JSME International Journal Series A Solid Mechanics and Material Engineering
Online ISSN : 1347-5363
Print ISSN : 1344-7912
ISSN-L : 1344-7912
Volume 44 , Issue 1
Showing 1-25 articles out of 25 articles from the selected issue
PAPERS
Material Evaluation
  • Norimitsu HIROSE, Shin-ichi TANAKA, Junichi ASAMI, Tohru KOHNO
    2001 Volume 44 Issue 1 Pages 1-7
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    In this study, the model in which Young’s modules(E) depends on only porosity (P) was investigated by the experimental results of sintered iron, and new equation of Young’s modulus against porosity was derived. The equation was confirmed by applying to the relationship between Young’s modulus and porosity obtained by the acoustic pulse method and the resonance frequency method. The derived equation is E(P)=E0(1−kE·P)·(1−P), where kE and E0 are a positive constant and E at P=0, respectively. This equation shows that the term of 1−P represents the porosity effect and the term of 1−kE·P represents the pore morphology effect, such as shape, size and interconnection of pores. Furthermore, for P below 0.2, a new nondestructive method to calculate partial porosity and propagation distance, such as defect depth and sample thichness, was developed by using of the ratio of longitudinal wave velocity to shear wave velocity obtained by the acoustic pulse method.
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  • Hidetaka NISHIDA, Hiroshi YAMAGUCHI, Masashi YOSHIDA
    2001 Volume 44 Issue 1 Pages 8-16
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    Creep damage in SUS304 steel samples fabricated by a hot isostatic press (HIP) at 1050°C was evaluated using the electromagnetic acoustic resonance (EMAR), noise energy and ultrasonic spectroscopy (first moment) methods. The coefficients of attenuation of 1.1 to 5.4 MHz electromagnetically excited acoustic waves in the samples were investigated. By reducing diffraction loss and the loss to the electromagnetic acoustic transducer due to transmission of the ultrasonic waves via mechanical contacts, it was possible to detect shear wave attenuation coefficients as low as 2.5×10-4/microsecond at 1.1 MHz. With specimens fabricated at pressure of 170MPa, the attenuation coefficient increased in proportion to frequency up to 5.4 MHz, whereas with specimens fabricated at pressure lower than 80 MPa the attenuation coefficient increased rapidly above 4 MHz. The void fraction was found to be greater in specimens fabricated under lower pressure, which may be responsible for the markedly higher ultrasonic attenuation in the specimens fabricated at pressure lower than 80MPa. It was apparent from the experiments that the EMAR method detects creep voids with greater sensitivity than the other methods. Accordingly we hope it will be possible for the EMAR method to be used on real facilities in the future.
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  • Lingfeng HE, Shoichi KOBAYASHI
    2001 Volume 44 Issue 1 Pages 17-22
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    In the present paper, 1) a new non-contact ultrasonic stress measurement technique is proposed based on acoustoelasticity, in which ultrasonic wave motion is detected by use of a laser Doppler velocimeter, and 2) the stress-acoustic coefficients of Rayleigh wave for aluminum alloy and structural steel are determined by the technique. In the measurement system, Rayleigh waves are emitted into the specimen by a wedge-type piezoelectric transducer and vertical velocities of the surface motions of the traveling Rayleigh waves are detected by the laser Doppler velocimeter at two points of 4 cm apart. In order to measure the traveling time of the wave between the two points, the converted voltage signals are supplied both to i) a sing-around unit and ii) to a digital oscilloscope. The time-of-flight over the distance between the two points is obtained either by subtracting the sing-around periods measured at the two points or by direct reading at zero-cross of the overlapped images of the two waves on the CR display of the oscilloscope. Both measurements are made at the same time under increasing or decreasing loads. The stress-acoustic coefficients obtained are −1.2×10-5/MPa and −0.21×10-5/MPa for aluminum alloy 5052 and structural steel SS400, respectively. These results are in good agreement with those determined using two knife-edge contact piezoelectric transducers. This study shows that the proposed non-contact measuring technique by use of laser velocimetry is applicable to determining the stress-acoustic coefficients.
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  • Sung-Jin SONG, Ji-Ung CHOE, Dong-Ho BAE
    2001 Volume 44 Issue 1 Pages 23-30
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    For reliable quality assurance of filament-wound composite rocket motor (FCRM) cases by use of acoustic emission during hydroproof (AE/H) testing, it is necessary both to detect defects introduced in fabrication process and to monitor damage done due to hydraulic pressurization. The circumferential wave, which has strong directivity and weak anisotropy in wave propagation in the particular FCRM case under present investigation, has high potential to address such a need. To explore these outstanding capabilities of the circumferential wave, two kinds of experiments were conducted. The pitch-catch measurements of the circumferential wave that propagated through the artificial slits demonstrated its high potential for detection of flaws. The experiments for AE source location using a particular, triangular layout of AE sensors declared its capability for monitoring of damages. Inspired from the experiments, new approaches to nondestructive evaluation of the given FCRM case were proposed using the circumferential wave: 1) an ultrasonic pitch-catch scanning and 2) an AE/H testing with a suitable triangular layout of AE sensors.
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  • Kui-Seob KIM, Naotake NODA
    2001 Volume 44 Issue 1 Pages 31-36
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    The transient temperature solution for a functionally graded material (FGM) is formulated by Green’s function based on the Galerkin method. An approximate solution that satisfies the homogeneous boundary condition is substituted into the governing equation to yield an eigenvalue problem. To solve the eigenvalue problem, the eigenfunctions are approximated by a series of polynomials satisfying the homogeneous boundary condition. The Galerkin method is used to determine the coefficients of eigenfunctions. The transient temperature solution for a general heat conduction equation with a source and nonhomogeneous boundary conditions is obtained by using Green’s function, which is expressed by eigenvalues and corresponding eigenfunctions. Transient thermal stresses in a FGM plate and a FGM hollow circular cylinder are discussed.
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  • Hideo AWAJI, Hiromitsu TAKENAKA, Sawao HONDA, Tadahiro NISHIKAWA
    2001 Volume 44 Issue 1 Pages 37-44
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    This paper presents a numerical technique for analyzing one-dimensional transient temperature and stress distributions in a stress-relief-type plate of functionally graded ceramic-metal based materials (FGMs), in relation to both the temperature-dependent thermal properties and continuous and gradual variation of the thermo-mechanical properties of the FGM. The FGM plate is assumed to be initially in steady state of temperature gradient, suffering high temperature at the ceramic side and low temperature at the metallic side associated with its in-service performance. The FGM plate is then rapidly cooled at the ceramic side of the plate by a cold medium. The transient temperature and related thermal stresses in the FGM plate are analyzed numerically for a model alumina-nickel FGM system. The proposed analytical technique for determining the temperature distribution is quite simple and widely applicable for various boundary conditions of FGMs, compared with methods recently proposed by other researchers. The optimum composition of FGMs is also discussed to reduce the thermal stresses in the FGM plate, relating to the function of the volume fraction of the metal across the thickness.
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  • Yajun YIN, Mingde XUE, Shouwen YU
    2001 Volume 44 Issue 1 Pages 45-56
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    A lower bound rigid plastic constitutive model for porous materials has been published recently, but its reliability and accuracy is still kept unknown. Therefore, this paper is confined to examine this model by comparing it with other ones such as the upper bound one and experimental-based one. Under three loading states (i.e. uniaxial stress condition, biaxial equal stress condition and uniaxial strain condition), the sintered copper’s ductility, compressibility, strength property, deformation characteristics, stress∼strain curves and damage evolution process predicted by these models are systematically compared. The advantage of the lower bound model in describing the yield property and its limitations in evaluating the ductility, compressibility, strength variation and damage evolution process of porous materials are clarified. Systematical analysis reveals that these limitations may be attributed to the short of void interaction mechanism in the lower bound model. This discovery lays the foundation for further improvement and modification of the lower bound model in the future research.
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Deformation
  • Masaki KUSAGAWA, Toshiya NAKAMURA, Yasuhide ASADA
    2001 Volume 44 Issue 1 Pages 57-63
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    Fundamental deformation and deformation-recovery behaviors of Ni-Ti-Nb shape memory alloy were experimentally investigated, especially their dependency on test temperature. Monotonic tension tests and deformation-recovery tests were conducted at low and high temperatures ranging from 253 K to 473 K. Stress-strain response above 323 K is quite different from that below the temperature. The yield stress shows the positive temperature-dependency below 323 K and the opposite trend is observed above this temperature. From this result, it is estimated that the dominant mechanism of inelastic deformation is the plastic deformation above 323 K and the pseudo-elastic one below it. The temperature at which strain recovery starts strongly depends on the maximum prestrain and prestraining temperature. Residual strain is always observed in the deformation-recovery tests. Also it is found that the larger recovery strain can be obtained for the larger prestrain and at the lower prestraining temperature.
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  • Yoichi OBATAYA, Takamoto ITOH, Takaya KATO
    2001 Volume 44 Issue 1 Pages 64-70
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    The multiple strata plasticity model, which was presented in the previous paper by introducing the tensor Q expressing the activating state of slip systems in the polycrystalline metal, has been developed with a new internal state factor B5. This factor B5 denotes the ratio of the grains with activating slip systems greater than five to the total grains. The variation features of B5 and Q can be computationally evaluated with a virtual polycrystalline specimen. The microstructure state, such as dislocation density can be reflected in the internal structure variable A in this model. The relationship between B5 and A has been examined on the tensile flow curve of pure copper. As a result, a simple formula connecting A with B5 was determined. Moreover, it has been clarified that this formula is applicable for describing the plastic deformations under other proportional loadings.
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  • (Case of Nonlinear Elastic-Plastic Deformation Field)
    Toshihisa NISHIOKA, Hiroki IKEGITA, Kazutaka TAMAI, Naoki KOBAYASHI
    2001 Volume 44 Issue 1 Pages 71-81
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    In previous studies, for the case of linear elastic deformation field, the authors developed a hybrid experimental-numerical method based on a variational principle minimizing experimental measurement errors. In this study, for the case of nonlinear elastic-plastic deformation field, first, an incremental variational principle is derived to minimize the errors and noises associated with a full-field displacement measurement such as moire interferometry. Next, on the basis of this variational principle, an intelligent hybrid experimental-numerical method is developed for the case of nonlinear elastic-plastic deformation field. The intelligent hybrid method demonstrates automatic detection and elimination of the measurement errors and smooth visualization of stress and strain contours. Furthermore, the intelligent hybrid method automatically achieves the path independence of the T* integral, restoring the path-dependence caused by the measurement errors. This paper provides the foundation of an intelligent measurement and visualization of displacement and stress fields in actual structural components.
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  • Ken-ichi OHGUCHI, Katsuhiko SASAKI
    2001 Volume 44 Issue 1 Pages 82-88
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    This paper shows the temperature effect on the deformation of 60 Sn-40 Pb solder alloys and a simulation using a constitutive model for viscoplasticity. First, a series of tests, such as creep, pure tension, and cyclic tension-compression loading were conducted to clarify the temperature effect on the deformation of 60 Sn-40 Pb solder alloys. The test results showed that the deformation of the solder alloys has a large temperature dependence. Simulations of the temperature effect on the deformation were also conducted by a constitutive model previously proposed. The parameters used in the model can be determined simply from pure tension and cyclic tension-compression loading tests at several temperatures. It was also found that the constitutive model describes not only the temperature effect on the pure tension and the cyclic tension-compression loading but also the creep curves after cyclic tension-compression preloading.
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  • (In Case of a Square Frame with Rigid Joints)
    Atsumi OHTSUKI, Fernand ELLYIN
    2001 Volume 44 Issue 1 Pages 89-93
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    In elements used as flexible linking devices and structures, the main characteristic is a fairly large deformation without exceeding the elastic limit of the material. This property is of both analytical and technological interests. Previous studies of large deformation have been generally concerned with a single member (e.g. a cantilever beam, a simply supported beam, etc.). However, there are very few large deformation studies of assembled members such as frames. This paper deals with a square frame with rigid joints, loaded diagonally in either tension or compression by a pair of opposite forces. Analytical solutions for large deformation are obtained in terms of elliptic integrals, and are compared with the experimental data. The agreement is found to be fairly close.
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  • Toshiyuki HAYASHI, Akihito MATSUMURO, Tomohiko WATANABE, Toshihiko MOR ...
    2001 Volume 44 Issue 1 Pages 94-99
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    Plastic deformation of TiN thin films with (111) and (200) preferred orientation was determined based on their hardness anisotropy. Hardness was measured by means of the nano-indentation technique. Plastic deformation of TiN films was caused by the indentation of the trigonal diamond tip, and evidence of this phenomenon was provided by cross-sectional scanning electron microscopy (SEM) observation and transmission electron diffraction (TED) analysis. The influence of the differences in residual stress and grain size on hardness anisotropy was restrictive, and hardness anisotropy can be explained by the anisotropy of yield stress as calculated using Schmid’s law. This relationship suggests the existence of a {100}‹110› slip system in the TiN crystal. Transmission electron microscopy (TEM) observation of brittle cracks in TiN films confirmed that these cracks are caused not cleavage fractures but by intergranular fractures.
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Damage & Fracture
  • Alexander ZOLOCHEVSKY, Yoichi OBATAYA
    2001 Volume 44 Issue 1 Pages 100-108
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    A constitutive model is proposed to describe the damage development in aluminum alloys under creep conditions for both isothermal and nonisothermal processes. Special emphasis is laid on four specific phenomena: tension-compression asymmetry of creep, damage induced anisotropy, unilateral creep damage and damage deactivation. Within the framework of the phenomenological approach in the Continuum Damage Mechanics, the nonlinear tensor constitutive equation for creep deformation and damage evolution equation are proposed to account for different orientation of microcracks in aluminum alloys under tensile and compressive loading types. After a determination of the material parameters in the obtained constitutive equation and damage growth equation, the proposed model is applied to the describing creep behavior of the aluminum alloy under uniaxial nonproportional and multiaxial nonproportional loading for both isothermal and nonisothermal processes.
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  • Kasivitamnuay JIRAPONG, Toshiya NAKAMURA, Yasuhide ASADA
    2001 Volume 44 Issue 1 Pages 109-116
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    A nonlinear damage model, previously developed for creep-fatigue life evaluation of Mod. 9Cr-1Mo steel and 316FR stainless steel in a high vacuum environment was applied to 2 1/4Cr-1Mo steel. In the damage model, the damage accumulation process is considered to be composed of three basic processes: fatigue, creep and creep-fatigue. The fatigue damage process consists of a crack initiation period and a crack growth period. The creep damage process consists of nucleation of creep cavities. Damage interaction is fatigue cracks propagate from creep cavities. Application of this damage model to creep-fatigue tests under complex waveforms and complex strain histories showed a good correlation with the experimental results within a factor of 2.
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  • Pey-Shiuan SONG, Bor-Chiou SHEU, Li CHANG
    2001 Volume 44 Issue 1 Pages 117-122
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    This paper modifies the Wheeler retardation concept to improve the accuracy of crack growth predictions after a single overload. This modification is primarily based on the number of delay cycles, Nd, and on the overload affected crack length, ad. Nd is used to calibrate the Wheeler exponent, m, and ad is employed to assess the effective overload plastic zone size. In addition, the regression equations for m and ad are established in terms of the overload ratio OLR. The modified Wheeler model together with these equations performed well in reproduction of the actual crack growth behavior in 5083-O aluminum alloy. All predictions of the number of delay cycles were within a factor of 1.1 of the experimental observations.
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  • (Application of Fractal Dimension of AE Peak Amplitude Distribution)
    Noboru SINKE, Akira YONEKURA, Masanori TAKUMA, Hideaki YOSHIDA, Seiki ...
    2001 Volume 44 Issue 1 Pages 123-129
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    Many of the ceramic coating materials can be exposed to harsh working conditions because of their excellent characteristics. Therefore, for producing superior coating materials, it is necessary to establish a methodology for evaluating the mechanical properties of these materials. In this study, nine kinds of TiN film of 5 μm thickness, which were deposited on different parent materials under different N2 partial pressures, were scratched and a method for evaluating the properties of the coating materials with acoustic emission (AE) was developed. The results were as follows: (1) A scratch testing apparatus that can detect AE signals emitted from the TiN film during scratch testing was fabricated. (2) The relationship between the characteristics of the detected AE signals and the fracture modes of the film (i.e., cracking, chipping and flaking) was investigated. (3) It was clarified that fractal dimension m determined from AE peak amplitude distributions become greater than three when the fracture mode was cracking, the dimension became between one and three when the mode was chipping, and the dimension became less than one when the mode was flaking. From these experimental results, it was found that the strength and the adhensivity of thin film can be evaluated from fractal dimension m determined by scratch testing.
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  • Yinsheng LI, Daisuke KATO, Katsuyuki SHIBATA
    2001 Volume 44 Issue 1 Pages 130-137
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    In this paper, the results of the sensitivity analyses of failure probability are presented for pressure vessel under PTS (Pressurized Thermal Shock) transient, by using our new probabilistic fracture mechanics analysis code. The pressure, the fluid temperature at the inner surface of pressure vessel and other analysis conditions are based on an international benchmark study on PTS. The temperature and stress distributions through wall thickness during transient are obtained by a heat transfer and an elasto-plastic stress analysis with FEM. The new models and methodologies introduced in our code are summarized and the effect of overlay clad and behavior of semi-elliptical crack on the failure probability are investigated especially.
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  • (Effect of Oxide-Induced Crack Closure)
    Terutoshi YAKUSHIJI, Masaharu KAGE, Hironobu NISITANI
    2001 Volume 44 Issue 1 Pages 138-143
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    Rotating bending fatigue tests were carried out in air and in pure argon to clarify the effect of oxide-induced crack closure on crack non-propagation. The materials used were Ti added extra-low-carbon steel with no strain ageing, 0.02% carbon steel with a single ferrite phase, and annealed 0.19% carbon steel. The results obtained are as follows. (1) Threshold stress for fatigue crack growth is lower for Ti added extra-low-carbon steel in argon than for that in air, though the fatigue crack has a tendency to propagate more slowly in argon than the air. The above result shows that oxide-induced crack closure plays a significant role in crack non-propagation. (2) It seems that the influence of oxide-induced crack closure on crack non-propagation is smaller than the influence of strain ageing and microstructure. (3) A non-propagating crack is observed in the steel with a single ferrite phase even when strain ageing and oxide-induced crack closure do not occur.
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Computational Mechanics
  • Akira MATSUDA, Takashi KATO, Yoshio MIYAKAWA, Tooru KAWABE, Akinori NA ...
    2001 Volume 44 Issue 1 Pages 144-151
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    In large-scale numerical simulations based on a rigid-plastic finite element method, most of the processing time is used in solving linear equations called stiffness equations. Therefore, the method of parallel processing is introduced to speed up the process of solving stiffness equations. The conjugate gradient method is adopted for solving the equations and is parallelized on a cluster of workstations. In the present paper, we present a new cluster of workstations which uses broadcasting to transmit data from a master processor to all slave processors, instead of unicasting. Broadcasting can transmit data to all slaves by one transmission regardless of the number of slaves. Therefore, the cluster of workstations with broadcasting can reduce communication time sharply compared with sequential data transmission by conventional unicasting. We achieved more than twelvefold increase in speed for solving the stiffness equations with twenty slave processors.
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  • Satoshi IZUMI, Takashi KAWAKAMI, Shinsuke SAKAI
    2001 Volume 44 Issue 1 Pages 152-159
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    A new method combining the finite element method (FEM) and the molecular dynamics (MD) for the complicated diamond-like structure of silicon is proposed. For simultaneous simulation, the patch model was used to exchange displacement information in both directions. A one-to-one correspondence of atoms and nodes is impossible for a silicon lattice, therefore, the atom was embedded in an isoparametric element. The influence of internal displacement which is a additional displacement to the continuum one, was taken into consideration. Martin’s method was applied to calculate the internal displacement and elastic constants. The conjugate gradient method was used for MD, the Newton-Raphson method was used for FEM to efficiently find the stable state, and the acceleration condition was set to raise convergence. The verification model showed that the smooth transition of displacement and stress was realized in the boundary region of FEM and MD. These values showed good agreement with the elastic solution.
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  • Akihiro MATSUDA, Yasuki OHTORI, Shuichi YABANA, Kazuta HIRATA
    2001 Volume 44 Issue 1 Pages 160-166
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    The purpose of this paper is develop the evaluation method for mechanical properties of laminated rubber bearings. We use nonlinear finite element method considering the volumetric deformation of natural rubber material. Relationship between pressure and volumetric strain of the natural rubber is obtained from volumetric tests and is introduced into user-subroutine of finite element code. Simulations of natural rubber bearing of different geometric shapes are conducted. Effects of finite element mesh and compressibility of rubber material on simulated results are investigated. Horizontal and vertical stiffness by numerical calculations are simulated with enough accuracy. Vertical deformation caused by shear deformation and vertical stiffness with static offset shear strain is also simulated well.
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Optimal Design
  • Osamu KUWAZURU, Nobuhiro YOSHIKAWA, Shigeru NAKAGIRI
    2001 Volume 44 Issue 1 Pages 167-174
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    A design methodology of flexible structures is presented for the enrichment of the structural function by utilizing the mechanism realized by the large elastic deformation of flexible members. The flexible member is modeled by straight bar segments linked by virtual coil springs. Its large deformation is analyzed by the Lagrange multiplier method, the functional of which is constituted in line with the principle of stationary potential energy with constraints on geometrical boundary conditions. The sensitivity analysis is formulated especially for the solution obtained by the Lagrange multiplier method so as to approximate the change of the structural response in the first-order sense with respect to design variables. Owing to the sensitivity analysis, governing equation of the design variables to realize the prescribed structural performance is derived as the linear simultaneous equations. The solution of design variables is determined by the Moore-Penrose generalized inverse since the coefficient matrix becomes rectangular.
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  • Qinzhong SHI, Ichiro HAGIWARA, Futoshi TAKASHIMA
    2001 Volume 44 Issue 1 Pages 175-184
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    A considerable number of functional evaluations may be required in the process of optimization. Although approximation models constructed by response surface methodology can significantly reduce functional evaluations, the design accuracy may be strongly dependent on the type of activation functions and designs used. In this paper, we propose techniques to search the design space containing the global optimal design using designs by conditioned random seeds, and techniques for determining more accurate approximations by employing a sequential approach called the most probable optimal design (MPOD) method. The MPOD method is a response surface methodology based on the holographic neural network, which uses the exponential function as an activation function. In the MPOD method, extrapolation is employed to make the technique available for general application in structural optimization. The formula to estimate the necessary functional evaluations under certain conditions is expressed. Application examples show that the MPOD method is an effective methodology, which is expected to determine the optimal design with multiple local optima.
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  • —A Finite Element Modeling
    Li-Tung CHANG, Chih-Han CHANG, Guan-Liang CHANG
    2001 Volume 44 Issue 1 Pages 185-192
    Published: 2001
    Released: September 27, 2002
    JOURNALS FREE ACCESS
    Optimized assessment of the adequacy of fit conditions between a motorcycle helmet and head size in relation to prevention of head injury remains unclear and is complicated by wide variations in the size and shape characteristics of helmet and wearer’s heads. A finite element model (LS-DYNA3D) based on realistic geometric features of a motorcycle helmet was established to simulate the standard shock absorption test for evaluating the dynamic response and fit effects of a helmet. The model was used to simulate crown, rear and side sites impacts of the helmet. The peak acceleration and Head Injury Criterion (HIC) were employed to assess the protective performance of the helmet against head injuries. The results show that this helmet model had various dynamic responses at different impact sites due to its geometric shape, but that the impact-absorbing capability did not vary markedly within these sites. The fit conditions between the headform and the helmet dramatically affected the assessment of the impact-absorbing capability of the helmet in the standard shock absorption test. However, for a motorcyclist, the helmet fit would have only minor influence on the protection against head injuries. This observation suggests that a better fitting helmet with stable fixation should provide more protection against head injury.
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