To evaluate reliability indices is an important step in the structural reliability analysis. It is, however, difficult to evaluate reliability indices by conventional optimization methods in case local optimal solutions exist or the limit state function is not differentiable. In order to solve these problems, this paper presents genetic algorithm-based optimization methods to evaluate structural reliability indices. Two methods, the penalty function method and the backtracking method, are applied to the GA to deal with constraints. A local GA is also added to the program to accelerate convergence to the optimal solution. Through the application of the developed program to typical structural reliability problems, it is revealed that the GA-based optimization methods can evaluate the reliability indices even when conventional methods fail, and the local GA is quite effective to reduce CPU time and to increase the stability of convergence for both the penalty function method and the backtracking method.

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Molecular Dynamics is one of the powerful tools to investigate the relation between the microscopic structures of the crystal and these elastic properties. In this paper, the calculation method for the elastic constants proposed in the former paper, is extended in order to estimate the elastic properties of the inhomogeneous structures. The elastic constants are divided into homogeneous and inhomogeneous term, in consideration of the difference in definition of the strain between continuum and atomic system and the difference in the displacement. Newly defined method is applied to the silicon crystal with the grain boundary. It is found that the elastic constants of the stable coincidence boundary are almost same as the bulk values. However in the case of unstable grain boundary, those are lowered mainly by the effect of the internal displacement. It is found that the peculiar internal displacements, which appears within 1∼2 nm around the grain boundary, make the crystal softer.

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When a sphere is suddenly subjected to an instantaneous axisymmetric body heating, stress waves occur at the surface of the sphere the moment thermal impact is applied. The stress waves in a sphere proceed radially inward to the center of the sphere. The wave may accumulate at the center and give rise to very large stress magnitudes, even though the initial thermal stress should be relatively small. This phenomenon is called the stress-focusing effect. In this paper, we analyze the phenomenon theoretically. The results give a clear indication of the mechanism of thermal stress-focusing effects and clarify the order of singularity of the stress-focusing effects in a sphere.

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This paper describes a new type of impact test system using the Air-Slide that has less friction force generated between its moving part and the guide way. Dynamic force acting on a test piece is evaluated by measuring the velocity-time history of the moving part by an optical interferometer. Some cases on dynamic three-point bending tests for pure Aluminurn bars are carried out. The dynamic force is estimated by both the proposed experimental way and the finite element analyses. Although a slight deviation between the results related to the fixed condition can be observed, the results show the validity of the test system especially for observing the material behavior under spontaneous unloading.

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Previously, we presented the determination method of a dynamic constitutive equation with strain- rate sensitivity from the results of an uniaxial longitudinal impact compression test. The proposed method could derive an appropriate strain-rate sensitive dynamic constitutive equation as a polynomial without consideration based on underlying material physics. However, the method needs a lot of test results over a wide strain-rate range. This paper presents the determination method of a dynamic constitutive equation from a few impact test results. The proposed method determines unknown coefficients involved in a polynomial that appears dynamic constitutive relations of a material by nonlinear least squares method. Furthermore, the proposed method determines the optimum degree of a polynomial constitutive equation by AIC (Akaike information criterion). This paper, further, presents the split Hopkinson pressure bar method using a stepped striker bar. The proposed method can obtain the relations between the dynamic stresses and strain-rates in a wide range from a single impact test.

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It is known that Young's modulus and tensile strength of composite depend on loading direction, and those are commonly the highest in the loading direction parallel to fiber. But it isn't clear in C/C composite. In this paper, uniaxial and biaxial tensile deformation properties of C/C composite in the direction inclined to fiber were examined by using the strip-type and the cruciform-type specimens with slots . C/C composite showed the large breaking strains and the lower tensile strengths when the tensile loading was given in the direction inclined to fiber. In addition, the linear relationship holds between ε_x and ε_y in elastic region and it is independent of biaxial stress ratios T=0, 0.5 and 1.0 under biaxial loading in this material.

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It is well known that intermetallic compounds are embrittled by exposuring to the hydrogen environment. However, there are only a few reports investigating the effects of hydrogen-embrittlement on TiAl intermetallic compounds. In this study, the effects of hydrogen and metal-hydride on mechanical properties in TiAl intermetallic compounds were investigated. Hydrogen was introduced to specimens by corroded in acidic solution, and a metal-hydride formed on the specimen surface was analyzed by an X-ray diffraction method. To dissolve the metal-hydride, some specimens were heat -treated at 623 K in vacuum. Some mechanical properties—fracture stress, fracture strain, hardness and so on—were compared by using with or without corrosion and heat-treated specimen. Also, the hydrogen microprint technique was applied to visualize the behavior of hydrogen in TiAl. It is found that fracture strength and fracture strain were decreased by effects of hydrogen. Also silver grains indicating hydrogen emanation were revealed along slip lines and around crack tips.

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The friction welding for copper and aluminium is useful to minimize the diffusion thickness at the interface between copper and aluminium. The minimization of diffusion thickness is required for maintaining the joint strength. However, it is well known the growth of brittle intermetallic compounds, such as CuAl₂, CuAl and Cu₉Al₄, during the use at high-temperature. The effect of intermetallic compound layers on joint strength characteristics has not fully been clarified. In this study, elastic properties of intermetallic compound layers were investigated using depth-sensing micro-indentation technique. And stress singularities at the intersection of free surfaces and the interface between copper and aluminium were investigated using singular analysis. It was informed that the no free-edge stress singularities existed at the nelghborhood of apex angles of 55°and 125°. It was also confirmed using finite element analysis that the reduction of stress concentration could be observed by paying attention to the condition of no free-edge stress singularities in case of the copper and aluminium friction welded joint with brittle intermetallic compound layers.

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Metal powder assembles show some volumetric change under irreversible deformation, and their yield function (yield curve) is represented in the space of equivalent deviatoric stress q and hydrostatic pressure p. In the mechanics of metal powder assembles yield curves are assumed to be contours of constant density. However, there is something questionable in this concept. We already examined the consistency of the concept referring to the experimental data of tri-axial compression tests carried out in the past by other investigators, who compressed cylindrical specimens subjected to constant lateral pressure. The results showed that the concept of constant density was inconsistent with the actual constitutive behaviour under the assumption that associated flow rule is applicable and yield curve expands in a similar shape with increasing of compressive deformation. Here, we newly referred to the data obtained from compaction tests by Brown and A-Chedid who collected them using uni- and tri-axial strain control systems. In these compaction tests the direction of stress vector (p, q) do not change largely, and so whole view of yield curve is not obtainable. But the correspondence between directions of stress and strain rate vectors can be determined in the compactions. The comparison of the results with those in compression tests does not show good agreement. That suggests the necessity of inspection on the suitability of constitutive model used in the mechanics of metal powder assembles. Hardening strain rate ε_y is defined here which is composed of volumetric strain rate ε_p and equivalent deviatoric one ε_q accompanying with constant parameter. Finally, it is found that this definition is valid from the view point of inherent characteristic of hardening.

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High-temperature and high-pressure water corrosion behaviors of Y₂O₃ doped Zirconia (ZrO₂) were investigated in term of with the microstructural changes, residual strength characteristics and tetragonal to monoclinic phase transformation. Sintered ZrO₂ ceramics bodies with 2, 3, 4, 6 and 8 mol% Y₂O₃ doped ZrO₂ were made using a pressure less sintering method at 1 450°C, respectively. These ZrO₂ ceramics were corroded in high-temperature and high-pressure water condition such as 150°C∼300°C. Corrosion damage of the decrease Y₂O₃ dope ZrO₂ ceramics occurred preferentially on ZrO₂ particle after long-term immersion in high-temperature and high-pressure water environment. The tetragonal to monoclinic phase transformation occurred in decrease Y₂O₃ doped tetragonal ZrO₂ ceramics due to high-temperature and high-pressure water corrosion. For improvement of the corrosion characteristics (thermal stability) of the ZrO₂ ceramics, are important increasing the Y₂O₃ doped within a range in which no remarkable residual strength degradation is recognized. CeO₂ doped ZrO₂ ceramics were useful to improvement the thermal stability of Y₂O₃ doped ZrO₂ without loss of the fracture strength. CeO₂ doped ZrO₂ ceramics also have superior residual strength characteristics compared to Y-ZrO₂ ceramics. In addition design concepts used to obtain water-corrosion-resistant high strength and high toughness ZrO₂ ceramics were proposed.

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In this paper, the characteristics of the stress field near a corner of a T-shaped junction are studied by the eigenfunction expansion method. The eigenequation for the asymptotic behavior of stresses at the corner tip is given in an explicit form. The analysis is based on an assumption that the stresses in each plate can be approximated by a plane stress condition. This assumption was verified by numerical results of the finite element method.

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Evaluation of crack growth behavior under cyclic loading is often required in the structural integrity assessment of cracked components. Closing and re-opening of the crack give large influence on crack growth rate through the change of fracture mechanics parameters. Elastic-plastic finite element analysis was conducted by a computer program incorporating a two-surface-type constitutive model, for a wide range of loading conditions. It was found that variation of stress and strain ranges due to crack growth and crack opening behavior were properly simulated by the present analysis. Predicted deformation behavior was in good agreement with experimental data.

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Evaluation of crack growth behavior under cyclic loading is often required in the structural integrity assessment of cracked components. Closing and re-opening of the crack give large influence on crack growth rate through the change of fracture mechanics parameters. Based on the finite element analysis for a center-cracked plate, dependency of crack opening ratio on applied stress range and mean stress was examined. Simple formulae for representing the results were derived for plane stress and plane strain conditions.

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Evaluation of crack growth behavior under cyclic loading is often required in the structural integrity assessment of cracked components. The effective J-integral range is widely used in characterizing crack growth under cyclic loading with large plastic deformation. However, methods for estimating it for cracks in actual components are not well established because of several difficulties. In this study, a formulation for evaluating the effective J-integral range by the finite element analysis was presented and applied to center cracked plate specimens. Then a simplified estimation method using the reference stress and the crack opening ratio was proposed and its effectiveness was demonstrated through a comparison with the results of finite element analysis.

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The plane strain dynamic singular stress problem for a piezoelectric ceramic plate having a central crack is considered. The Laplace and Fourier transform techniques are used o formulate the problem in terms of a singular integral equation. The singular integral equation is solved by using the Gauss-Jacobi integration formula. A numerical Laplace inversion routine is used to recover the time dependence of the solution. Numerical calculations are carried out, and the main results presented are the variation of the stress intensity factor as functions of the geometric parameters and the piezoelectric material properties of the plate.

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In this paper, the effect of crack size, number and distance between the centers of surface cracks on the fatigue behavior of alumina has been investigated. A single crack and two or three collinear surface cracks were introduced by Vickers indenter. The crack size was changed by several indentation load. The fatigue tests were carried out under static tensile loading (stress ratio R=1) and cyclic loading (R=0 and -1). The main results were as follows. ( 1 ) For any surface crack size, appreciable decrease in fatigue lifetime due to cyclic load was observed, and the degree of the decrease was almost the same for each crack size. ( 2 ) No appreciable effect of stress ratio was observed in the decrease in fatigue lifetime due to cyclic loading. ( 3 ) An interaction of multiple surface cracks, that is to say, the decreasing fatigue lifetime by the interaction with the increase in the nurnber of crack or the decrease of the distance between those cracks was more clearly observed in the case of small crack size, while no interaction was observed In the case of large crack. Such a result seems to be caused by the reason that the interaction of multiple surface cracks may be cancelled by the higher crack propagation resistance owing to bridging effect in crack tip.

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In this paper, we examine the interaction between crack propagation and tilt grain boundary under uniaxial tension using molecular dynamics simulations. The Johnson potential for α-iron is used in these simulations. A structural transition from bcc to hcp induced by hydrostatic stress and brittle crack propagation are observed in a system including a crack whose direction is [010] in the (101) plane. In a system including both the crack and a [110] (112) grain boundary which is symmetric and stable, not only the phase transition but also the crack propagation are restrained by the grain boundary. In a system including both the crack and a [110] (111) grain boundary which is asymmetric and unstable, intergranular crack propagation occurs after the crack tip reaches the grain boundary.

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The fatigue strength and the fracture mechanism of ion-nitrided steel S 15 CK were investigated by using three types of the specimen, that is, the annealed specimen, the nitrided specimen and the specimen that the surface layer was removed. The last specimen is called the surface removed specimen and its surface layer was removed 0.1 mm in radius after nitriding. The fatigue limit of the nitrided specimen was higher than that of the annealed specimen, but was almost the same as that of the surface removed specimen. This is related to the hardness distribution of the specimen. In the case of the nitrided specimen, the hardness is recorded maximum at the surface layer and decreased in the internal section. The estimated strength from the hardness of that specimen is higher than the measured fatigue limit within the 0.1 mm thickness layer from the surface. At the internal section deeper than 0.1 mm from the surface, the estimated strength is expected to be lower than the fatigue limit. Thus the fatigue limit and strength of the nitrided specimen are determined by the material properties of internal site. So, the fatigue limit and strength of the nitrided specimen are not decreased even if the surface layer is removed. In the case of the nitrided specimen, the crack was difficult to be observed from the surface because the crack initiation occurred from the internal site. To remove the surface layer makes ease to observe the crack on the surface. In the case of the surface removed specimen, many cracks were initiated and propagate along the needle shaped participate Fe₄N and finaly these cracks made coalescence each other. The similar mechanism is expected to be operated at the internal site of the nitrided specimen from the observation of the fracture surfaces.

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Ceramics have many excellent mechanical characteristics. However, it is being taken up that the bending strength is decreased by the micro crack of the ceramics surface as a serious problem. Therefore, the repair of the cracks is one of the important technologies for the practical use of ceramics. This paper describes the repair of the crack for Al₂O₃ ceramics with pulsed YAG laser irradiation and the evaluation of its conditions with Acoustic Emission (AE) method. The specimens of Al₂O₃ Ceramics (i.e. smooth, pre-cracked and laser irradiated specimen) were prepared. The AE signals were measured during 4-point bending test of these specimens. As a result of evaluating the repair condition of the crack with the laser irradiation based on the microscopy and AE method, the following facts became clear. ( 1 ) In the detected AE signals during the bending test of the smooth, the pre-cracked and the laser irradiation specimen, there were features. ( 2 ) In the 270 V-applied voltage or less, the repair effect of the crack was not obtained. There was the recovery of the strength in the 310 V-applied voltage ; however, the behavior of AE signals resembled the feature of the pre-cracked specimen because the damage by the laser irradiation was intense. On the other hand, the bending strength and the AE signals of the irradiation specimen of the 290 V-applied voltage were similar to the features of the smooth specimen. ( 3 ) In addition to the bending strength and the microscopy, it was proven that making observation of the behavior of the detected AE signals during the bending test could carry out the detailed evaluation on the repair condition of the crack.

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In order to investigate the effect of nitriding condition on the fatigue strength at elevated temperature, rotating bending fatigue tests for dies steels nitrided by 5 kinds of nitriding treatment were carried out at room temperature and 450°C. Fatigue strength increased by nitriding at both room temperature and 450°C, due to the surface hardening and the compressive residual stress induced by nitriding. Although the fracture origin was surface slip bands or defects in the nonnitrided steel, i.e. the quenched and tempered steel, all the fractures in nitrided steels originated from inner inclusions at both temeratures. The crack initiation sites in the nitrided steels were inclusions at non-hardened layer or near the boundary between hardened and non-hardened layers.

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Simultaneous damage of cavitation erosion, and slurry wear of SUS 304 was studied by using a test apparatus with the combination of an ultrasonic vibratory cavitation device and a slurry jet device. The tests were carried out in slurry having concentrations from 0 mass percent to 6 mass percent. The slurry was made of tap water and silica sand. Volume loss rate first kept constant irrespective of concentration and then began to increase remarkably at a certain concentration. Thus, a critical concentration was found beyond which silica sand has an effect on slurry wear. Moreover, the volume loss rate of the simultaneous damage is higher than that of slurry wear without cavitation at concentrations exceeding the critical concentration. The acceleration ratio of volume loss rate (simultaneous damage/slurry wear) is as much as 2.4, when the flowing velocity is low (5 m/s) and the slurry is low near the critical concentration.

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