Equivalent stiffness of clamped plates should be prescribed not only to evaluate the strength of bolted joints by the scheme of "joint diagram" but also to make structural analyses for practical structures with many bolted joints. We estimated the axial stiffness and bending stiffness of clamped plates by using Finite Element (FE) analyses while taking the contact condition on bearing surfaces and between the plates into account. The FE models were constructed for bolted joints tightened with M8, 10, 12 and 16 bolts and plate thicknesses of 3.2, 4.5, 6.0 and 9.0mm, and the axial and bending compliances were precisely evaluated. These compliances of clamped plates were compared with those from VDI2230 (2003) code, in which the equivalent conical compressive stress field in the plate has been assumed. The code gives larger axial stiffness for 11% and larger bending stiffness for 22%, and it cannot apply to the clamped plates with different thickness. Thus the code shall give lower bolt stress (unsafe estimation). We modified the vertical angle tangent, tan φ, of the equivalent conical by adding a term of the logarithm of thickness ratio t_1/t_2 and by fitting to the analysis results. The modified tan φ can estimate the axial compliance with the error from -1.5% to 6.8% and the bending compliance with the error from -6.5% to 10%. Furthermore, the modified tan φ can take the thickness difference into consideration.

View full abstract

The main topic of this study is a shape design of structure considering various load conditions. In order to evaluate such a performance, we formulate a criterion based on the norm of the stiffness matrix of the structure to be designed. A shape design of crane-hook is introduced as an example. We formulate a multi-objective optimization problem based on an FEM analysis. The displacement at the specified force applying point, the ratio between the total displacement against various load conditions and the structural volume of the crane-hook are adopted as the evaluation items to be minimized. The cross-section and the contour shape are adopted as the design variables. Continuous change of these design variables is expressed in terms of orthogonal polynomials and the Fourier series. The Particle Swarm Optimization (PSO) is used as the optimization method. The obtained crane-hook design has a taperd shape similar to those of existing designs.

View full abstract

We developed an analysis system for simulating birefringence of an annealed ingot of CaF_2 single crystal caused by the residual stress after annealing process. The analysis system comprises the heat conduction analysis that provides the temperature distribution during ingot annealing, the stress analysis to calculate the residual stress after ingot annealing, and the birefringence analysis of an annealed ingot induced by the residual stress. The finite element method was applied to the heat conduction analysis and the stress analysis. In these analyses, the temperature dependence of material properties and the crystal anisotropy were taken into account. In the residual stress calculation, we considered the time-dependent nonlinear deformation behavior of a material called creep. In the birefringence analysis, the distributions of optical path difference were calculated by using the average stress method with consideration of the crystal anisotropy. We can perform the birefringence analysis of an ingot of CaF_2 single crystal with any growth direction, using this analysis system, and we performed the analyses of the crystals with the <100> and <111> growth directions. From these analyses, we obtained reasonable results of optical path difference in comparison with the experimental results.

View full abstract

Although three-dimensional finite element analysis has become a common tool in the industries to perform their design analyses, there still exist some difficulties in performing three-dimensional fracture analyses. One of them is that although automatic mesh generation techniques are available for tetrahedral finite elements, hexahedral finite elements are commonly used in three-dimensional crack analyses. The other is that the analysis models tend to be large in their scales. Therefore performing a three-dimensional fracture analysis takes much manual labor in the analysis model generation processes and computational time. In present research, the authors have been developing a fracture mechanics analysis system that minimizes the manual labor. The key components in the analysis system are a mesh generation software and the virtual crack closure-integral method (VCCM) for the second-order tetrahedral finite element to evaluate the crack parameters (the energy release rates and the stress intensity factors). Based on them, the software system was carefully designed to handle large scale analysis model. In this paper, as the first report, the outlines of the system and the finite element model procedures are described.

View full abstract

The finite element modeling for polypyrrole films previously proposed by the authors, which combines the one-dimensional ionic transport modeling and the three-dimensional electrochemical-poroelastic modeling, has been applied to the simulation of the electrochemical-poroelastic behaviors of polyaniline fibers in HCl or HBF_4. The parametric studies for the evaluation of parameters such as the viscous coefficient and phenomenological parameters have also been conducted. The validity of the present computational modeling has been demonstrated by comparing the calculated results with the experimental results.

View full abstract

Although a lot of interface crack problems were previously treated, few solutions are available under arbitrary material combinations. This paper deals with an edge interface crack in a bonded infinite plate and semi-infinite plate. Then, the effects of material combination on the stress intensity factors are discussed. To obtain the interface stress intensity factor very accurately, a useful method is presented on the bases of the stress values at the crack tip calculated by the finite element method. The stress intensity factors are indicated in charts under arbitrary material combinations. For the edge interface crack, it is found that the dimensionless stress intensity factors are not always finite depending on Dunders' parametersα, β. For example, they are infinite when α(α-2β)>0. And they are finite when α(α-2β)=0, and zero when α(α-2β)<0.

View full abstract

In the piping wall of the industrial machinery such as the chemical plants, the ceramics coating materials which are excellent in the heat-resistance and the corrosion resistance are used. However, in the wall of which the speed and pressure of the flowing fluid become high, the cavitation is generated, and the adverse effect of the performance deterioration, the noise, the vibration and the cavitation damage is being brought about. Especially, the serious adverse effect is the cavitation damage, and the large accidents by the fracture of the piping will be caused. Therefore, the life prediction of the film is required in order to prevent the accidents. In this study, the life prediction of the film was tried by the features of the detected AE signals in the cavitation erosion experiment. Then, the FBG (Fiber Bragg Grating) sensor which is a kind of optical fiber sensor was used, because the AE measurement by the sensor using the piezoelectric device which handles the electric signals is very difficult in the environments where the explosion-proof and the water resistance such as the chemical plants are required. As the results, the following things became clear: (1) From the frequency analysis results of the detected signals, it was possible to evaluate the fracture types of the film. (2) The spectrum analysis is effective for the analysis method of the detected signals, and the life of the film is the case in which the spectrum exponent reached about 2.5. Also the similar results were observed, when the heat history was given to the film.

View full abstract

In order to evaluate residual stress distributions in a reactor pressure vessel (RPV) due to weld-overlay cladding and post-weld heat-treatment (PWHT), thermal-elastic-plastic-creep analysis considering the changes of material properties in heat-affected zone (HAZ) has been performed. By considering the material property changes in HAZ, stress distributions obtained from the analysis agree well with the experimental results measured from weld-overlay cladded plate. Applying the analysis method, residual stress distributions for a vessel model caused by weld-overlay cladding, PWHT, hydrostatic test, operational load and transient loads during pressurized thermal shock (PTS) events have been computed. Effects of the residual stresses on the structural integrity of RPVs have been evaluated by comparing the difference in stress distributions and the values of stress intensity factor for a postulated crack during PTS events.

View full abstract

In this paper, the average load for circular tubes subjected to axial impact is investigated, based on finite element analysis. It is found that the load for a strain rate sensitive material can be predicted from that for a strain rate insensitive material by taking into account of the modified yield stress and the plastic hardening coefficient. Here, these parameters can be calculated using the average strain rate at the apex of the second wrinkle from the fixed end of a tube during folding. Also, the average strain rate is found to be a function of a ratio of impact velocity and tube radius, and an approximate technique for evaluating the average strain rate is proposed. Moreover, based on our numerical results, an approximate equation for evaluating the average load is proposed, and checked the validity of the approximation.

View full abstract

In the framework of atomistic-level modeling, a new technique for predicting the interface strength of the resin-mold structure has been proposed. We show that our proposed interfacial fracture energy is effective in determining the adhesion strength of three kinds of interfaces between epoxy resin and metals (Cu, Fe and Al). We also discuss the multi-scale connection between nano-scale adhesion and macro-scale interface strength in terms of interface roughness. When the interface with roughness is subject to shear load under vertical residual stress in experiments, FEM analysis taking account of the effect of interface roughness shows that local debonding (mode I) due to the tensile stress induces global delamination. Our proposed method for dealing with local mode-I debonding by use of molecular dynamics is found to be effective in simulating the interfacial fracture with the shear load.

View full abstract

Microstructure control in martensitic stainless steel compacts fabricated by metal injection molding was investigated. The specimens were made by injecting the mixture of gas-atomized powder and a polymer binder into a metallic mold. The injection moled compacts were debound in air at a temperature between 533K and 593K for 7.2ks. They were sintered in vacuum at 1573K and 1623K for 7.2ks. The carbon content of the sintered compacts decreased linearly with increasing the debinding temperature. This means that the debinding temperature can control the carbon content of the sintered compacts accurately. The microstructures of the sintered compacts drastically changed from the debinding temperature of 573K. They became α-phase and γ-phase lower than 573K and only α-phase higher than 573K. The mechanical properties of the sintered compacts corresponded to the change in the microstructure.

View full abstract

Recently, it has been demanded to develop the lightened automobile with high stiffness and strength for the fuel economy and the vehicle safety performance. Therefore, it has been considered to apply the ultra high strength steel. A few reports indicate the fatigue strength of the spot welded joint using the high strength steel was not improved compared with the conventional mild steel. Moreover, it has been investigated the proper number of spot and the arrangement which were regarded the strength reliability for the energy saving and the cost reduction. In order to solve these problems, it is important to acquire the knowledge about the fatigue characteristics of spot welded joint. In this study, in order to investigate the fatigue characteristics, fatigue tests were conducted under the shear loading condition and internal fatigue crack behavior around the spot area was observed in detail. Test results showed that spot welded joints had the low fatigue strength compared with the strength of base metal. Additionally, specimens indicated the different fracture types; button fracture and base metal fracture. Then, 3-dimensional observation of the fatigue crack propagation was conducted on each fracture type. As this result, it was clarified spot welded joints used in this study require many cyclic loading to initiate the fatigue crack around the spot area. Furthermore their fatigue crack propagation indicates the similar behavior in major part of life independently of the stress level.

View full abstract

In order to investigate the brittle-striation formation mechanism of a low carbon steel JIS S10C fatigued in hydrogen gas environment, fractographic observations were conducted on the visualized fracture phenomena at some processes of brittle striation formation. The results showed as follows. A striation line was formed during the loading part of the cycle as a trace of blunting by slip. Then, stable ductile crack growth started. These processes were similar to those in the normal ductile fracture from a crack, that is, ductile tearing process in tension. Based on the experimental results, a brittle striation formation model in which hydrogen only enhances the microscopic ductile tearing process just ahead of a crack tip is proposed. The model rationally explains the peculiar load-frequency effect in quasi-cleavage range on the fatigue crack growth which reveals lower growth rate in spite of lowering the load-frequency.

View full abstract

In order to investigate the brittle-striation formation mechanism of low carbon steel JIS S10C in hydrogen gas environment, TEM/EBSD observations corresponding to fractography were conducted. Main results are as follows. (1) Quasi-cleavage (QC) facets plane with brittle striations does not coincides with (100) cleavage plane. (2) Slip deformation distributions reflecting the brittle striation formation processes are observed with TEM. One of the conceivable brittle-striation formation mechanisms which can explain these results is as follows. A striation line is formed during the loading part of the cycle as a trace of blunting by slip. Then, stable ductile crack growth starts. These processes were similar to those in the normal ductile fracture from a crack, that is, ductile tearing process in tension.

View full abstract

In resin-metal composite structures such as electronic products with the epoxy resin molding insulator, a thermal fatigue of the epoxy resin during thermal cycles is a critical issue. Since a thermal stress in the epoxy resin and its strength change simultaneously during thermal cycles, an estimation method for the thermal fatigue reliability in consideration of the temperature dependence of fatigue strength is required. In this paper, the technique of predicting thermal fatigue crack growth rate in epoxy resin is studied and we proposed an estimation method using strain intensity factor K/E as an evaluation parameter. In order to evaluate the proposed method, both the crack propagation test by thermal cycles and the thermal stress evaluation by viscoelastic FEM analysis were carried out. As a result, it was found that the proposed method estimated the crack growth rate under thermal fatigue test less than that of iso-thermal fatigue test.

View full abstract

High-density electronic package employs elerctroplated copper foils for micro-wiring. To ensure the reliability of the micro-wiring, the characteristics of deformation of electroplated copper foils must be clarified. Then, in this study, inelastic deformation characteristics of an electroplated copper foil were investigated by conducting tests under several loading conditions. Though all the tests were conducted at room temperature, stress-strain curves in tensile tests were affected by strain rate, and clear creep curves were obtained by creep tests. Moreover, stress relaxation by strain maintenance and ratchetting deformation by cyclic tension-unloading occurred. To discuss the mechanism of these time-dependent deformations, an elasto-plastic-creep constitutive model for the electroplated copper foil was constructed and simulations of the deformations were conducted by employing the model. As a result, the strain rate effect, the creep and the stress relaxation were almost described by the constructed model, and it was found that the creep strain controlled the characteristics of the time-dependent deformation. Meanwhile, the model could not describe the ratchetting deformation well. This suggests that a parameter which expresses damages due to cyclic loading should be considered in the constitutive model for the electroplated copper foil to predict the ratchetting deformation precisely.

View full abstract

Strength evaluation of conductive adhesive paste for die bonding during reflow soldering is important, as the fracture in the paste is one of the main causes of package failure. First, we assumed that the fracture was caused by thermal stress and vapor pressure at the interface between the resin and the copper lead frame. Next, we measured the fracture strength of a bonded specimen with the paste using a three-point bending test. Then, we calculated the stress intensity factor of a corner in the paste during the reflow soldering process. Finally, we compared it with the fracture strength of the paste, and evaluated the possibility of the fracture in the paste. The strength evaluation analysis results were consistent with the results of the package reflow test. Therefore, we confirmed that the fracture in the paste can be predicted with a high accuracy by using this method.

View full abstract

The present study evaluated the yield stress distribution in the surface layer and the fatigue properties of a steel SUS316L modified by cavitation peening (CP). The yield stress was evaluated based on micro-indentation tests using a spherical indenter. Especially, a technique of inverse analysis using elastic-plastic finite element analyses was utilized to determine the yield stress from the results of indentation tests, at which the effect of compressive residual stress introduced by CP was excluded. The identified results revealed that the surface with 50μm in thickness was remarkably work-hardened by CP, and that the yield stress was increased within 500μm in depth from the surface. Moreover, the plane bending fatigue tests demonstrated that the fatigue limit of the specimen was increased from 277MPa to 362MPa by CP. Therefore, we can conclude that CP is effective for introducing a high yield stress on the metal surface, which suppresses the initiation of surface crack during fatigue cycles.

View full abstract

Lumbar spinal fusion with unilateral pedicle screw and rod system (unilateral PS) has received favorable clinical reports. However, there are very few reports about mechanical properties of this system. The purpose of this study was to investigate the mechanical properties of the lumbar spinal fusion with unilateral PS. Functional spinal units (FSUs) from wild boar cadavers were used. Bending tests in a total of eight directions were performed to clarify the range of motion (ROM) for each FSU using a 6-axis material testing machine. The results showed that while the fixation for bilateral PS model is high in all bending directions, the ROM for the unilateral PS model depends on the bending directions, and the fixation in the diagonal direction of pedicle screw insertion is low. Furthermore, it was considered that the factor of the low fixation in the diagonal direction is the relative rotation of the screw toward the vertebral body.

View full abstract

Ceramics are expected as the structural materials but they are brittle materials. Therefore there is a possibility of rapid fracture when a contact stress intensively acts on the surface of materials. Therefore improvement of reliability is needed for ceramics used under a local contact stress. In response, the crack healing ability of ceramics is extremely effective. If the crack-healing ability of ceramics is used on structural components, great benefits can be anticipated, such as an increase in the reliability of structural ceramic members and a decrease in the inspection, machining and polishing costs of ceramic components. In the present study, the effects of the crack-healing on contact strength were investigated for Si_3N_4/SiC which was subjected to various machining process. The evaluation of contact strength was done by sphere indentation test that used acoustic emission (AE) together. As a result, the contact strength of Si_3N_4/SiC was improved by crack-healing with a combination of rapping even for the material that had machining cracks induced by a heavily machining process.

View full abstract