dislocations close to fatigue cracks was observed by using the etch pit technique. The grain size dependence on the fatigue strength for pure copper and Cu-3.5at%Al alloys disappears beyond 106cycles, whereas the fatigue lives of Cu-6.8at%Al and Cu-14.8at%Al alloys uniformly increase with grain refining over all ranges. Relationship between the fatigue strength at 107cycles and aluminum concentration for Cu-Al alloys of fcc solid solution can be approximately represented by a straight line as well as the shear stress. The fatigue strength of Cu-14.8at%Al alloys markedly decreases with increasing temperature and becomes almost half of that for room temperature at 773K. The results based on the etch pit observation indicate that the dislocations multiplied under cyclic loading pile up against the grain and twin boundaries, where large cracks along both boundaries form and propagate due to the stress concentration.
  • 脇田 将見, 久野 隆紀, 長谷川 智也, 猿木 勝司, 田中 啓介
    2008 年 57 巻 8 号 p. 800-807
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Torsional fatigue tests were conducted for a spring steel, SUP7, with Vickers hardness of 480. The effect of shot peening on the torsional fatigue strength of smooth and artificially notched specimens in the corrosive environment and air was studied. The increase of the fatigue strength of smooth specimens in air by shot peening was relatively small, and the surface roughness due to peening did not reduced the fatigue strength. On the other hand, the fatigue strength of notched specimens increased greatly by shot peening and the notch sensitivity was reduced. In corrosion fatigue, the fatigue strength increased by shot peening in the medium life range. The strength of shot-peened specimens was reduced in the long life range around 107cycles and was approached to that of not-peened specimens. A similar improvement was observed for notched specimens in the medium life range, and the notch sensitivity was reduced by shot peening. The compression zone made by shot peening extended about 0.3mm below the surface, and relaxed by fatigue loading. The observation results on the cracking behavior and residual stress distribution were used to interpret the effect of shot peening on the fatigue strength of spring steel.
  • 植松 美彦, 栗田 達夫, 村田 瞬亮, 戸梶 惠郎
    2008 年 57 巻 8 号 p. 808-813
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    This paper presents the fatigue behaviour and fracture mechanisms of resistance spot welded joints of Al-Mg-Si alloy sheet. Prior to fatigue test, microstructure, Vickers hardness and tensile-shear strength were evaluated. Based on those results, tensile-shear fatigue tests have been performed and the fatigue behaviour and fracture mechanisms were discussed. The microstructure of a cross section of the welded joints consisted of three distinct regions of equiaxed grain structure, columnar crystal structure and base alloy structure. In the equiaxed grain structure, grains were considerably refined. The Vickers hardness in the nugget was lower than that in the base alloy because of the dissolution of precipitates. The average tensile-shear strength was 5241N and final fracture occurred by shear of the nugget. The tensile-shear fatigue strength at 107 cycles was 800N and the fatigue fracture mode was dependent on applied load. When maximum load Pmax = 2500N, final fracture took place by shear of the nugget, while when 1000N < Pmax < 2000N, it occurred in the base sheet. It was indicated that such transition of fracture modes was attributed to the stress state around the nugget induced by the crack initiation and the following crack opening displacement around the nugget.
  • 荒井 政大, 田中 博己, 松下 寿暁, 多田 耕三, 川久保 洋一, 杉本 公一
    2008 年 57 巻 8 号 p. 814-819
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    In the present paper, press molding for thermo-plastic resin toughened by carbon nanofiber were discussed. Polycarbonate (PC) and vapor grown carbon nanofiber (VGCF) are used for the resin matrix and the reinforcement. In order to investigate and simulate the process molding process of the PC/VGCF composite by finite element method, the thermo-viscoelastic property of the composites are estimated using unidirectional compression creep test. The creep functions were converted into relaxation function based on the convolution integral form of the basic equation using Laplace transform. Experimental testing for press molding of the PC/VGCF composites using V-shape die has been carried out. The results of the V-shape profiles and those obtained by FEM analysis were compared to confirm the validity of the present analysis method based on thermo-viscoelastic theory. The effect of the carbon nanofiber for the formability improvement of the PC/VGCF composites was discussed in detail.
  • 小林 秀敏, 堀川 敬太郎, 岸 真人
    2008 年 57 巻 8 号 p. 820-825
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Recently, a kind of titanium alloy which has outstanding properties, such as low elastic modulus, high strength, super elasticity and super plasticity has been developed by TOYOTA Central R & D, Inc. and named “gum metal”. Because of a new alloy, the mechanical properties of gum metal has not been clarified enough. In this paper, therefore, impact compression tests on gum metal were carried out at a nominal strain rate of about 1200s-1 and at various ambient temperatures (T = 77K∼773K) . For comparison, quasi-static compression tests were also performed at 6.7 × 10-4s-1. The flow stress σ at a strain of ε = 0.05 increased with an decrease in ambient temperature, especially σ ≈ 2050MPa at T = 77K. In static tests, work hardening could be observed in a plastic region. In dynamic tests, however, work softening could be seen. It may be considered that adiabatic heating is one of the largest reasons.
  • 田宮 洋一, 井上 正哉
    2008 年 57 巻 8 号 p. 826-831
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Synchronous reluctance motors (SynRM) have an effect for saving energy and reducing CO2 emissions by reason that the efficiency of them is higher than of induction motors. But now, SynRM are not much put a thing to practical use because that the high-precise fatigue design method of the rotor at centrifugal force is required for the preliminaly functional design. Accordingly, it is indispensable to establish the high-precise fatigue design method in order that SynRM are applied in market, safely and economically. In this paper, rotating fatigue failure tests were carried out using the SynRM rotor samples, and stress distribution of the rotor is calculated by finite element method (FEM). Then, experimental results were analyzed using the FEM results and the fatigue data of simple shaped specimens. The conclusions are obtained as follows :
    (1) Failure phenomenon and strength limit of SynRM rotor at centrifugal force can be turned out clearly.
    (2) Validity of FEM model of the rotor is confirmed by a comparison of experimental results and FEM results.
    (3) And, using the FEM simulation, high-precise fatigue design method of SynRM rotor can be established.
    Moreover, this method can be adopted for fatigue design of interior permanent magnet synchronous motors (IPMSM) and permanent magnet assisted SynRM, too.
  • 伊藤 義康, 須山 章子, 深沼 博隆
    2008 年 57 巻 8 号 p. 832-837
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    A cold spraying process has been developed for dense and clean and tightly bonded coatings in contract with other established thermal spraying processes. The cold sprayed copper is particularly well suited for metallic coatings, such as electric conductor and heat-sink. However, the microstructure of copper coatings formed by the cold spraying have not always been clarified everything. Two kinds of free-standing copper specimens, which were formed by the cold spray process and an atmospheric plasma spray process, were machined from the thick coatings and were used for measuring the density, microstructure, oxygen distribution and pore size distribution. As a result, it was confirmed that the cold sprayed copper showed remarkable high density and low oxygen content in comparison with the atmospheric plasma sprayed copper. Also, these experimental results showed that the pores below the size of a few μm in diameter could not be observed in case of the cold sprayed copper though the pore size distribution of the atmospheric plasma sprayed copper was extensive.
  • 森田 辰郎, 大友 隆行, 加賀谷 忠治, 田中 信一, 辻 宣佳
    2008 年 57 巻 8 号 p. 838-845
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    This study was conducted to improve fatigue strength of Ti-6Al-4V alloy by hybrid surface modification which was composed of plasma carburizing and fine-particle bombarding (FPB) treatment. Plasma carburizing at the first stage was conducted for 14.4ks at 1023K to form a hardened layer with no remarkable growth of microstructure in the substrate. Subsequent FPB treatment was carried out to eliminate the brittle compound layer (TiC) formed on the outermost surface by plasma carburizing and to introduce compressive residual stress. The result showed that the plasma carburizing slightly increased the fatigue strength of the titanium alloy. However, this treatment greatly reduced fatigue life because fatigue cracks rapidly propagated along the compound layer. When FPB treatment was further performed for the plasma-carburized material, the compound layer was perfectly eliminated and a relatively high compressive residual stress was introduced. As a result, initiation of fatigue cracks from the surface was strongly suppressed so that the crack initiation site existed under the hardened layer. At the same time, the fatigue life and the fatigue strength were greatly improved and the maximum percentage of the improvement in the fatigue strength reached to 30%.
  • 中村 士郎, 阪上 隆英, 原田 修輔, 久保 司郎
    2008 年 57 巻 8 号 p. 846-851
    発行日: 2008/08/15
    公開日: 2008/08/21
    ジャーナル フリー
    Data processing of sequential thermal data from concrete structure using infrared thermography is studied. Estimating a depth and a shape of the delamination defect in concrete, it is shown that phase value from sequential thermal data using Fourier analysis is effective. It can be also reduced disturbance depends on some noise from thermography or inhomogeneity of surface condition of concrete for detecting the defect.
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