Rotating bending high-cycle fatigue tests were carried out on austempered ductile cast iron (ADI) specimens with different levels of surface hardness. Special attention is paid to the relationship among the surface hardness, shape of S-N curve and fracture mode transition. For the specimen with Vickers hardness H_v<400, the S-N curve is of two-step bending type, similar to that of high-strength steel. That is, a surface-initiated crack dominates the fatigue process for short fatigue life, while so-called fish-eye pattern fractures or internally initiated cracks control the fatigue process in the very long fatigue life region of N_f>10⁷. In contrast, the specimen with H_v>400 shows a typical S-N curve of single bending type. This means that increasing H_v shortens the initiation life of the internal crack and results in the fish-eye pattern occurring even for shorter fatigue life.

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Rotating bending and torsional fatigue tests were carried out on specimens of cast stainless steel. The initiation and propagation processes of microcracks were investigated using the plastic replica method. The main results are summarized as follows. (1) The critical cracks appeared at a microshrinkage cavity in rotating bending fatigue, but not in torsional fatigue. Therefore, the ratio of fatigue limits in cast stainless steel (τ_w/σ_w≒0.77) is larger than that of defect-free isotropic carbon steel (τ_w/σ_w≒0.65). (2) The crack growth laws in rotating bending and torsional fatigue are expressed by dl/dN∝σ^7.8l and dl/dN∝τ^9.0l, respectively. (3) The difference between the dependences of N_f on the stress amplitude in rotating bending fatigue and in torsional fatigue can be explained by the above crack growth laws.

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In this study, an attempt to analyze the fatigue damage in Cr-Mo-V rotor steel based on a probability theory or statistical mechanics was made. The following results were obtained: (a) A model of distributed "driving force" with uniform "fracture resistance" is applicable for the analysis of two phenomena, i.e., the fatigue crack initiation and the decrease of half value breadth of X-ray diffraction profiles with an increase of fatigue damage in Cr-Mo-V steel forging (b) The driving force shows an exponential distribution. The mean value of the distribution appears to be a monotonic function of the nominal strain of fatigue test. (c) It is reasonable to consider that the change of half-value breadth of X-ray diffraction profiles corresponds to the accumulation of fatigue damage before crack initiation.

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Plane bending fatigue tests of an annealed 0.42% carbon steel were carried out on specimens with a small blind hole and smooth specimens, with round or square cross sections, in order to investigate the effect of cross-sectional shape on the fatigue strength. The fatigue strength of a round-sectioned specimen is higher than that of a square one under the same stress. This is mainly caused by the difference in the crack growth rates. The crack growth rate can be expressed by the term σⁿl (small-crack growth law) in each cross-sectional shape, separately. Moreover, by transforming the nominal stress amplitude σ into the plastic strain amplitude ε_p using the cyclic stress-strain curve measured at the specimen surface, the crack growth rate can be uniquely determined by the term εⁿ'_pl, regardless of the shape of the cross section. Based on these results, a conventional prediction method of fatigue crack growth rate and fatigue life is proposed using the small-crack growth law and the cyclic stress strain curve.

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Push-pull cyclic fatigue tests were performed for silicon nitride ceramics with stress-rate-controlled triangular and trapezoidal stress waves at 1 200°C at stress ratio R=-1.0. Then the effects of stress rate and stress hold time on the cyclic high-temperature fatigue were examined. The cyclic fatigue life is shorter under trapezoidal stress wave loading than under triangular stress wave loading, and becomes shorter with increasing hold time under the trapezoidal stress wave loading. Cyclic fatigue lifetime is successfully estimated through the crack extension law based on a slow crack growth mechanism, showing a small cyclic stress effect and dominant stress-application-period effect on cyclic fatigue lifetime. This was also evident from scanning electron microscope observations of fracture surface. Cyclic fatigue life was influenced little by stress waves which have hold time on both the tension and compression sides or only on the compression side. Thus compressive stress hold seems not to induce cyclic fatigue damage.

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The influence of cooling rate on the fatigue reliability of austempered ductile iron (ADI) was investigated with special attention given to a mesoscopic factor of fracture origin. The main results were as follows. (1) The fatigue strength of ADI could be improved by increasing the cooling rate. (2) Different kinds of casting defects were observed at the origin of fatigue crack in three types of ADI, which were produced through rapid, medium and slow cooling rates (designated as R, M and S materials, respectively). These are larger dross, spheroidal graphite and porosity in R, M and S materials, respectively. (3) The mesoscopic factors of fracture origin, which consisted of casting defects and matrix microstructure, could be quantitatively related to the fatigue life.

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An examination of the effect of shot-peening (SP) conditions on the improvement of fatigue reliability of metal-molded ADI was performed with a mesoscopic observation of fracture morphology. The main results obtained were as follows: (1) The 10⁶ endurance limit could be improved with an increase of SP strength, but the endurance limit of 1.0mmA-Sp material was as high as that of 0.8-SP material. On the other hand, the 2×10⁷ endurance limit as not improved with SP treatment. (2) The origin of fatigue cracks of metal-molded ADI as not the pores, since the cooling rate of metal-molded ADI was slower than that of sand-molded ADI. (3) The origin of fatigue cracks was transferred from the surface to the inside with an increase of SP strength, due to the effect of compressive residual stress induced by SP treatment. (4) Although the depth affected by SP treatment of 1.0-SP material was greater than that of 0.8-SP material. The influence of SP strength on the endurance limit of metal-molded ADI was only effective up to 0.8-SP strength because of the existence of a stress gradient.

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A large-scale database MSDRD on mechanical properties of structural materials such as metals, composites and ceramics was constructed by The Research Group for Statistical Aspects of Materials Strength. In this study, fatigue data of carbon steels and alloy steels for machine structural use are obtained from the present database and distribution characteristics of fatigue properties are analyzed by a statistical procedure developed by the authors. It is found that fatigue lives of the respective steels have a complicated distribution pattern depending on the stress level and that mixed-mode Weibull distribution is preferable to represent the fatigue life distribution in a wide variety of steels and a wide range of stress levels. In fact, statistical fatigue properties thus analyzed are in good agreement with the experimental data filed in MSDRD and analytical results are applicable to the reliability design of mechanical structures.

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Fatigue process of bovine compact bone was investigated to clarify the effects of bone tissue on crack initiation and growth behavior during the fatigue process. Then, computer simulation of the fatigue process was performed. The main results were summarized as follows. (1) Fatigue cracks initiated mainly from havers, lacunae and volkmann in bone. (2) Crack growth rate decreased when the crack tip reached bone tissues such as havers, lacunae and volkmann due to blunting of the crack tip. When the crack tip reached the interface between the bone matrix and cement lines, crack growth direction was deflected to the direction of the cement line. (3) Computer simulation of the fatigue process of bone was conducted, in which the above-mentioned interactions between bonetissues and crack initiation and growth behavior were considered. The results coincided well with the experimental results, and showed a strong effect of bone density on the crack growth life.

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In a previous paper, we proposed methods to predict the two-dimensional fatigue crack propagation life in residual stress fields. One method called the "U (crack opening ratio)-estimation" method used the superposition of the effects of external load and residual stresses to evaluate the effective mechanical load acting in the vicinity of the crack tip and the crack opening ratio. This method was extended to three-dimensional cracks. In this paper the extended method was applied to the prediction of propagation life of a semi-elliptical surface crack in a clad steel plate with residual stress. The predicted results were compared with experimental ones conducted by Ogura et al. The predicted crack propagation behavior was in good agreement with the experimental results.

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Although a number of models for the initiation of intergranular cracks under fatigue have been proposed, most of them are not considered to be sufficient qualitatively. In this study, the mechanism of intergranular crack initiation in fatigued copper is investigated on the basis of the observation of dislocation structures and surface topography by means of transmission and scanning electron microscopy. At the intersections of ladder-like structures with the grain boundary, extrusion-type intergranular deformations are frequently observed. It is also found that the ladder-like structures are sometimes formed along the grain and twin boundaries. On the basis of these observations, a mechanism of crack initiation at the grain boundary is proposed.

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Fatigue tests under axial loading were carried out on specimens with a small blind hole in order to investigate the validity of small-crack growth law for large cracks. When the stress level is high, the crack growth rate is proportional to the crack length. However, the relation, dl/dN∝l, is limited to within about 1mm under a constant stress amplitude. The validity of the relation exceeds 3mm if the boundary correction factor is considered in the stress evaluation; that is, regardless of the crack length, small-crack growth law holds under high stress levels similarly to Paris law under the condition of small-scale yielding.

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Explicit expressions for the mode III asymptotic solutions of an unsteadily fast-propagating crack tip had not been derived before. In the present study, first, the governing equations for this problem were derived. Next, to obtain the solutions of the governing equations, a symbolic manipulation system was used. The explicit expressions for the mode-III transient stress and displacement fields were successfully derived for the first five terms (n=0∼4) in asymptotic series. It was found that the transient effects do not appear in the singular and constant-stress terms. but appear only in the higher-order terms greater than or equal to the third order (n&ge;3). These features of the asymptotic solutions agree with those of the in-plane mixed-mode asymptotic solutions obtained in a previous study.

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In order to clarify the statistical characteristics of scatter in the crack growth rate of an age-hardened Al alloy 6061-T6, rotating bending fatigue tests of smooth specimens were carried out at several stress levels. Fifteen or seventeen specimens were fatigued at each stress level and the initiation and propagation behaviors of major cracks were examined for all the specimens. The crack growth rates were calculated based on the smoothed growth curves. The crack lengths at which the growth rates were calculated were 0.03, 0.1, 0.5, 1 and 3mm. The statistical treatments of each set of growth rate data were performed by assuming the Weibull distribution. The scatter characteristics of these growth rate distributions were investigated based on the CV vs σ_a relation (CV : coefficient of variation, (σ_a : stress amplitude).

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To investigate the safety margin in the design of nuclear plant piping, plastic collapse behavior of statically indeterminate beams with a crack was numerically simulated as the preliminary study. From the simulations, the following results were obtained. When no crack propagation occurs prior to plastic collapse, the ultimate strength of a statically indeterminate beam is much higher than that of a statically determinate beam. As a crack propagates in a statically indeterminate beam before plastic collapse, load does not decrease provided that dj/da of materials is higher than a critical value which is determined by J_ic, yield strength, crack length, length and flexural rigidity of the beam, because sufficient redistribution of the bending moment occurs in the beam under the condition. In this case, the reduction of the ultimate strength caused by cracking is small. Thus statically indeterminate structures have a higher safety margin.

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Many small pores, which are nucleated through sintering and thermal treating processes, remain in alumina crystals. It was observed that when a crack crossed a pore, the pore divided the crack plane into two planes having different height level, and then formed a crack surface misfit. On the fracture surface, this misfit originating at a pore was observed as though "tail" Therefore, this crack surface misfit was named "Pore tail". Pore tail has two important parameters. One is tail length, and the other is tail direction. Pore tails on fracture surfaces of sintered Al₂O₃ were observed precisely and the fracture origin could be found by analyzing the morphology of pore tail. It is concluded that "pore tail" is an useful sign as well as the "Mirror", "Mist" and "Hackle".

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The fracture toughness of glass-fiber-reinforced thermosetting resin, molded by the transfer molding process is measured experimentally. The glass fiber is oriented unidirectionally near the surface of the plate and randomly inside the plate. The specimen is modeled by the two-layer model. The fracture toughness is measured using three-point bending specimens with different thicknesses. They are made by cutting off the surface of the bulk plate. The specimen made of the surface layer shows higher fracture toughness than that made of the central part. Three-dimensional FEM analyses are conducted considering the difference of the fiber orientation. It is found that the existence of the surface layer in the thickness direction has little effect on the fracture toughness, and that in the width direction increases the apparent fracture toughness. Residual stress distributions in the plate are measured by the layer removal method, coupled with the FEM calculation. The effect of residual stress is studied by two-dimensional FEM. It is shown that crack tip stress decreases with residual stress, which agrees with the experimental results qualitatively.

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A recently proposed infrared radiation heating method is successfully applied to float glasses to evaluate thermal shock parameters. This method makes it possible to measure the parameters directly from the electrical power charge. This paper discusses how the thermal shock parameters should be estimated as one of the physical properties of the material concerned, and as a function of the temperature tested. The stress intensity factors are also analyzed numerically for a disk with an edge crack where the disk is charged by a constant heat flux. The thermal shock fracture toughness, R₂c, of a float glass is measured by the IR heating technique, and is compared with the estimated thermal shock fracture toughness, λK₁c/Eα, which is combined by the temperature-dependent material properties, such as thermal conductivity, λ, fracture toughness of the material, K₁c, Young's modulus, E, and coefficient of linear thermal expansion, α, where these values are measured individually. It is shown that the experimental thermal shock fracture toughnesses are coincident with the estimated values.

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Tantalum (Ta) has excellent corrosion resistance, but is rather expensive. Therefore, Ta has been used for critical components in chemical plants operating in highly corrosive environments. However, one typical degradation phenomenon of Ta is hydrogen absorption embrittlement. The object of this paper is to present the case study of a failure analysis on a Ta sheath tube of a thermometer used in a sulfuric-acid-containing environment. In the analysis, the nondestructive evaluation (NDE) method for estimation of hydrogen concentration and the changes in mechanical properties of Ta with hydrogen absorption was examined. Furthermore, with the aim of life extension of the components of interest, a hydrogen desorption procedure to recover the mechanical properties of the samples was also examined. Based on NDE of hydrogen concentration of Ta and hydrogen desorption heat treatment on service-exposed materials, life estimation and management procedures of components in chemical plants are proposed.

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This paper is concerned with a new boundary element method for two-dimensional orthotropic thermoelasticity in steady heat conduction. The fundamental solution can be obtained through coordinate transformation for the present case where the principal axes of orthotropy do not coincide with the coordinates. The boundary integral equations are regularized up to a weakly singular order. In this case, the singularities of the fundamental solution can be cancelled out in advance, and the resulting integrals can be evaluated very accurately using the standard Gaussian quadrature. The proposed solution procedure is applied to some examples, and through discussion of the results obtained, its usefulness is demonstrated.

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The boundary element method formulation is applied to a forced steady-state vibration problem of thin-walled plate structures. The integral equations for in-plane motion are derived using three types of fundamental solution : one for elastostatics, and the conventional one for elastodynamics and its expanded form. On the other hand, the static fundamental solution is employed for the derivation of the integral equations for out-of-plane motion. All the integral equations used in the analysis are regularized up to an integrable order and then discretized by means of the boundary-domain-element method. The entire system of equations for the plate structures composed of plate elements is obtained by assembling the equations for each plate element to satisfy the equilibrium and compatibility conditions on the assembled edge. Numerical analysis is carried out for a few examples, and the computational aspects of the different formulations of in-plane motion are discussed.

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This paper is concerned with an approximate three-dimensional analysis of thermal stresses in a nonhomogeneous plate with temperature variation and nonhomogeneous properties only in the thickness direction. The nonhomogeneous plate is approximated as a laminated plate consisting of different homogeneous and isotropic layers which are perfectly bonded to each neighboring layer. The transient temperature field is analyzed by Vodicka's method for a heat conduction problem in one-dimensional composite regions. The nonhomogeneous thermal and elastic properties are restricted to those symmetric with respect to the mid-plane of the plate. The three-dimensional thermal stresses are analyzed using the solutions developed by Rogers and Spencer, which are expressed in terms of the solution of the approximate, two-dimensional, thin-plate, governing equations for an equivalent homogeneous plate.

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In this paper, making use of the strain increment theorem, the transient thermoelastoplastic bending problem of an infinite plate is discussed. In order to develop the analysis for the temperature field, the methods of Fourier cosine and Laplace transforms are introduced, and the theoretical solution is obtained. On the other hand, for the development of the thermoelastoplastic field, both the analytical technique of Airy's stress function method for thermoelastic behavior and the numerical technique of the finite difference method for the plastic behavior are applied. However, difficulty in applying the finite difference method may occur. It is clearly impossible to cover an infinite region with a finite difference grid. Thus we have introduced the assumption that plastic deformation is restricted to a local region. Thereafter, based on Dixon's experimental results that the stress field outside the plastic region is the same as the elastic stress field, namely, the elastic solution prevails outside the plastic region, the finite difference method is applied to the finite region predicted to show the plastic deformation. Some numerical results, such as the plastic deformation in the out-of-plane direction, are shown in figures and discussed briefly.

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In this paper, the Mooney-Rivlin model is employed as the hyperelastic potential function and three methods are discussed for introducing the incompressibility constraint in FE formulation, i. e., the Lagrange multiplier method, the penalty method, and the mixed interpolation with projection, which incorporates the features of the former two methods. Their similarities and differences are also pointed out. Then basic problems are systematically analyzed using these mixed finite elements and performance-based comparisons of resultant numerical behaviors are subsequently applied to clarify the desirable characteristics required to carry out reliable finite element analysis of incompressible hyperelastic materials

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When a sheet is drawn over a die corner, the sheet is subjected to stretch bending and subsequent unbending. This paper deals with the deformation behavior of sheet metal laminates in the stretch bending/unbending process. For two-ply laminates of aluminum/SUS430 and three-ply laminates of SUS304/aluminum/SUS430, the behavior of sheet thickness change and the loss of stretching force (i. e. the plastic energy dissipation) in the process are discussed based on the experimental results and the corresponding numerical simulations. The results show that the amount of change in sheet thickness increases with decreasing die radius. Especially for the two-ply laminate of aluminum/SUS430, the deformation behavior is strongly affected by the sheet-set condition of whether a die-contact metal is aluminum or SUS430. The loss of stretching force is affected by the die radius and the friction characteristics of the die-contact metals.

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The analysis of the stress distribution around a defect near the contact area and the effect of the defect on the contact pressure distribution is one of the most important problems in tribology. In this work, for a contact problem of an elastic half-plane containing a circular hole, the hoop stress around the hole is calculated using the singular integral equations method. For the contact pressure distribution, two models are considered. One is the Hertzian contact pressure distribution, and in the other, the effect of the circular hole on the contact pressure distribution is taken into consideration. In these analyses, the circular hole is replaced with a continuous array of edge dislocations and the traction-free condition on the surface of the circular hole is reduced into a set of singular integral equations in which the dislocation density functions are unknown. In solving the singular integral equations numerically, the Hilbert inversion formula is utilized. From the results of the numerical calculations, it has been clarified that the hoop stress around the hole becomes tensile at some parts, and that in some cases, the contact pressure deviates from the Hertzian one due to the existence of the circular hole.

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This paper discusses the dynamic strain and stress distribution of adhesively bonded joints having different configurations. The selected configurations are tapered lap and scarf joints. The dynamic deformation of the joints under impact loading is calculated using the finite element method. The calculated strains are compared with the experimentally measured dynamic strains. The bending effect for the tapered lap joints are more remarkable than those in the scarf joints when the joints are subjected to impact loading. Stress waves propagate in a complicated manner due to the bending effect and the reflection of the waves for the tapered lap joints; however the waves propagate smoothly for the scarf joints. The dynamic stress in the adhesive layer increases for the tapered lap joints with increasing in time, but the change in dynamic stress with respect to time is negligible for the scarf joints.

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The spot-welded joint is widely used as a structural part. Therefore, the nondestructive inspection of the welded region is very important for quality control. In this study, by parallel scanning over the welded region by ultrasonic inspection with a point-focussed probe, its geometry in resistance spot welding of a two-plate joint was inspected. As a result of the inspection, the periphery of the spot weld was identified by the sharp minimum pattern in the scanning graphs of a sound spot weld. when the spot weld included a shrinkage cavity or blow hole in the welded region, different patterns appeared in scanning graphs, so that the tension shearing load determined by testing was also different in the case of these patterns. Therefore, it is probable that the information for evaluating the load is included in the scanning graph obtained by ultrasonic inspection. Our aim is to develop a methodology for nondestructive evaluation of the function of the spot-welded joint. The present study is an attempt to construct an evaluation system for tension shearing load of the joint. The system consists of an ultrasonic inspection device and neural network. The network learned the pattern sets dealing with the interaction between the scanning graphs and the tension shearing load of spot-welded joints. The output from the system was examined by comparing it with the experimental results, and the validity of the system was confirmed.

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Isotactic polypropylene (PP) samples were subjected to stress-relaxation experiments after simple tensile tests at four different strain rates and at three levels of temperature. The relaxation moduli were determined in the range of temperature between 20 and 80°C with a relaxation period of 1 200_s duration. The activation energy value of the shift factor was determined using the time-temperature superposition principle. The calculated stress-relaxation curves and the calculated stress-strain ones were obtained from constitutive equations based on an overstress theory in which the temperature dependence of viscosity and the activation energy were considered. The temperature dependence of viscosity could be represented by an Arrehnius-type equation. The calculated stress-relaxation curves and the stress-strain ones were obtained using the proposed constitutive equations and compared with the experimental data. The proposed constitutive equation based on the overstress model explains well the viscoelastic plastic behavior of PP samples.

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In this paper the stress concentration problems of fillet in round and flat bars under tension and bending are considered. First, the body force method (BFM) is applied and the stress concentration factors (SCF_s) are systematically calculated with good accuracy. Second, SCF_s of fillet in a semi-infinite plate (K_tf) are presented in the form of formulae and charts on the basis of the results of the BFM. Third, using K_tf, expended Neuber's approximate formulae are proposed for fillet in a simple form which are useful for a wide range of geometries of fillet within l0% error. Next, as a result of comparison of these Neuber formulae and the results of BFM, correction factors are given in the form of formulae by applying the least squares method. Finally, it is found that the correction factors with the Neuber formulae give accurate SCF_s within 1% error for a wide range of geometrical conditions.

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The split Hopkinson bar (SHB) method has been widely used to study mechanical behavior of materials at high strain rates. In this paper, we present a new kind of SHB technique to study dynamic behavior of materials having low characteristic impedance, such as polymers or composite materials. To enable better matching of characteristic impedance between specimens and stress bars, polymethyl methacrylate (PMMA) bars, which are considered to be viscoelastic, and whose viscoelastic properties were determined in advance through preliminary experiments, are used as the input and output stress bars. In this method, the wave analysis of the stress pulses is executed in the frequency domain. Strain histories of the PMMA stress bars resulting from a SHB test are resolved into frequency components by the fast Fourier transform, and mean stress and strain of the specimen are subsequently determined as functions of frequency. Employing the Fourier inverse transformation, the stress-strain relation of the specimen can be obtained. It is found that the proposed SHB method provides reasonable estimations of the dynamic properties of materials having low impedance, such as polymers.

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Tensile stress field induced by fibroblasts in collagen gel are investigated by the cell culture method. Specimens of thin collagen gel membrane, within which cells proliferate, are subjected to various initial and boundary conditions, and cell stretching, cell orientation, cell migration, cell proliferation, induction of tensile stress field in collagen gel, and geometry transformation of specimens are observed. It is demonstrated that fibroblasts induce tension, change their orientation along the tensile direction, and create structures composed of collagen fibers. A hypothetical mechanism for such automorphogenesis is proposed wherein fibroblasts induce tension and produce tense collagen fibers. Cells then stretch along the tense fibers, which increases tension in this direction. Thus this mechanism functions as a positive feedback loop which enables cells to form structures with collagen fibers according to the mechanical environment.

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Residual stress at butt-welded pipe joints and butt-welded plate joints was studied using inherent strain analysis. Through-thickness residual stress at an 18.2-mm-thick multipass butt-welded pipe joint, which was assumed to be axisymmetric, was obtained by measuring elastic strain due to cutting and by calculating the distribution of inherent strain. The stress distribution obtained from the inherent strain analysis agreed with the values measured directly from the specimen surface. The measured values are not used as the input data for inherent strain analysis. The inherent strain in the pipe joint resembles that in a plate joint with the same thickness and welding conditions. Residual stress distribution in the pipe joint calculated using the inherent strain of the corresponding plate joint agrees well with the original distribution. These results indicate that the inherent strain analysis is applicable to a great variety of joint configurations.

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The effects of various spin tensors on strain localization phenomena predicted by a noncoaxial viscoplasticity model are investigated by finite-element analysis. The noncoaxiality between the plastic strain rate and the stress arises from a directional dependence of the plastic strain rate on the stress rate. The spins considered here are continuum spin, plastic spin, Green-Naghdi spin and Euler spin. Using these spins, the simple shear problem and tension and compression problem involving shear band formations are analyzed. The spins that can produce monotonic stress responses in simple shear noticeably restrain the shear band formations. Actually, in large torsion tests of metals which were newly carried out, we observe monotonic torque responses. Consequently, it is found that it is difficult for the noncoaxial model to predict realistic strain localization phenomena if we choose a spin on the bases of the results of torsion tests. This inconsistency should be recognized as an important problem that must be subjected to further investigations.

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Based on experimental data using SCM435 steel obtained by us, it was found that flow stress and relaxation properties of metallic materials depend on both viscoplasticity, which responds quickly to strain rate change, and strain ageing, which responds slowly. In this study, both flow stress under nominal stress-controlled tensile loading in which strain rate increases gradually, and creep properties which show a gradual decrease in strain rate, are investigated experimentally and theoretically. It is shown that the viscoplastic constitutive model proposed by us, which takes the time dependence of strain ageing into consideration, has good applicability in describing stress-controlled inelastic behavior, including the creep property.

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Plastic deformation processes in metalworking may be classified into only a few categories on the basis of the type of force applied to the workpiece. These categories are direct-compression, indirect-compression, tension, bending, and shearing processes. In the numerical analysis of super-plastic deformation in these metalworking processes, strain rate sensitivity m determined from a tensile test is considered as a unique reliable constant of mechanical properties. Superplastic flow is believed to be observed in the simultaneous appearance of grain boundary sliding and some accommodated process. m value is a macroscopic material constant, and can be determined through simple calculation by using tensile test data. It is natural to think that the extent of both grain boundary sliding and accommodation may alter the m value. Each superplastic material has its own m value under a specific strain rate range and temperature. The different strain rates from uniaxial stress, however, can naturally change the extent of grain boundary sliding and accommodation, and therefore, change the value of m also. This phenomenon has never been studied. Strain rate sensitivity determined from compression test appears to be useful for analysis of compressive processes such as forging, rolling, extrusion and drawing. In this study, special attention will be paid to the determination of strain rate sensitivities in uniaxial compressive deformation. The friction at the tool-workpiece interface leads to a barreled specimen profile, and it is difficult to measure compressive stress and strain. A much more suitable test for determining strain rate sensitivity is the Siebel type compression test. The advantage of this test is that it eliminates inhomogeneous deformation by using a cone-shaped punch and a corresponding specimen geometry at the same time. The state of compression was analyzed by using the finite element method (FEM). Upon comparison of the m values in compression and tension tests, a small difference of the two m values is recognized and they are almost the same level.

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In this paper, we describe the sensitivity analysis of buckling load of the simply supported symmetric laminated plate with respect to the material constants and the orientation angle of each ply. The objective is to clarify the dominant factor for the buckling load. Since the buckling load is obtained as the minimum eigenvalue by Galerkin's method, the sensitivity of the buckling load is formulated by making use of the sensitivity analysis of the eigenvalue problem. When the buckling load corresponds to a multimodal eigenvalue, the sensitivity should be obtained by another method which utilizes the directional derivatives. However, in the typical case of a double eigenvalue for the laminated plate, it is shown that such a method is not required. Finally, the dominant effect on the buckling load sensitivity is evaluated through numerical simulations. Then, it is demonstrated that the coupling term plays an important role in buckling sensitivity.

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Optical computed tomography is a medical imaging modality in which red and near infrared light are used to probe biological tissues. Images are obtained by mapping the absorption coefficient of the tissue. Realization of clinically viable optical computed tomography depends on the development of reliable and fast methods of obtaining inverse solutions of the photon diffusion equation which describes the photon migration in biological tissues. In this paper we use a linearization and regularization technique and a finite element method to invert the diffusion approximation, and we propose the use of a zooming method as a way to improve the resolution of the images. We compare the performance of three regularization methods, i. e., the truncated singular value decomposition, Tikhonov regularization, and Phillips-Twomey regularization, The usefulness of the zooming method is demonstrated.

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To examine the fundamental characteristics of the geometric and mechanical structure of amorphous material on an atomic scale, molecular dynamics simulations for the melting-rapid quenching process of a single-component atomic system are carried out under the constant-pressure condition. The interatomic interaction is assumed to be described by the FS-potential for α-Fe, which is a many-body potential, and reproduces a bcc crystal. Voronoi polyhedron analysis and calculation of the radial distribution function and the mean square displacement are performed to study the transition of characteristic features of the atomic structure in the process. It is proven that the obtained quasi-stable atomic structures have a radial distribution with the second peak separated into two apexes and are almost the Voronoi polyhedron which contains more than 5 pentagonal planes. Strong correlation between the volume of the polyhedron and the first invariant of the stress is recognized and the atom which has the (0 0 12 0)-type polyhedron is subjected to high hydrostatic pressure of up to 30 GP_a.

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Stress-induced migration near the grain boundary in Al thin interconnect is studied by means of an ab initio molecular dynamics simulation formulated on the basis of the effective-medium theory. The consistent grain boundary model is used, whose geometry is characterized by Σ-value. It is shown that besides Σ-value, existence of high-energy sites near the grain-boundary is an important factor which promotes nucleation and growth of voids. In the bulk state, grain boundary decohesion due to coalescence of voids is dominant in the high-boundary-energy case, and growth of a small number of voids leads to breakage of the Al wire with the grain boundary of low energy and high consistency. If there exists a free surface, marked decrease in thickness near the grain boundary occurs and a narrow groove develops in the high-energy case.

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For the design and production of new advanced composite materials and structures, the need for numerical simulations of their mechanical behaviors is increasing. There are two requirements for the simulation. First, microstructural architectures must be considered. The homogenization analysis method, which is an applied mathematical method developed from late 1970's to early 1980's, seems to be the most appropriate method for modelling microstructures in current literature. The second requirement is the solution of nonlinear problems. Therefore, we have developed a nonlinear homogenization method where both geometrical nonlinearity due to large deformation of microstructures and material nonlinearity due to microscopic fractures are considered. The new formulation of the nonlinear homogenization method is based on a stepwise linear algorithm with initial stresses. Two examples, i. e., biomechanical problem and fracture behavior of woven fabric composite material, were solved.

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An acoustic emission (AE) technique was developed to study the adhesional characteristics of TiN thin film deposited on SKD and SKH steel using plasma CVD technique. Pin-on-disk type wear testing machine with acoustic emission sensor was used to measure the relationship between the AE count rates monitored through the pin and the wear rate of the steel materials deposited by the thin film and to investigate the adhesional strength of the film. In addition, the applicability of the Root Mean Square (RMS) value derived from the AE signal to the evaluation of the wear and frictional properties of the steel was discussed, The experimental results showed that the relationship between the wear rate and the RMS value was significantly dependent upon the mechanical properties of the pin with a deposition of the film. The frictional coefficient derived from the interface between the pin and the disk increased slightly with an increase in the amount of indentational load applied to the pin. It may also mentioned that the present results demonstrate sufficient promise of the ability of the AE technique to be applied to the evaluation of the thin film deposited on metallic materials.

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Wire bonding, which is a process of connecting a semiconductor chip and a lead frame by a thin metal wire, is one of the important processes in electronic packaging. This paper presents the damage estimation of a G_aA_s chip during the gold (Au) wire bonding process. The Au wire bonding process is carried out by pressing an Au ball made at a tip of the Au wire on a semiconductor chip and vibrating it by ultrasonication. High contact pressure is useful for shortening the process cycle; however, it sometimes causes damage to the semiconductor chip. Elastic plastic large-deformation contact analysis is performed and the distributions of the stress in a G_aA_s chip is investigated. The possibility of fracture under the usual wire bonding pressure is expected in a G_aA_s chip.

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Volume resistivity of carbon-fiber-filled silicone oil was measured under shear flow. A rheometer with double-cylinder-type sample cell was employed to measure the volume resistivity. Both the outer and inner cylinders were used as electrodes. The reproducibility of volume resistivity measurements under shear flow using the double-cylinder-type cell was better than that of the parallel-plate-type cell. The critical values for a sudden increase in volume resistivity were found when both shear rate and volume fraction of fibers were varied. These experimental results were interpreted on the basis of a quantitative model for the formation and destruction of an electric circuit by fibers. The results were also correlated with the interaction and orientation of fibers. A new method of measuring volume resistivity was established as a means of investigating the fiber orientation distribution in the matrix for the conductive-fiber-filled insulating matrix composite materials.

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For design and development of advanced textile composite materials, evaluation of their bending strength is essential. However, as far as we know, there has been no literature on the numerical simulation predicting the bending strength of textile composites. In this paper, we show a methodology for calculation of the bending strength for woven fabric composite materials by the homogenization method. Both the location and the cause of microscopic fracture in a fiber bundle and resin are discussed in detail. The effect of the pitch of woven fabrics on the bending characteristics is also noted.

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