The fatigue properties of the structural steels were investigated based on “Database on Fatigue Strength of Metallic Materials” published by the Society of Material Science, Japan. The rotating bending fatigue test results for pure iron and structural steels JIS S10C-S58C were extracted from the database and hyperbola regression model was applied based on the JSMS standard, “Standard Evaluation Method of Fatigue Reliability for Metallic Materials-Standard Regression Method of S-N Curve-”. The correlations between obtained regression parameters were investigated and it is revealed that there were strong correlation between (1) tensile strength σB and fatigue limit E, (2) slope A and intercept B of inclined part of S-N curve, (3) slope A and the critical number of stress cycles giving fatigue limit Nw. In addition, a number of fatigue data extracted were normalized by tensile strength of σB and pooled altogether. Using these data sets, the P-S-N property was also analyzed and it is confirmed that the fatigue life distribution shows complex pattern and changes at each stress level for evaluation, while fatigue strength distribution shows relatively simple pattern and were approximately identical at different number of cycles.
Evaluation of fatigue crack nucleation and propagation behavior in anisotropic metals is necessary for improving an accuracy of structural integrity assessment. In this paper, high cycle fatigue tests and polycrystalline FE analyses were carried out to investigate a relationship between crack nucleation positions and stress distribution. Fatigue tests of a rectangular specimen were conducted by four-point bending under load control. Fatigue crack nucleation and propagation in the gauge region was carefully observed during the tests. Grain morphology and crystallographic orientation of tensile surface were characterized using electron beam backscattering diffraction pattern (EBSP) analysis. The size of characterized area was determined to sufficiently cover the gauge region of four-point bending inner span. Stress distribution of the specimen was computed by polycrystalline finite element analysis. Resolved shear stresses were calculated in expected twelve slip systems. Fatigue cracks were tend to be initiated in areas indicating relatively high resolved shear stresses in several slip systems.
Natural fiber twisted yarn has high potential as a reinforcement of green composites, because the yarn is produced as a continuous form, which is necessary for exhibiting reinforcing mechanism in a composite. In a twisted yarn, spun yarn filaments meander from the twisted yarn surface to inner along the yarn axis. This phenomenon is called ‘migration’, and yarn structural mechanics has been developed by taking an ideal migration into consideration. In the yarn mechanics, yarn tensile properties are often expressed as a function of twist angle, while yarn structure depends strongly on performance of twisting machine. Thus, the present study deals with the effect of twist contraction ratio (TCR), one of the most important parameters in yarn structure, on tensile properties of twisted yarns and green composite reinforced with twisted yarn. Ramie spun yarn filaments were used as a test material, and the twisted yarn were produced using auto or hand twisting machine. The results show that relation between mechanical properties of the twisted yarn and yarn structure can reasonably be comprehended when choosing TCR as an evaluation parameter. Furthermore, this study additionally deals with relation between theoretical elastic modulus, and twist angle or TCR based on the experimental data of green composites as well as twisted yarns.
The recent trend in circuit assemblies is to bond larger IC packages to PCB (Printed-circuit Board) by BGA (Ball Grid Array) solder joint and underfills. This would induce more intricate CTE (Coefficient of Thermal Expansion) mismatches among the PCB, the IC package, the solder joint and the underfill. The underfill, which fills the gap between solder balls, is usually used to increase mechanical strength, and to improve the reliability of the solder joint. In this research, 1) The effects of the stiffness of underfills on stresses and strains on solder joint in which CTE mismatch and thermal deformation are involved, are discussed based on finite element analysis. 2) The effects of size of packages on stresses and strains on solder joint are presented. By the procedures, about 50% lower strains of solder joint are obtained, as compared to those of solder joints in which the underfill has 2.5 times larger elastic modulus, or the package has about 28% smaller size. 3) An optimum design method is applied to minimize the thermal deformation of solder joint taking properties (Young's modulus, Poisson's ratio and CTE) of underfill as design variables. The optimization method developed here can be used to optimize the underfill properties, and to improve effectively the reliability of solder joint. The method is useful to evaluate underfill materials for circuit assemblies.
Cellular automaton simulation was performed for a study case in which fire occurs at Umeda underground arcade in Osaka city. In the simulation, pedestrian behavior was divided into 5 types, namely, normal, subordinate, slow, panic and needing assistance. In addition simulation parameters, such as, pedestrian viewpoint, evacuation behavior, objective, simulation time step and diffusion of smoke, were modeled. The aim of the present study is to define the risk factors during evacuation, and to propose the key factors concerning safety performance when designing underground malls under similar conditions as that of Umeda underground arcade. To define the ranks calculated reliability of evacuation is tried. Then, the performance matrix with reliability of evacuation by the simulation is applied at. In this study, the time slot in which evacuation is dangerous in case of fire was made clear together with studies carried out on the effects of the sign position on the reliability of evacuation in case smoke descends from the ceiling. In addition, the influence of the density of underground shopping malls pillars on the evacuation reliability was verified.
The present paper describes the maintenance methodology of deteriorating reinforced concrete piers supporting highway bridges. The highway network is considered, in order to plan the maintenance scheduling for many highway bridges. The Life Cycle Cost analysis with seismic risk is numerically carried out. The optimum maintenance plan is searched by GA. The numerical results show the maintenance plans for the scale of highway bridges, the elapsed years after construction, the seismic design level and so on. Also, it can be seen from the numerical consideration that the seismic strengthening for the bridge piers designed by former seismic design code is effective in the view of bridge management.
When traffic accident occurs on urban expressway network, lifesaving activity with emergency car is important that promptly executes arrival at the scene of the accident and transportation to a hospital. The execution of the lifesaving activity requires a lot of time by the cause such as traffic jam after accident occurs. Therefore, the fire fighting organization and the urban expressway organization should examine method of executing prompt the lifesaving activity even when traffic jam occurred by frequently executing the lifesaving training that assumes situation when the accident occurs. However, it is difficult on both sides of time and the cost to actually execute lifesaving activity in urban expressway. Then, in this study, we examined prompt lifesaving activity in urban expressway by using the traffic simulation system developed by authors. Specifically, by reproducing situation that the traffic accident occurs on urban expressway and the lifesaving activity executes with the system, we measured the arrival time that emergency car arrives at the scene of the accident. And, we examined the expected lifesaving effects by using the concept of Golden Hour Principle that shows relation between passage time since the injury and mortality rate, we made the problem of the present lifesaving activity clear. In addition, we proved that a lifesaving possibility improved by shortening time that the emergency car enters in the expressway.
Mechanical properties of single-walled carbon nanotubes with one-dimensional intramolecular junctions (CNT-IMJs) are investigated using first-principles density functional theory calculations. The influence of Stone-Wales (SW) defects (a pair of five- and seven-membered rings) at a junction on the Young's modulus, tensile strength and breaking strain of the CNT-IMJs are discussed from the charge density and interatomic distance. Our calculations reveal that deformation concentration on a seven-membered ring causes the decrease in the strength and elongation of the CNT-IMJs. It is found that the tensile strength and breaking strain of the CNT-IMJs depend on the position of SW defects, while the number of SW defects hardly affects them. The applicability of AIREBO classical interatomic potential to simulate tensile deformation in the CNT-IMJs is also discussed.
To understand the nature of ferroelectric instabilities under finite electric fields, e.g., domain switching and polarization reversal, as an origin of critical failure of ferroelectric devices, it is essential to evaluate critical electrical and/or mechanical conditions where the ferroelectric state becomes unstable. However, the instability criterion for such a multi-physics system where the mechanical and electric properties interact each other has not yet been proposed. In this paper, we developed an analytical method to rigorously describe the multi-physics instability criterion for arbitrary atomic structures under finite electric field and/or mechanical load. According to the proposed method, the instability starts when the minimum eigenvalue of the Hessian matrix of the potential energy reaches zero. The corresponding eigenvector indicates the displacement of atoms at the instability. We applied the method to 180° domain walls in ferroelectric PbTiO3 under external electric fields, and our criterion can successfully describe the onset of instability, namely, the domain switching, as well as their displacement pattern at the switching. This clearly indicates the validity of our multi-physics instability criterion.
The simple method to analyze atomistic strain in molecular dynamics (MD) simulation is studied. The proposed method, here called atomic strain measure (ASM), is based on Green-Lagrangian strain measure which is traditionally defined in continuum mechanics. The ASM is formulated for the use in atomic system with some adequate assumptions. In our formulation, pairwise interatomic vectors of finite length are approximately substituted for infinitesimal continuum line segments between material's points. The obtained expression of ASM is very simple and easy to use. The authors are checking the validity, limitation and usefulness of ASM by actual MD models of aluminum. A perfect-crystal model results in qualitatively good results for strain evaluation, as for the response to homogenous deformation. The ASM is also applied to a polycrystal model, which has a lot of inhomogeneous nanostructures, i.e. crystalline defects such as grain boundaries (GBs) and triple junctions (TJs) among them. It is confirmed that a certain concentration of strain onto the vicinity of GB plane or TJ point is able to be clearly captured by using ASM. It is concluded that approach of the present ASM analysis for atomic simulation is very effective to obtain change of nanostructure.
In order to investigate the mechanism behind the improvement of fracture toughness in ultrafinegrained metals at low temperatures, the strain rate dependence of the transition of dislocationsources from crack tips to grain boundaries is studied by the combination of molecular dynamicssimulations and the linear elastic theory. As the strain rate decreases, grain boundaries becomeanother stress consented site due to the pile-up of dislocations against the grain boundaries. Theamount of stress concentration became larger than that of crack tip as the number of dislocationsemitted from the crack tip increases. It was clearly indicated that dislocations were impinged intothe grain boundary when a new dislocation was emitted from there. It indicates the transition ofdislocation sources from the crack tip to grain boundaries at lower applied stresses. The distributionof dislocations between the crack tip and grain boundary is strongly related to the strain rate ; namely, a larger number of dislocations are distributed very close to the crack tip as the strain rate increases.It induces the smaller stress concentration at the grain boundary since the number of piling-updislocations is decreased around the grain boundary. Consequently, as the strain rate increases, thematerial becomes brittle, indicating that it will fail in a brittle mode and no longer deform plastically.
Numerous studies have reported that solute hydrogen atoms and lattice defects have strong interactions, and that hydrogen atoms significantly change the stability and/or mobility of lattice defects. Although molecular dynamics (MD) simulations can treat complicated interactions of various lattice defects, the time scale is insufficient to treat hydrogen diffusion so as to influence the lattice-defect generation and cooperative motion of hydrogen atoms and lattice defects. Here we developed an interatomic potential for Fe with pseudo-hydrogen effects on lattice-defect energies and performed MD simulations of tensile loading. First, we estimated the lattice-defect energies of Fe and hydrogen-trap energies of lattice defects by using first-principle calculations and evaluated the lattice-defect energies under a practical gaseous hydrogen environment. Second, we refitted the existing embedded-atom-method potential for Fe to represent the lattice-defect energies amended by hydrogen effects. Finally, we confirmed that our potential is applicable for various phenomena by estimating the reproducibility of grain-boundary energies that are not employed for potential fitting. Our tensile-loading simulations of a nano specimen show that hydrogen reduces elongation at rupture.
A model composite method was proposed to evaluate the interfacial fracture toughness between fiber and matrix, where the resin fracture is not involved. In the composite, two glass fibers from 10 to 15μm in diameter were bonded by resin. In the present study, two kinds of surface treatments were conducted for the glass fibers. One was the acrylic-silane coupling, and the other one was the 3-methacryloxy-propyl tri-ethoxysilane coupling. The interfacial strength is low for the former and it is high for the latter. Two kinds of resins were also employed for the matrix, one was vinylester resin, and the other one was unsaturated polyester resin. The former is ductile and the latter is brittle. In-situ observation testing system was developed for Mode I fracture toughness tests of the model composite, where the resolution of applied force was about 0.01 mN. For the model composites consisted of either the low interfacial strength fiber and the ductile resin or the high interfacial strength fiber and the brittle resin, the crack propagation was unstable, and the interfacial fracture toughness could be evaluated. On the other hand, ductile fracture occurred in the resin and the interfacial fracture was not evaluated for the model composite consisted of the high interfacial strength fiber and the ductile resin.
This study was conducted to propose a design of cementitious material containing Ordinary Portland Cement (OPC) with Blast Furnace Slag (BFS), anhydrite (AH : CaSO4) and lime stone powder (LSP : mostly CaCO3), with improved sulfate resistance. This study was also focused on the influence of adding BFS, AH and LSP on sulfate resistance of OPC paste and mortar specimens at temperatures of both 23°C and 40°C and investigated the mechanism of sulfate attack on cementitious materials at both temperatures. The sulfate resistance was improved by adding BFS, AH and LSP; the best sulfate resistance was found for BFS 59.5%-OPC 25.5%-AH 5%-LSP 10% (BFS/OPC ratio = 70 : 30), at 40°C. It was concluded that (a) the amounts of substances responsible for expansion, CH and residual C3S, could be decreased with increasing the BFS replacement ratio (especially by adding more than 55% BFS) (b) a large amount of ettringite instead of monosulfate was produced before immersion in sulfate solution when 5% AH was added and (c) adding 10% LSP resulted in production of monocarbonate, which was stable in sulfate solution, instead of unstable monosulfate. AH and LSP could also improve early age strength. In addition, sulfate resistance of specimens was almost the same at both 23°C and 40°C when the BFS replacement ratio was more than 55%.
The effects of brazing filler metal diffusion on the mechanical strength of a ceramic/metal composite are described. First, a specimen of a SUS304/Cu/Si3N4 system, including brazing filler metals of AgCu and/or AgCuTi, was prepared by brazing in a vacuum furnace. Next, the vicinity of the joint parts was observed using field emission scanning electron microscopy. The brazing filler metals were found to diffuse into the Cu interlayer and ramify. In addition, a crack due to thermal stress resulting from a mismatch in thermal contraction between the materials present was observed around the AgCuTi/Si3N4 boundary. Energy-dispersive X-ray spectroscopic analysis revealed that Ag atoms diffused into the Cu interlayer to form the solid solutions of Ag and Cu. Micro-Vickers hardness tests were conducted on the surface of the Cu interlayer in order to examine the effects of brazing alloy diffusion on the hardening of the Cu interlayer. It was found that the diffusion of the brazing filler metals enhanced the hardness of the Cu interlayer. Finally, numerical simulations were performed to verify the increase in thermal stress due to hardening of Cu, and this behavior was discussed using simple equations. The results of the experiments and numerical simulations showed that the mechanical strength of the SUS304/Cu/Si3N4 composite is subjected to the diffusion of brazing filler metals.