Nobutada Ohno - Academia.edu (original) (raw)
Papers by Nobutada Ohno
Journal of the Society of Materials Science, Japan, 1995
When a cylinder is subjected to a temperature front moving cyclically in the axial direction, the... more When a cylinder is subjected to a temperature front moving cyclically in the axial direction, the circumferential plastic strain may accumulate with the increase of the number of cycles. This is a thermal ratchetting problem induced by a liquid surface moving in a cylinder, and it is important especially in designing fast breeder reactors. In the present paper, the effect of kinematic hardening rule on the thermal ratchetting analysis is discussed by implementing the following four kinds of kinematic hardening rules in a finite element analysis; the perfectly plastic model (PP), the linear kinematic hardening rule (LKH), the classical nonlinear kinematic hardening rule of Armstrong and Frederick (AF), and the rule proposed recently by Ohno and Wang (OW). It is shown that disregard of transient hardening after yieding leads to overstimating the thermal ratchetting, that a rule predicting larger mechanical rathcetting under uniaxial cyclic loading makes the thermal ratchetting more serious, and that the Ohno and Wang rule can render the analysis most realistic among them.
Journal of the Society of Materials Science, Japan, 2008
Stress distribution and the residual stress in the solidified materials are investigated by the n... more Stress distribution and the residual stress in the solidified materials are investigated by the numerical analyses based on the phase field method. The governing equations considering the coupling effects among phase transformation, temperature and stress/strain, including elasto-plastic constitutive relation, are formulated using the phase field model, and they are numerically solved by the finite element method. As a typical case of the microstructure formation, the dendritic growth processes are studied. In spite of several assumptions employed for the material properties of liquid, the complicated stress distributions in dendrites are exhibited : Strong stresses are distributed at the bottom of the side branches. Due to the plastic behavior, residual stress distributions are obtained when the solidification and cooling are completed in the whole region. When a microstructure constructed with two dendrites is formed, high stresses are generated in the regions where the liquid is remained till the very last stage of the solidification. Subsequently, two types of the morphology of microstructures are investigated, and it reveals that the residual stress distribution and the maximum value are strongly affected by the microstructure.
International Journal of Solids and Structures, 2007
In this study, a method for three-dimensional microscopic interlaminar analysis of cross-ply lami... more In this study, a method for three-dimensional microscopic interlaminar analysis of cross-ply laminates is developed based on a homogenization theory to analyze microscopic interactions between unidirectional long fiber-reinforced laminae. For this, a unit cell of a cross-ply laminate, which includes interlaminar areas, is defined under the assumption that each lamina in the laminate has a transversely square fiber array. Then, showing that the laminate has a point-symmetric internal structure, the symmetry is utilized to introduce half of the unit cell as the domain of analysis. Moreover, the domain of analysis is divided into substructures using a substructure method combined with the homogenization theory, significantly reducing the computational costs. The present method is then applied to the analysis of interlaminar stress distributions in a carbon fiber/epoxy cross-ply laminate subjected to in-plane uniaxial tension. It is shown that microscopic shear stress noticeably occurs at the interface between the 0°-and 90°-plies. It is also shown that the microscopic interaction between the two plies is observed only in the vicinity of the interface.
TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICAL ENGINEERS Series A, 2013
This paper describes the homogenization of fin layers for structural analysis of heat exchanger c... more This paper describes the homogenization of fin layers for structural analysis of heat exchanger cores composed of flat tubes and wavy fins. It is assumed that uniform deformation prevails at the bonded interfaces between fins and tubes, and also that the fins have the Y-periodicity in the fin layers. The homogenization method for 3D periodic solids is then shown to be applicable to the homogenization of fin layers, provided that the uniform deformation condition is imposed at the bonded interfaces between fins and tubes. The resulting homogenization method is applied to designed and real shapes of outer and inner fins in an intercooler. The evaluated, homogenized elastic stiffnesses of fin layers are verified by performing finite element analyses using full-scale and fin-homogenized models of tube-fin connected pieces subjected to uniaxial compression. An experiment of a tube-fin laminated block under uniaxial compression is further performed to demonstrate that the real shapes of outer and inner fins are definitely important for the homogenization of fin layers.
Journal of the Atomic Energy Society of Japan / Atomic Energy Society of Japan, 1991
Journal of Solid Mechanics and Materials Engineering, 2008
In this study, a linearization method is used to develop an implicit integration scheme for a cla... more In this study, a linearization method is used to develop an implicit integration scheme for a class of high-temperature inelastic constitutive models based on non-linear kinematic hardening. A non-unified model is first considered in which the inelastic strain rate is divided into transient and steady parts driven, respectively, by effective stress and applied stress. By discretizing the constitutive relations using the backward Euler method, and by linearizing the resulting discretized relations, a tensor equation is derived to iteratively achieve the implicit integration of constitutive variables. The implicit integration scheme developed is shown to be applicable to a unified constitutive model in which back stress evolves due to static and dynamic recoveries in addition to strain hardening. The integration scheme is then programmed for a subroutine in a finite element code and applied to a lead-free solder joint analysis. It is demonstrated that the integration scheme affords quadratic convergence in the iterations even for considerably large increments, and that the non-unified and unified models give almost the same results in the solder joint analysis.
Journal of the Society of Materials Science, Japan, 2008
The axial stresses of the fiber in single carbon fiber/epoxy resin model composites under change ... more The axial stresses of the fiber in single carbon fiber/epoxy resin model composites under change of surroundings, e.g., temperature and water, have been measured by micro-Raman spectroscopy. The specimens were soaked in hot air, water and hot water, and returned to the beginning state to cause cyclic changes of temperature and water absorption : (1) heating and cooling at atmospheric environment, (2) immersing in the water of the room temperature and drying, and (3) immersing in the hot water of 80 and drying at the room temperature. In the case of (1) and (2), the fiber axial stresses after the cyclic change of the surroundings recover almost the same before the heating or the immersion. On the other hand, the fiber axial stress after the drying is more compressive than that before the hygrothermal exposure in the case of (3). Hygrothermal effects on the residual stress are discussed based on fiber stresses caused at elevated temperature over the glass transition temperature of the epoxy resin.
Journal of the Society of Materials Science, Japan, 2005
In this study, we develop a homogenization theory and its finite element discretization based on ... more In this study, we develop a homogenization theory and its finite element discretization based on the nonlocal crystal plasticity theory of Gurtin (J. Mech. Phys. Solids 48 (2000) ; J. Mech. Phys. Solids 50 (2002)), which includes slip gradients and additional slip boundary conditions with respect to microforce balances for each slip system. Although it is difficult to determine slip boundary conditions, the development of the homogenization theory for the nonlocal crystal plasticity makes it possible to obtain the unified treatment of slip boundary conditions on the periodic boundary in periodic materials. Furthermore, by applying a tangent modulus method to the finite element discretization of rate-dependent small deformation, a strong coupling boundary value problem is derived from homogenization equations which consist of the stress balance, the microforce balances and the macroscopic relation. Finally, the effectiveness of the proposed theory and method is confirmed through finite element analyses of a double slip single crystal model with periodic obstacles.
Journal of Solid Mechanics and Materials Engineering, 2010
In this study, we examine the variation in residual stresses in polymer matrix composites with cy... more In this study, we examine the variation in residual stresses in polymer matrix composites with cyclic changes in hygrothermal environments by measuring the axial stresses in the fiber of single-carbon fiber/epoxy resin model composites using Raman microspectroscopy. To change the temperature and water absorption in the specimens separately and simultaneously, the specimens were (1) heated and cooled at atmospheric environment, (2) immersed in water and dried at room temperature, and (3) immersed in hot water (80°C) and cooled to room temperature in water and then dried. All specimens were thus restored to their initial states and then evaluated. In the cases of (1) and (2), the fiber stresses reverted to almost the same as their initial states, but in the case of (3) the axial stress after hygrothermal cyclic change was more compressive compared to the initial state. The differences among the results of the three cases are discussed based on the fiber stresses at elevated temperature, which were evaluated by Raman microspectroscopic measurement performed at temperatures ranging from room temperature to those above the glass-transition temperature of the epoxy matrix resin.
Journal of the Society of Materials Science, Japan, 2008
Journal of the Society of Materials Science, Japan, 2004
Journal of the Society of Materials Science, Japan, 2010
In this study, we develop an extended version of homogenization theories for composite materials ... more In this study, we develop an extended version of homogenization theories for composite materials consisting of inclusions and a matrix. The extension allows discrete dislocations to move in a matrix based on dislocation dynamics. Macroscopic stress-strain response and microscopic stress distribution in representative volume elements (RVEs) are analyzed on the assumption that the periodic arrangement of RVEs is subjected to macroscopic uniform deformation. The periodicity, which enforces the periodic distributions of microscopic stress and strain as well as dislocations, is used as boundary conditions on the surface of RVEs. It is shown that elastic fields of periodically arranged dislocations in an infinite medium have no contribution to macroscopic relations, and that discontinuous displacements resulting from dislocation motions in each RVE are introduced as a macroscopic plastic strain. From these relations, a set of homogenization equations is derived, in which two boundary value problems in integral form are solved for analyzing perturbed displacements due to the presence of inclusions and due to the interaction of inclusions with dislocations, respectively. Finally, the influence of periodic boundary conditions on macroscopic and microscopic responses is investigated by performing the RVE analysis of a model composite.
Transactions of the JSME (in Japanese), 2015
In this study, the thermal fatigue life of substrate with Cu through-hole is evaluated by conside... more In this study, the thermal fatigue life of substrate with Cu through-hole is evaluated by considering the mechanical properties of Cu thin film and glass fiber cloths structure. We first conducted tensile tests of Cu thin film and found that the rate-dependence of inelastic property varies abruptly with temperature. An inelastic constitutive model for Cu thin film is then proposed by combining both rate-independent and rate-dependent models. The proposed inelastic constitutive model is introduced in a Finite Element Method based analysis of glass epoxy substrate with Cu through-hole. Moreover, low cycle tests of Cu thin film are carried out by using repeated 4-point bending to evaluate its isothermal fatigue properties. Through our analysis we verified the capability of the proposed model to predict thermal fatigue life of Cu through-hole using the isothermal fatigue properties. The results show that the glass fiber cloths structure of the substrate needs to be considered in order to successfully predict the thermal fatigue life of the Cu through-hole.
Journal of Nanomaterials, 2014
The effects of wavenumber and chirality on the axial compressive behavior and properties of wavy ... more The effects of wavenumber and chirality on the axial compressive behavior and properties of wavy carbon nanotubes (CNTs) with multiple Stone-Wales defects are investigated using molecular mechanics simulations with the adaptive intermolecular reactive empirical bond-order potential. The wavy CNTs are assumed to be point-symmetric with respect to their axial centers. It is found that the wavy CNT models, respectively, exhibit a buckling point and long wavelength buckling mode regardless of the wavenumbers and chiralities examined. It is also found that the wavy CNTs have nearly the same buckling stresses as their pristine straight counterparts.
Journal of the Society of Materials Science, Japan, 1997
The paper deals with estimation of thermal ratchetting strain in cylinders subjected to short axi... more The paper deals with estimation of thermal ratchetting strain in cylinders subjected to short axial travelling of temperature distribution. The estimation is made by considering partly pressurized cylinders as a model. It is assumed that the pressurized region, which is taken to occupy the temperature travel region, is ruled by the deformation theory with a constant secant modulus so as to estimate the thermal ratchetting strain conservatively. This assumption enables us to utilize a linear elastic solution for thin-walled cylinders. The secant modulus is specified using the analytical solution derived in the long travel case by assuming Masing's rule and no cyclic hardening. It is shown that the model gives good estimates to finite element analysis of the thermal ratchetting strain as well as to experiments on 316FR steel cylinders done by Kitade et al. An extension of the model, in which plastic yielding outside the travel region is taken into account, is also discussed.
Physical Review B, 2010
Electronic structures of ͑6,0͒, ͑8,0͒, and ͑10,0͒ single-walled boron nitride nanotubes ͑SWBNNTs͒... more Electronic structures of ͑6,0͒, ͑8,0͒, and ͑10,0͒ single-walled boron nitride nanotubes ͑SWBNNTs͒ subjected to tension, torsion, and flattening are investigated using first-principles calculations. Energy bands and charge distributions of the SWBNNTs are calculated within the density-functional theory and forces required to deform the SWBNNTs are estimated using energy variation with deformation. Our calculations show that all the deformation modes decrease the energy gaps of the SWBNNTs because of a decrease in the conductionband minimum ͑CBM͒ energy, which is caused by an overlap of CBM charge densities between circumferentially neighboring boron atoms. It is found that flattening with a force smaller than that applied for tension or torsion causes a larger decrease in energy gaps of the SWBNNTs and that the force required for flattening SWBNNTs is not unrealistic.
Modelling and Simulation in Materials Science and Engineering, 2009
Shape-memory behaviour in multi-grain material is simulated using a molecular dynamics method. An... more Shape-memory behaviour in multi-grain material is simulated using a molecular dynamics method. An embedded-atom-method potential for NiAl alloy is applied, and a sequence of conditions including loading, unloading, heating and cooling is imposed. Two types of grain arrangement are used, and the deformation and shape recovery due to phase transformation are observed for both models. The stress-strain relation is revealed to draw a hysteresis loop, and the individual curves are smoother than those previously obtained from a single-crystal model. The deformation mechanism during loading is discussed using local structure analysis. Local deformation is initiated at the grain boundaries, and the deformed region propagates along the twin plane in the grain. The propagation is then obstructed by the grain boundaries, and a band pattern of the deformed area is formed. The influence of the grain shape and distribution, as well as the crystal orientation of each grain, on the deformation behaviour is also investigated. Qualitatively common features in the deformation mechanism and stress-strain relation are observed despite different grain distributions, while the critical values in stress vary, owing to the crystal orientations of the grains.
MATERIALS TRANSACTIONS, 2007
A computational multiple scattering simulation method was applied to analyze the characteristics ... more A computational multiple scattering simulation method was applied to analyze the characteristics of the ultrasonic shear wave that propagates in unidirectional carbon-fiber-reinforced epoxy composites with its polarization direction parallel to the fibers. The numerical simulations were carried out for regular as well as random fiber arrangements and for different fiber volume fractions. The results were combined with the one-dimensional theory describing the macroscopic propagation behavior, in order to identify the phase velocity and the attenuation coefficient of the composite. The phase velocity and the attenuation coefficient were found to depend significantly on the fiber volume fraction, but less so on the fiber arrangement in the frequency range examined here. Furthermore, the present analysis showed a good agreement with the experimental data.
JSME International Journal Series A, 2005
As a background of the ultrasonic characterization of contact interfaces, ultrasonic wave propaga... more As a background of the ultrasonic characterization of contact interfaces, ultrasonic wave propagation characteristics at contacting solid surfaces have been studied numerically and experimentally. The wave propagation along the contact interface has been simulated by the two-dimensional finite difference method, assuming a spring-type interface with normal and tangential stiffnesses. The analysis has verified that the antisymmetric-mode interface wave can be generated by the shear excitation at one end of the interface, and its phase velocity can be evaluated with good accuracy from the waveform received at the other end. Measurement of the phase velocity of the interface wave has been carried out for contacting PMMA surfaces under different contact pressures, together with the shear-wave reflection measurement to evaluate the interface tangential stiffness. The measured relation between the phase velocity and the tangential stiffness has been shown to agree well with the theoretical relation for the antisymmetric-mode interface wave.
JSME International Journal Series A, 2006
Molecular dynamics simulations of the shape-memory effect are carried out to investigate the atom... more Molecular dynamics simulations of the shape-memory effect are carried out to investigate the atomistic behavior during deformation and shape-recovery processes. The embedded-atom-method potential function and parameters for Ni-Al alloy are applied. The initial configurations of atoms are set on the lattice points of the martensite structure, in which the distribution of the variant orientation is limited to the two-dimensional direction for simplicity. When the shear load is imposed toward the x direction, parallel to the variant interface, the deformation of the variants occurs, and finally, all variants settle into the uniform orientation. The deformed state is maintained after the load is released, and the original shape is recovered through heating and cooling processes because of phase transformation to bcc and martensite. In the loading process, the stress-strain curve exhibits a zigzag shape consisting of repeated stress increase and abrupt release. The interval of the stress peaks is revealed to be smaller as the model size becomes larger. Deformation observed in variant layers seems to occur at the same time at every points in the layer for a small model. However, the simulation with a large model indicates a nucleation and propagation behavior in each layer.
Journal of the Society of Materials Science, Japan, 1995
When a cylinder is subjected to a temperature front moving cyclically in the axial direction, the... more When a cylinder is subjected to a temperature front moving cyclically in the axial direction, the circumferential plastic strain may accumulate with the increase of the number of cycles. This is a thermal ratchetting problem induced by a liquid surface moving in a cylinder, and it is important especially in designing fast breeder reactors. In the present paper, the effect of kinematic hardening rule on the thermal ratchetting analysis is discussed by implementing the following four kinds of kinematic hardening rules in a finite element analysis; the perfectly plastic model (PP), the linear kinematic hardening rule (LKH), the classical nonlinear kinematic hardening rule of Armstrong and Frederick (AF), and the rule proposed recently by Ohno and Wang (OW). It is shown that disregard of transient hardening after yieding leads to overstimating the thermal ratchetting, that a rule predicting larger mechanical rathcetting under uniaxial cyclic loading makes the thermal ratchetting more serious, and that the Ohno and Wang rule can render the analysis most realistic among them.
Journal of the Society of Materials Science, Japan, 2008
Stress distribution and the residual stress in the solidified materials are investigated by the n... more Stress distribution and the residual stress in the solidified materials are investigated by the numerical analyses based on the phase field method. The governing equations considering the coupling effects among phase transformation, temperature and stress/strain, including elasto-plastic constitutive relation, are formulated using the phase field model, and they are numerically solved by the finite element method. As a typical case of the microstructure formation, the dendritic growth processes are studied. In spite of several assumptions employed for the material properties of liquid, the complicated stress distributions in dendrites are exhibited : Strong stresses are distributed at the bottom of the side branches. Due to the plastic behavior, residual stress distributions are obtained when the solidification and cooling are completed in the whole region. When a microstructure constructed with two dendrites is formed, high stresses are generated in the regions where the liquid is remained till the very last stage of the solidification. Subsequently, two types of the morphology of microstructures are investigated, and it reveals that the residual stress distribution and the maximum value are strongly affected by the microstructure.
International Journal of Solids and Structures, 2007
In this study, a method for three-dimensional microscopic interlaminar analysis of cross-ply lami... more In this study, a method for three-dimensional microscopic interlaminar analysis of cross-ply laminates is developed based on a homogenization theory to analyze microscopic interactions between unidirectional long fiber-reinforced laminae. For this, a unit cell of a cross-ply laminate, which includes interlaminar areas, is defined under the assumption that each lamina in the laminate has a transversely square fiber array. Then, showing that the laminate has a point-symmetric internal structure, the symmetry is utilized to introduce half of the unit cell as the domain of analysis. Moreover, the domain of analysis is divided into substructures using a substructure method combined with the homogenization theory, significantly reducing the computational costs. The present method is then applied to the analysis of interlaminar stress distributions in a carbon fiber/epoxy cross-ply laminate subjected to in-plane uniaxial tension. It is shown that microscopic shear stress noticeably occurs at the interface between the 0°-and 90°-plies. It is also shown that the microscopic interaction between the two plies is observed only in the vicinity of the interface.
TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICAL ENGINEERS Series A, 2013
This paper describes the homogenization of fin layers for structural analysis of heat exchanger c... more This paper describes the homogenization of fin layers for structural analysis of heat exchanger cores composed of flat tubes and wavy fins. It is assumed that uniform deformation prevails at the bonded interfaces between fins and tubes, and also that the fins have the Y-periodicity in the fin layers. The homogenization method for 3D periodic solids is then shown to be applicable to the homogenization of fin layers, provided that the uniform deformation condition is imposed at the bonded interfaces between fins and tubes. The resulting homogenization method is applied to designed and real shapes of outer and inner fins in an intercooler. The evaluated, homogenized elastic stiffnesses of fin layers are verified by performing finite element analyses using full-scale and fin-homogenized models of tube-fin connected pieces subjected to uniaxial compression. An experiment of a tube-fin laminated block under uniaxial compression is further performed to demonstrate that the real shapes of outer and inner fins are definitely important for the homogenization of fin layers.
Journal of the Atomic Energy Society of Japan / Atomic Energy Society of Japan, 1991
Journal of Solid Mechanics and Materials Engineering, 2008
In this study, a linearization method is used to develop an implicit integration scheme for a cla... more In this study, a linearization method is used to develop an implicit integration scheme for a class of high-temperature inelastic constitutive models based on non-linear kinematic hardening. A non-unified model is first considered in which the inelastic strain rate is divided into transient and steady parts driven, respectively, by effective stress and applied stress. By discretizing the constitutive relations using the backward Euler method, and by linearizing the resulting discretized relations, a tensor equation is derived to iteratively achieve the implicit integration of constitutive variables. The implicit integration scheme developed is shown to be applicable to a unified constitutive model in which back stress evolves due to static and dynamic recoveries in addition to strain hardening. The integration scheme is then programmed for a subroutine in a finite element code and applied to a lead-free solder joint analysis. It is demonstrated that the integration scheme affords quadratic convergence in the iterations even for considerably large increments, and that the non-unified and unified models give almost the same results in the solder joint analysis.
Journal of the Society of Materials Science, Japan, 2008
The axial stresses of the fiber in single carbon fiber/epoxy resin model composites under change ... more The axial stresses of the fiber in single carbon fiber/epoxy resin model composites under change of surroundings, e.g., temperature and water, have been measured by micro-Raman spectroscopy. The specimens were soaked in hot air, water and hot water, and returned to the beginning state to cause cyclic changes of temperature and water absorption : (1) heating and cooling at atmospheric environment, (2) immersing in the water of the room temperature and drying, and (3) immersing in the hot water of 80 and drying at the room temperature. In the case of (1) and (2), the fiber axial stresses after the cyclic change of the surroundings recover almost the same before the heating or the immersion. On the other hand, the fiber axial stress after the drying is more compressive than that before the hygrothermal exposure in the case of (3). Hygrothermal effects on the residual stress are discussed based on fiber stresses caused at elevated temperature over the glass transition temperature of the epoxy resin.
Journal of the Society of Materials Science, Japan, 2005
In this study, we develop a homogenization theory and its finite element discretization based on ... more In this study, we develop a homogenization theory and its finite element discretization based on the nonlocal crystal plasticity theory of Gurtin (J. Mech. Phys. Solids 48 (2000) ; J. Mech. Phys. Solids 50 (2002)), which includes slip gradients and additional slip boundary conditions with respect to microforce balances for each slip system. Although it is difficult to determine slip boundary conditions, the development of the homogenization theory for the nonlocal crystal plasticity makes it possible to obtain the unified treatment of slip boundary conditions on the periodic boundary in periodic materials. Furthermore, by applying a tangent modulus method to the finite element discretization of rate-dependent small deformation, a strong coupling boundary value problem is derived from homogenization equations which consist of the stress balance, the microforce balances and the macroscopic relation. Finally, the effectiveness of the proposed theory and method is confirmed through finite element analyses of a double slip single crystal model with periodic obstacles.
Journal of Solid Mechanics and Materials Engineering, 2010
In this study, we examine the variation in residual stresses in polymer matrix composites with cy... more In this study, we examine the variation in residual stresses in polymer matrix composites with cyclic changes in hygrothermal environments by measuring the axial stresses in the fiber of single-carbon fiber/epoxy resin model composites using Raman microspectroscopy. To change the temperature and water absorption in the specimens separately and simultaneously, the specimens were (1) heated and cooled at atmospheric environment, (2) immersed in water and dried at room temperature, and (3) immersed in hot water (80°C) and cooled to room temperature in water and then dried. All specimens were thus restored to their initial states and then evaluated. In the cases of (1) and (2), the fiber stresses reverted to almost the same as their initial states, but in the case of (3) the axial stress after hygrothermal cyclic change was more compressive compared to the initial state. The differences among the results of the three cases are discussed based on the fiber stresses at elevated temperature, which were evaluated by Raman microspectroscopic measurement performed at temperatures ranging from room temperature to those above the glass-transition temperature of the epoxy matrix resin.
Journal of the Society of Materials Science, Japan, 2008
Journal of the Society of Materials Science, Japan, 2004
Journal of the Society of Materials Science, Japan, 2010
In this study, we develop an extended version of homogenization theories for composite materials ... more In this study, we develop an extended version of homogenization theories for composite materials consisting of inclusions and a matrix. The extension allows discrete dislocations to move in a matrix based on dislocation dynamics. Macroscopic stress-strain response and microscopic stress distribution in representative volume elements (RVEs) are analyzed on the assumption that the periodic arrangement of RVEs is subjected to macroscopic uniform deformation. The periodicity, which enforces the periodic distributions of microscopic stress and strain as well as dislocations, is used as boundary conditions on the surface of RVEs. It is shown that elastic fields of periodically arranged dislocations in an infinite medium have no contribution to macroscopic relations, and that discontinuous displacements resulting from dislocation motions in each RVE are introduced as a macroscopic plastic strain. From these relations, a set of homogenization equations is derived, in which two boundary value problems in integral form are solved for analyzing perturbed displacements due to the presence of inclusions and due to the interaction of inclusions with dislocations, respectively. Finally, the influence of periodic boundary conditions on macroscopic and microscopic responses is investigated by performing the RVE analysis of a model composite.
Transactions of the JSME (in Japanese), 2015
In this study, the thermal fatigue life of substrate with Cu through-hole is evaluated by conside... more In this study, the thermal fatigue life of substrate with Cu through-hole is evaluated by considering the mechanical properties of Cu thin film and glass fiber cloths structure. We first conducted tensile tests of Cu thin film and found that the rate-dependence of inelastic property varies abruptly with temperature. An inelastic constitutive model for Cu thin film is then proposed by combining both rate-independent and rate-dependent models. The proposed inelastic constitutive model is introduced in a Finite Element Method based analysis of glass epoxy substrate with Cu through-hole. Moreover, low cycle tests of Cu thin film are carried out by using repeated 4-point bending to evaluate its isothermal fatigue properties. Through our analysis we verified the capability of the proposed model to predict thermal fatigue life of Cu through-hole using the isothermal fatigue properties. The results show that the glass fiber cloths structure of the substrate needs to be considered in order to successfully predict the thermal fatigue life of the Cu through-hole.
Journal of Nanomaterials, 2014
The effects of wavenumber and chirality on the axial compressive behavior and properties of wavy ... more The effects of wavenumber and chirality on the axial compressive behavior and properties of wavy carbon nanotubes (CNTs) with multiple Stone-Wales defects are investigated using molecular mechanics simulations with the adaptive intermolecular reactive empirical bond-order potential. The wavy CNTs are assumed to be point-symmetric with respect to their axial centers. It is found that the wavy CNT models, respectively, exhibit a buckling point and long wavelength buckling mode regardless of the wavenumbers and chiralities examined. It is also found that the wavy CNTs have nearly the same buckling stresses as their pristine straight counterparts.
Journal of the Society of Materials Science, Japan, 1997
The paper deals with estimation of thermal ratchetting strain in cylinders subjected to short axi... more The paper deals with estimation of thermal ratchetting strain in cylinders subjected to short axial travelling of temperature distribution. The estimation is made by considering partly pressurized cylinders as a model. It is assumed that the pressurized region, which is taken to occupy the temperature travel region, is ruled by the deformation theory with a constant secant modulus so as to estimate the thermal ratchetting strain conservatively. This assumption enables us to utilize a linear elastic solution for thin-walled cylinders. The secant modulus is specified using the analytical solution derived in the long travel case by assuming Masing's rule and no cyclic hardening. It is shown that the model gives good estimates to finite element analysis of the thermal ratchetting strain as well as to experiments on 316FR steel cylinders done by Kitade et al. An extension of the model, in which plastic yielding outside the travel region is taken into account, is also discussed.
Physical Review B, 2010
Electronic structures of ͑6,0͒, ͑8,0͒, and ͑10,0͒ single-walled boron nitride nanotubes ͑SWBNNTs͒... more Electronic structures of ͑6,0͒, ͑8,0͒, and ͑10,0͒ single-walled boron nitride nanotubes ͑SWBNNTs͒ subjected to tension, torsion, and flattening are investigated using first-principles calculations. Energy bands and charge distributions of the SWBNNTs are calculated within the density-functional theory and forces required to deform the SWBNNTs are estimated using energy variation with deformation. Our calculations show that all the deformation modes decrease the energy gaps of the SWBNNTs because of a decrease in the conductionband minimum ͑CBM͒ energy, which is caused by an overlap of CBM charge densities between circumferentially neighboring boron atoms. It is found that flattening with a force smaller than that applied for tension or torsion causes a larger decrease in energy gaps of the SWBNNTs and that the force required for flattening SWBNNTs is not unrealistic.
Modelling and Simulation in Materials Science and Engineering, 2009
Shape-memory behaviour in multi-grain material is simulated using a molecular dynamics method. An... more Shape-memory behaviour in multi-grain material is simulated using a molecular dynamics method. An embedded-atom-method potential for NiAl alloy is applied, and a sequence of conditions including loading, unloading, heating and cooling is imposed. Two types of grain arrangement are used, and the deformation and shape recovery due to phase transformation are observed for both models. The stress-strain relation is revealed to draw a hysteresis loop, and the individual curves are smoother than those previously obtained from a single-crystal model. The deformation mechanism during loading is discussed using local structure analysis. Local deformation is initiated at the grain boundaries, and the deformed region propagates along the twin plane in the grain. The propagation is then obstructed by the grain boundaries, and a band pattern of the deformed area is formed. The influence of the grain shape and distribution, as well as the crystal orientation of each grain, on the deformation behaviour is also investigated. Qualitatively common features in the deformation mechanism and stress-strain relation are observed despite different grain distributions, while the critical values in stress vary, owing to the crystal orientations of the grains.
MATERIALS TRANSACTIONS, 2007
A computational multiple scattering simulation method was applied to analyze the characteristics ... more A computational multiple scattering simulation method was applied to analyze the characteristics of the ultrasonic shear wave that propagates in unidirectional carbon-fiber-reinforced epoxy composites with its polarization direction parallel to the fibers. The numerical simulations were carried out for regular as well as random fiber arrangements and for different fiber volume fractions. The results were combined with the one-dimensional theory describing the macroscopic propagation behavior, in order to identify the phase velocity and the attenuation coefficient of the composite. The phase velocity and the attenuation coefficient were found to depend significantly on the fiber volume fraction, but less so on the fiber arrangement in the frequency range examined here. Furthermore, the present analysis showed a good agreement with the experimental data.
JSME International Journal Series A, 2005
As a background of the ultrasonic characterization of contact interfaces, ultrasonic wave propaga... more As a background of the ultrasonic characterization of contact interfaces, ultrasonic wave propagation characteristics at contacting solid surfaces have been studied numerically and experimentally. The wave propagation along the contact interface has been simulated by the two-dimensional finite difference method, assuming a spring-type interface with normal and tangential stiffnesses. The analysis has verified that the antisymmetric-mode interface wave can be generated by the shear excitation at one end of the interface, and its phase velocity can be evaluated with good accuracy from the waveform received at the other end. Measurement of the phase velocity of the interface wave has been carried out for contacting PMMA surfaces under different contact pressures, together with the shear-wave reflection measurement to evaluate the interface tangential stiffness. The measured relation between the phase velocity and the tangential stiffness has been shown to agree well with the theoretical relation for the antisymmetric-mode interface wave.
JSME International Journal Series A, 2006
Molecular dynamics simulations of the shape-memory effect are carried out to investigate the atom... more Molecular dynamics simulations of the shape-memory effect are carried out to investigate the atomistic behavior during deformation and shape-recovery processes. The embedded-atom-method potential function and parameters for Ni-Al alloy are applied. The initial configurations of atoms are set on the lattice points of the martensite structure, in which the distribution of the variant orientation is limited to the two-dimensional direction for simplicity. When the shear load is imposed toward the x direction, parallel to the variant interface, the deformation of the variants occurs, and finally, all variants settle into the uniform orientation. The deformed state is maintained after the load is released, and the original shape is recovered through heating and cooling processes because of phase transformation to bcc and martensite. In the loading process, the stress-strain curve exhibits a zigzag shape consisting of repeated stress increase and abrupt release. The interval of the stress peaks is revealed to be smaller as the model size becomes larger. Deformation observed in variant layers seems to occur at the same time at every points in the layer for a small model. However, the simulation with a large model indicates a nucleation and propagation behavior in each layer.