A ball indenter was impressed on low index planes in pure Mg, Mg-0.5at%Zn alloy, Mg-0.5at%Al alloy, Mg-0.5at.%Y alloy, Mg-0.9at.%Y alloy single crystals and pure Zn single crystals, and the formations of the indentations were investigated. In pure Mg and Mg alloy single crystals, indentation forms isotropic round shape when indented on (0001). The indentations elongated to [0001] were observed when indented on (1100) and (1120). Basal slip and (1012) twin were observed around the indentations. The obtained results were discussed with the critical resolve shear stress: CRSS of basal slip and second ordered prismatic slips and twins. The indentation size depends on a CRSS of basal slip indented on (0001) and on both activity of basal slips and twinning indented on (1100) and (1120).

View full abstract

To understand deformation mechanisms of non-basal slip in magnesium, [1010] tensile tests were carried on pure magnesium single crystals in various temperatures. While the crystal fractured as brittle manner at room temperature, the crystals were deformed by {1011}<1123> first order pyramidal slip and showed ductility above room temperature. Yield stress due to the first order pyramidal slip showed high temperature dependence. These results indicate that high ductility of magnesium at high temperature would be caused by activation of first order pyramidal slip.

View full abstract

A large number of tiny particles hit the specimen to improve the mechanical properties of treated materials in Fine Particle Peening (FPP) process. The induced surface modification is influenced by many parameters such as material properties, particle size, particle amount, air pressure and so on. Especially, it is well known that particle velocity significantly influences the results of surface modification. Thus, we focused on the particle velocity and measured it by a high-speed camera. Also, we developed the model of particle motion by considering the effect of jet flow and calculated the particle velocity after spouting from the nozzle. Then, we compared the experimental results with the calculated results and those agreed well each other. It is concluded that the particle velocity can be estimated by our proposed model and it is helpful information when one decides the process condition on FPP treatment.

View full abstract

In the Persson's contact mechanics theory, the friction model of a sliding rubber has been proposed, accounting for the surface property on multiscale asperities; the friction coefficient which is equivalent to hysteresis loss is mathematically derived by the rubber's viscoelasticity and the surface roughness power spectrum. In this study, we conduct friction tests under two types of normal loads. We compare the friction coefficients calculated from the Persson's theory with those of experiments so that the hysteresis friction is discussed as a factor of the friction mechanism based on the sliding velocity dependence of the friction coefficients.

View full abstract

Micropolar theory, one of the generalized continuum mechanics theories, is characterized by rotational degrees of freedom φ, which are independent from displacepent u. In this study, we apply micropolar theory to deformation analysis of a elastic body which has internal structures. The micropolar model and the classical model considering internal structures explicitly is simulated to examine the appropriateness of the micropolar model. Relationship of characteristic length, the new elastic constant introduced micropolar theory, and the scale of internal structures is illustrated.

View full abstract

Graphene has exhibited great potential for the application of next-generation electronic devices. However, undesirable interface stress induced during graphene transfer and device fabrication process causes the deformation of graphene, resulting in the change in its electronic properties. In this study, the effect of three-dimensional mechanical loading such as tensile, bending and folding deformations on the electronic states of armchair graphene nano-ribbons (AGNRs) is investigated based on density functional theory (DFT) calculation. It was found that the electronic structure of AGNRs shows differential sensitivity to various deformation modes. The effect of uniaxial tension on the electronic conductivity of AGNRs was also analyzed using quantum transport simulation and it was found that the uniaxial tensile strain changes the band gap of AGNRs, resulting in the change in the electronic conductivity. The current-strain (I-ε) relationship of AGNRs strongly depends on the length of the strained area. This understanding paves the way for the development of highly sensitive GNRs strain sensors.

View full abstract

Since carbon nanotubes (CNTs) have attractive electronic and mechanical properties, there have been many efforts to develop CNTs-based electronic devices and sensors. The authors have also validated the possibility of a highly sensitive strain sensor using vertically aligned multi-walled CNTs (MWNTs). However, the strain sensitivity (gauge factor) of our developed strain sensor was fluctuated largely from 0.3 to 100. Therefore, the effect of strain on their electronic properties was analyzed by the combination of molecular dynamics and the first principles calculation. In this study, the effect of radial strain on them was investigated in detail. It was found that the space distribution of state density of CNTs was localized at high-curvature region when a radial strain was applied. In addition, the localization of a state density decreased its state energy and increased the current through the CNT.

View full abstract

Safety margin for fatigue life is a critical factor in designing machinery in the appropriate quality. To determine the safety margin rationally, estimation of the fatigue life is very important. Surface finish is one of the factors that affects the fatigue life. In this research, we try to clarify the influence of the surface finish, especially on the surface damage on low cycle fatigue life. We prepared several Inconel 718 specimens with different surface-finishing conditions and obtained low cycle fatigue life. Also we obtained Local Misorientation Parameters (LMPs) using EBSD observation and measure the residual stress.

View full abstract

Molecular dynamics (MD) study on copper sulfide (Cu_2S) crystal which is subjected to sliding condition is performed. The hexagonal crystal structure of Cu_2S is determined by first-principle calculation, first. Values on the optimized structure are adopted to the construction the potential function which includes three-body term as well as pair term, resembling MoS_2 case. In MD simulations, a Cu_2S crystal undergoes relaxation and compression processes at first, then it is applied shear deformation and is slid on the hexagonal basal plane. The dependency on sliding direction with regard to hexagonal slip system and on sliding velocity is investigated. It is found that layers of Cu_2S slide without retaining their prescribed crystal stacking and shape. Under various temperature conditions, the sliding of the crystal is simulated and the shear resistance (averaged shear stress) shows a certain temperature-dependency, indicating stick-slip motion in some cases.

View full abstract

In order to make clear the mechanism of the directional coarsening (rafting) of y' phases in Ni-base superalloys under uni-axial tensile strain, molecular dynamics (MD) analysis was applied to investigate dominant factors of strain-induced anisotropic diffusion of Al atoms. The diffusion constant of Al atoms was changed drastically by the dopant elements and their contents. When the lattice constant of the γ phase was increased and its melting point was decreased by the addition of Cr or Al atoms, the strain-induced anisotropic diffusion of Al atoms in the γ' phase was accelerated. On the other hand, the addition of Co decreased the diffusion significantly. Therefore, the changes of lattice constant and melting point depending on the chemical composition of the γ/γ' interface are the dominant factors controlling the strain-induced anisotropic diffusion of Al atoms in the Ni-base superalloy.

View full abstract

We analyze lubricant viscosity using the non-equilibrium molecular dynamics simulation. We construct the analysis technique of the viscosity of the polyolester oil by fitting the velocity function of the Poiseuille flow. In order to investigate the structural dependence of viscosity, two constrained models are introduced. Their molecular potentials are artificially changed. They show obviously different viscosity from that of the original molecular model. We consider this effect comes from the characteristic structure of the lubricant molecule. Especially, we found that structure relates to the core-like structure formed by the active groups in the polyolester oil.

View full abstract

In this paper the loosening and coming out of a roller is considered by using FEM analysis when a ceramics sleeve is shrinking fitted to two steel shafts. It should be noted that only small shrink fitting ratio can be applied for the connection because of the brittleness of ceramics. However care should be taken for coming out of the shafts during rotation under such small shrink fitting ratio. In this study therefore the finite element analysis is applied to simulate this behavior. Then, the coming out behavior during rotation can be realized by the simulation where the rotation of the roller is replaced by the shift of the load at an interval of the rotation angle. Under smaller shrink fitting ratio the shaft comes out, but under larger shrink fitting ratio the shaft does not. The effects of the magnitude of the load, friction coefficient, and stiffness of the shaft are also discussed.

View full abstract

The ΔK value is calculated by using the local stress distribution at contact edge and 'intrinsic crack model'. Fretting fatigue initiation and limit is determined by theΔK_<eff, th> andΔK_<th> of the material. Fretting fatigue occur in compression stress field due to the surface pressure. However, ΔK_<th> in the compression stress field is unclear. Therefore Organize fretting fatigue data using this concept, to determine the mean stress effect of ΔK_<th>. By using this evaluation method, it was possible to estimate with an accuracy of ±20% for fretting fatigue limit.

View full abstract

In this study, fatigue tests of aluminum alloy bolts have been conducted to investigate the influence of clamping force of the bolted joint on fatigue characteristics. The initial clamping force was set at 2 kN, 4 kN and 6kN. The results showed that the fatigue limits decreased with an increase in clamping force. The results indicate that it was not suitable for aluminum bolts to be tightened with a higher clamping force although the steel bolt has been recommended to be tightened with a higher clamping force.

View full abstract

Welding is widely used to connect dissimilar materials, but an intermetallic compound produced in the interface is concerned to decrease the joint strength. It is important to evaluate the strength of welded joint for designing machine and structure. In this study, single lap specimens with welding joint of Fe and Al plates were prepared for a tensile shear test and a fatigue test. Influence of rotation caused by tension and distribution of interfacial strength were investigated by modifying specimen length, number of tabs, and shape of welding joint. It was found that the rotation did not have significant influence on interfacial strength, but fracture behavior varied and indicated uneven strength distribution along interface.

View full abstract

Dissimilar materials lap joint between PET and SUS304 was carried out with fiber laser. Bubbles formed in PET nearby the interface significantly affected joining strength. Size of the bubble changed by applying static and cyclic contact force. Effect of bubbles on joining strength was well evaluated by the maximum stress occurred at edge of a bubble. Reliability of strength in plastic/metal dissimilar materials joint can be improved by applying evaluation method for effect of bubbles and controlling method for formation behavior of bubbles proposed in the present study.

View full abstract

In this paper, the crack tip stress method (CTSM), which is the method for accurately calculating the stress intensity factors based on the usefulness of the stress values at a crack tip calculated by the finite element method (FEM), is extended to the tensile problems of the bi-material cylinder. In order to investigate the differences of the stress states between the bi-material cylinder and the bi-material plate, FEM analyses are performed on them under the same material combinations and the same mesh patterns. The asymptotic solution of the cylinder is derived from the FEM analysis results, and the boundary conditions such as the traction-free surface and the perfectly boned interface. Analyses of the intensity of the singular stress field are performed on the various bi-materials which have the same Dundurs' parameter by using the bi-material plate subjected to the tensile stress under the plane strain as the reference model.

View full abstract

This paper deals with the effect of adhesion layer on the stress intensity factor for small edge interface crack in bonded plates under uniaxial tension. In this study, the stress intensity factors of the small edge interface crack are analyzed by the crack tip stress method with varying the crack length, adhesive thickness and material combination. This method is based on the fact that the singular stress field near the interface crack tip is controlled by the stress values at the crack-tip-node calculated by FEM. The calculation shows that the normalized stress intensity factors of the small edge interface crack under uniaxial tension are related to the ratio between the crack length and the adhesive thickness (a/h) and the singularity at the interface edge without the crack λ. Then, it is found that the normalized stress intensity factors can be expressed as F_<1,2>=C_<1,2>(h/a)^<1-λ> and the parameters C_1 and C_2 are constant values when a/h<10^<-3>.

View full abstract

Advanced electronic device packaging has several kinds of joint structures of metal, ceramic and polymer. It is well known that the stress singularity occurs at the cross point and line of free surface and interface. Then, a crack initiates from the vertex and joints fail. Hence, a lot of studies on the joints have been carried out theoretically and experimentally. In the previous study, two-dimensional joints are extensively investigated, however, few studies on three-dimensional joint structures have been carried out until now. For instance, CSP has a three-dimensional bonded structure of IC chip and polymers, and delamination occurs frequently at an interface between IC and a resin. Therefore, a method for estimating the strength of interface at the vertex based on the three dimensional intensity of singularity is needed. In the present paper, energy release rate at an initial step of crack growth for four kinds of crack area in a three-dimensional joint is calculated and discussed..

View full abstract

In this study, singular stress at corners in an anisotropic two-dimensional joint structure under a tensile loading is analyzed using molecular dynamic (MD) method and the anisotropic elasticity theory. The analysis of stress fields near interface misfit dislocation in a two-phase anisotropic body considering interface stress and interface elasticity. The displacement and stress distributions at the interface are compared the analysis considering the interface properties and the MD method.

View full abstract

In this paper, failure evolution of thermal barrier (TBC) coated specimen relating to mechanical behaviors of the bond coat material under various temperatures and loading conditions will be discussed. As the first phase, monotonic tensile tests were carried out on bond coat material in the temperature range of 25 to 1000℃. Special emphasis put on establishing the ductile to brittle transition temperature (DBTT) of the bond coat. As the second phase, isothermal low cycle (IT-LCF) fatigue and thermo-mechanical (TMF) fatigue tests were carried out on thermal barrier coated ni-based super alloy in the temperature range of 500 to 900℃. After the testing, the specimens were sectioned to understand their failure mechanisms. Main importantly it has been observed that the number of cracks in the bond coat significantly changes by the test condition. This particular behavior may lead to a specific failure to the TBC specimen

View full abstract

The use of thermal barrier coatings (TBCs) for turbine blades in gas turbine engines has allowed higher engine operating temperatures exceeding the order of 1200 ℃ at the surface of the TBC top coat. More recently, it has been recognized that at these high temperatures the TBCs can be damaged by calcium-magnesium-alumino-silicates (CMAS) resulting from the ingestion of siliceous minerals (dust, sand, ash) with the intake air and from unclean fuels such as a syngas and biomass gas. The damage can accelerate the cracking and delamination of the TBCs and dramatically decrease its lifetime. In the present study, the CMAS damage was simulated in laboratory employing a synthetic CMAS product on Air Plasma sprayed (APS) and Electron Beam Physical Vapour Deposition (EB-PVD) thermal barrier coatings. The changes in microstructure were characterized, and the effect of the top coat spraying process on CMAS damage was discussed.

View full abstract

The purpose of this study was to understand the effects of thermal exposure on residual stress in thermal grown oxide (TGO) on CoNiCrAlY coatings, which were fabricated by air plasma spraying (APS), high velocity oxygen fuel spraying (HVOF) and low pressure plasma spraying (LPPS) on a nickel base alloy, respectively. The thermal exposure tests were conducted in air at 1000 ℃ and 1100 ℃ for up to 1000 hours using an electric furnace. X-Ray Diffraction (XRD) shows the presence of the γ phase and β phase in the coating. The residual stresses in the TGO layer were measured by photo-stimulated luminescence spectrum. It was shown that the residual stress in the TGO layer on APS coating was the highest and on LPPS coating was the lowest. The residual stress in the TGO layer decreased with an increase in thermal exposure time up to 300h.

View full abstract

The strength of thermal barrier coating (TBC) is usually evaluated from a free-standing coating because it is difficult to detect the damage from a TBC system which consists of a TBC, a bond coat and a substrate. In this study, evaluation method of the damage stress in TBC using change in the stress share of substrate was developed. Four point bending was applied to a TBC system specimen. The applied force and the substrate strain were measured. The substrate stress of a TBC system specimen became high when the TBC was damaged, because apparent Young's modulus of the TBC decreased. Therefore, the stress of TBC damage can be detected by comparing the substrate strain of a TBC system specimen with that of a bare substrate specimen. It was found that the evaluated damage stress agreed with that of a free standing TBC.

View full abstract

The wear performance between Partially Stabilized Zirconia (PSZ) plate and silicon nitride ball under dry conditions was investigated. The wear durability of PSZ with shot peening was higher than that of PSZ without shot peening in the sliding wear test under low Hertzian contact pressure. Due to shot peening, the transformation from tetragonal to monocryclinic phase with compressive residual stress occurred locally on the PSZ surface, and the wear durability on the surface of the PSZ plate was improved.

View full abstract

In this study, capability and reliability of advanced repairing technology based upon thermal spraying technique were examined. In order to achieve this purpose, the damage evolution progressed in the repaired part was clarified through continuous observation under tensile loading, and residual stress and the critical strain up to the delamination in the part were identified. For the tensile testing, the repairing geometry was assumed to be five types of the length under the same depth. As the results, residual stress increases to compressive side with the normalized repaired length, and the critical strain also increases with the length, which means that it is desirable to remove a large region from the damage part in the structure. It can be claimed that APS repairing technique has great capability in place of a traditional welding approach.

View full abstract

Fatigue strength of high speed tool steel, JIS SKH51, coated with TiAlN by AIP (Arc Ion Plating) and UBMS (Unbalanced Magnetron Sputtering) processing were investigated under four point bending fatigue tests. From the experimental results, Static bending strength of the coated steel by UBMS was same as that of the substrate. However the coated steel by UBMS has higher fatigue strength than the substrate over a range of number of cycles from 10^5 to 10^7. On the other hand, Static bending strength of the coated steel by AIP was lower than that of the substrate, and the coated steel by AIP has higher fatigue strength than that by UBMS. As a result of SEM observation, the difference of strength of coated steels by AIP and UBMS was explained by adhesion between the coating film and substrate.

View full abstract

This study aims to evaluate adhesion quality of oxide scale with a laser spallation technique. Adhesive strength was attempted to measure at room temperature and up to 200 ℃. The exfoliation can be detected by correlation coefficient calculated with a series of the waveform obtained at epicenter of laser irradiation. The adhesive strength was estimated with stress distribution from wave propagation calculated in the sample for taking an effect of reflection at the sample surface and the interface into account.

View full abstract

A new test method is proposed in this study to determine the apparent interfacial toughness of brittle thin films on substrate by using of the delamination area by indentation. The equivalent length of delamination crack is defined from the delamination area by indentation. The experimental results revealed that the ratio of the equivalent length of delamination crack and the radius of impression is related to the interfacial strength of thin films and independent with the indentation load. The equation to evaluate the apparent interfacial toughness of thin films from the ratio of the equivalent length of delamination crack and the radius of impression is proposed in this study based on the results of finite element analysis.

View full abstract

Al alloy has been used in several kinds of industries against a background of environmental problem and energy saving .However, Al alloy has poor wear-resistance, so in order to solve this problem, Diamond Like Carbon(DLC) coating which indicates high tribological properties was performed. In this paper, the effect of thin hard coatings on the tribological and fatigue strength of Al alloy was investigated. The coatings were DLC and Aluminum nitride (AlN) with Unbalanced Magnetron Sputtering (UBMS) method. Wear tests and fatigue tests were carried out. Al alloy coated DLC and AlN indicated high fatigue strength compared with Al alloy coated with AlN and substrate only.

View full abstract

In recent years, Aluminum alloy has been widely used because of their lightweight and easy adaptability for recycling. However, aluminum alloy has poor wear resistance. Therefore, it is necessary to improve this property to use aluminum alloys for machine parts. Aluminum chromium nitride (AlCrN) coating and Aluminum nitride (AlN) coating, which is high hardness and excellent wear resistance was performed. In this study presents an investigation of the influence of the tribological of hardness control AlCrN and AN films on A7075 alloy substrates using Unbalanced Magnetron Sputtering (UBMS).

View full abstract

This paper deals with investigation of fracture toughness of FGM consisting of partially stabilized zirconia(PSZ) and titanium(Ti) fabricated by spark plasma sintering(SPS). In the PSZ-Ti FGM, PSZ and Ti are placed on a surface and substrate, respectively, and then a PSZ content is reduced by 10-20% from 100% at the surface to 0% at the substrate. Specimen surface is observed with an optical microscope to investigate sintering condition of the specimen. After that, micro-Vickers and Vickers hardness tests are performed to determine distributions of Vickers hardness and fracture toughness, respectively. The dense compact of PSZ-Ti FGM is obtained by SPS because any large voids and defects are not observed and the relative density of the FGM is about 95%. The Vickers hardness increases with increasing PSZ volume fraction. The fracture toughness, however, does not improve although Ti volume fraction is increased.

View full abstract

A 0.4C-2Si-1 Cr-1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45° and 90° rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.

View full abstract

Strain gages to measure only the stress intensity factor are only one or two kinds so far. They are not easy nor practical to analyze the stress intensity factor. Therefore, this paper reports on the development of strain gages dedicated to analyze the stress intensity factor easily and practically. The authors have made the strain gages for trial purpose. The strain gage was pasted to the crack tip of the plate tension specimens, having the opening mode crack. The authors carried out the tensile tests of the test pieces that pasted such strain gauges. As results, the experimental values of the opening mode stress intensity factors to be able to count by calculator were obtained within ±10% compared with analytical values.

View full abstract

Strain gages to measure only the stress intensity factor are only one or two kinds so far. They are not easy nor practical to analyze the stress intensity factor. Therefore, the first paper reports on the development of strain gages dedicated to analyze the opening mode stress intensity factor easily and practically. The authors have made the strain gages for trial purpose. The strain gage was pasted to the crack tip of the plate tension specimens, having the mixed mode crack. The authors carried out the tensile tests of the test pieces that pasted such strain gages. As results, the experimental values of the mixed mode stress intensity factors to be able to count by calculator were obtained within ±10% compared with analytical values.

View full abstract

Solder joints in electronic packaging are being miniaturized year by year. The strength of such a miniaturized solder joint is affected by the presence of Cu/Sn intermetallic compounds (IMCs) generated at the interface between solder and copper wiring. Then, the strength reliability of miniaturized solder joint should be evaluated by the test using a solder joint specimen which contains the IMCs layer. In this study, a joint specimen was prepared by soldering two copper plates with Sn-3.0Ag-0.5Cu (SAC) solder. The IMCs layer was observed between the copper and the SAC solder in the joint part. Using the specimen, we conducted shear tests under four different strain rates. The results showed that the strain rate affected both the stress-strain relation and the cracking tendency of the specimen. Namely, the stress level of the stress-strain relation increased with increase in the strain rate. The cracking in the IMCs layer occurred in smaller deformation stage with increase in the strain rate.

View full abstract

The solder joint in electronic packages is increasingly miniaturized to achieve the high-density mounting of electronic parts. Such a miniaturized solder joint is thought to have different deformation characteristics from bulk solder specimens. Therefore, the finite element analysis for evaluating the strength reliability of the miniaturized solder joint should employ the material parameters estimated by conducting the test with miniature solder specimens. In addition, the material parameters must be estimated in a short period because the development period for electronic devices tends to become shorter. One of the authors has proposed an experimental method termed tension-strain maintenance test which can estimate the material parameters for viscoplastic material such as solder in a short time. Then, in this work, by applying the method to a miniature solder specimen, we estimated the material parameters which represent the elasto-plastic and creep characteristics of the specimen. Using a constitutive model with the estimated parameters, some simulations of the tensile tests with the miniature solder specimens were conducted to verify the availability of the estimation method.

View full abstract

To evaluate the strength reliability of electronic substrate by the accurate structural analysis, the method to evaluate the creep deformation of in-situ solder joints must be developed. The Indentation creep test is one of effective method to evaluate the creep deformation in microscopic specimen. It is required that the result obtained by the indentation creep test is should be expressed by the constitutive equation for the creep deformation under the uniaxial stress evaluated by the tensile creep test In this paper, the indentation creep test at IN loading for 9000s duration is conducted by experiment and the result obtained by the indentation creep test is compared with that obtained by the tensile creep test using the bulk specimen. As the result, the result obtained by the indentation creep test did not coincide with that obtained by the tensile creep test To explain the difference, the numerical indentation creep test is conducted. Finally, the method to determine the suitable reference area for calculating uniaxial stress of the indentation test is proposed. The reference area determined by the new indentation test is the effective to obtain the reasonable creep curves by the indentation test.

View full abstract

Aiming at understanding the thermal deformation characteristics of two solar cell configurations, finite element thermal stress analysis is carried out in this investigation. This study covers the manufacturing process and operating conditions including three thermal cycles of different environments in order to determine the long term effects of residual stresses. While the first solar cell model is tabbed by lead-free solder, the second model by conductive film (CF). A high temperature soldering process causes stress of the solar cell due to the different thermal expansion coefficients of materials; stress could weaken the bond and reduce reliability of the cell. Also when a photovoltaic (PV) module is placed under the sun, solar irradiation will generate a temperature distribution across its surface. The conductive film requires lower temperature for its bonding with beautifully finished bonded areas. In this study, first, finite element analysis (FEA) was carried out for manufacturing process using both solder and CF bonding. Then, three temperature cycles considering different environmental operating conditions were applied to the analysis to understand how thermal cycles affect the residual stress developed during manufacturing process of solar cell. Using times of six months for solder and one month for CF bonded cell were considered for the analysis. This investigation provides a comparison of thermal stress between solder and CF as bonding materials of solar cells in order to determine which offers best reliability in the long term.

View full abstract

The EBSD(Electron BackScatter Diffraction pattern) observations were conducted on unused and creep damaged materials of Mod.9Cr steel. GROD(Grain Reference Orientation Deviation) was used as the parameter to evaluate the grain inside distribution of deformation. The grain inside GROD distribution function was well approximated by the form of continuously distributed dislocation function. The coefficient A multiplied by grain size l was correlated with macroscopic creep strain and applied to evaluate strain distribution near creep void. Using A × l parameter, strains near creep voids were proved to be almost similar level of the macroscopic creep strains.

View full abstract

EBSD (Electron Backscatter Diffraction pattern) observations were conducted and GROD (Grain Reference Orientation Deviation) distribution patterns were obtained by drawing intersection lines settled on GROD maps for creep damaged samples of austenitic stainless steel SUS304HTB for boiler tube use. GROD distribution patterns were well fitted by continuum dislocation distribution functions and the representing parameter A of the distribution functions showed liner relationship against macroscopic creep strain. Using this relationship, creep strains near creep voids were evaluated as the similar levels of macroscopic creep strain.

View full abstract

Deformation behavior of adjacent crystal grains of a metal is observed using speckle interferometry. Coarse grained aluminum specimens which have two crystal grains in the observation region are made by a thermo-mechanical treatment for the observation by speckle interferometry. Grain boundaries are arranged 0°, 45° and 90° for the loading direction. Crystal orientations are measured by electron backscatter diffraction. Tensile tests are performed and the in-plane and the out-of-plane deformation behaviors of the specimen surfaces are observed using speckle interferometry. As the results, strain maps of ε_x, ε_y, and γ_<xy>, are obtained and heterogenous deformation behaviors in each crystal grain can be observed. Various values of yield stresses and strains of each crystal grain in each specimen are obtained. The values depend on Schmidt factor obtained from the crystal orientation analysis. A tendency of the variation to decrease with decreasing the angles of the grain boundaries is obtained. Therefore, the experimental investigation of deformations of adjacent crystal grains is made possible by speckle interferometry.

View full abstract

A technique to heal fatigue damage in a stainless steel by controlling high-density electric current have been studied. However, these mechanism was not investigated in detail. In this study, dislocation motion was observed by transmission electron microscope (TEM) before and after applying electric current in a sample cut out from fatigue specimen. When the current application direction was opposite, the dislocation motion direction also became opposite. The experimental results showed that dislocation motion was caused by electrons.

View full abstract

In this study, we investigate the effect of prestrains on swelling-induced buckling patterns in polymeric films with a square lattice of holes. To reproduce experiments by Zhang et al. (2008), finite element analysis is performed using an inhomogeneous field theory of polymeric gels in equilibrium, amd prestrains prior to swelling are given as uniaxial tension along a lattice direction. A periodic unit consisting of 10×10 unit cells is analyzed under a generalized plane strain assumption. The 10×10 unit cell shows that different buckling patterns appear depending on prestrains of ε=0, 20, 40 and 60%: a diamond plate pattern (ε=0%), a slightly distorted diamond plate pattern (ε=20%), a binary pattern of circles and lines (ε=40%), and a monotonous pattern of ellipses (ε=60%). These predictions are in very good agreement with experiments.

View full abstract

Deformation behaviors in Bimodal microstructure were studied by a crystal plasticity software code. The Bimodal microstructure consists of fine and coarse crystal grains. Some numerical models for such structure were constructed by the Voronoi dynamics software where growth process of crystal nuclei were simulated. Results showed that deformation of coarse grains took place first and then, deformation of fine grains occurred.

View full abstract

Foamed plastics is used for the purpose of improving the mechanical characteristics of industrial products by elastic and damping. The stress-strain curves is very complex under compression, because the included cell walls deform and crush flatly finally. In addition, it's said to affect the complexity of the behavior by the gas in the cell walls. This paper is constructed a numerical model of resin foams considering the microscopic structure based on the CT images. Moreover, the analysis of microstructure model using artificial viscosity is performed to verify the effect of friction and the included gas.

View full abstract

Bicrystalline micropillars (BCMs) involving several grain boundaries along the pillar axis were fabricated in an oxygen-free copper sample by focused ion beam milling. These pillars were compressed using a nanoindenter to investigate slip transfer across grain boundary (GB). Referring their stress-strain (SS) curves, the possible slip systems were resolved from scanning electron microscope images after deformation and GB interaction criteria with the Schmid factors (SFs) of each grain and the geometric relationship of intersections between slip planes and GB. The quite different SS behaviors were observed in the BCMs, of which one was the unstable elastic-almost perfect plastic deformation due to large strain bursts and the other was work-hardening accompanying several small strain bursts. The 1%-proofs obtained from SS curves were found to be roughly correlated with the GB interaction criteria the author already proposed except some GBs.

View full abstract

The authors recently developed a homogenization method for fin layers in tube-fm structures composed of flat tubes and wavy fins. In this paper, the resulting, homogenized elastic stiffness of fin layers with designed and real shapes are applied to finite element (FE) analysis of tube-fm specimens subjected to bending. By comparing the fin-homogenized and full-scale FE analysis results, the homogenized elastic stiffness of fin layers are shown to be valid even under bending although the homogenized method is of first-order. Bending experiments of tube-fin structures are further performed to examine the homogenized elastic stiffness of fin layers. It is shown that the bending strains are predicted well if the real shapes of outer and inner fins are considered in the fin-homogenized FE analysis. It is also shown that the Bernoulli-Euler assumption is not satisfied in the homogenized outer fin layers in the analysis.

View full abstract

Finite element homogenization (FEH) analysis is performed to investigate the effect of strain hardening on the homogenized elastic-viscoplastic behavior of a plate-fin structure with primary and secondary flow channels subjected to different pore pressures. A periodic unit cell is considered for the FEH analysis under uniaxial cyclic loading in the stacking direction. The plate-fin structure is assumed to be made of 316 stainless steel. The strain hardening and ratcheting properties of this base metal are represented using the Ohno-Wang kinematic hardening model. The FEH analysis then reveals that the strain hardening makes the mean pore pressure effective for the homogenized elastic-viscoplastic behavior. This is in accordance with one of the three special cases predicted using Hill's macrohomogeneity equation. The FEH analysis also reveals that the primary and secondary flow channels suffer from ratcheting due to the difference in two pore pressures despite of no ratcheting in the homogenized behavior.

View full abstract

In this study, tensile tests of plate-fin structures with a DIC measurement system are conducted to investigate validity of the homogenization analysis method developed by our research group. For this, tensile tests using plate-fin test specimens made of an epoxy are performed, and the tests are measured by a DIC measurement system. Then, corresponding elastic-viscoplastic analysis of plate-fin structures is carried out using the homogenization theory for nonlinear time-dependent materials. The analysis results obtained are compared with the experimental results to validate the present method.

View full abstract