The purpose of this study was to determine the effect of nitriding, used as a surface-modification method on the fatigue strength of metals. By conducting experiments for evaluation of the factors controlling the fatigue strength of nitrided pure iron and titanium, we obtained the following results. (1) The improvement in the fatigue strength of nitrided pure iron results from the fact that the hardened case existing under the compound layer suppresses the propagation of cracks. However, the fatigue strength of nitrided pure titanium decreases because a crack initiated in the compound layer at a low stress level directly results in the overall fracture of the brittle hardened case. (2) There is a Hall-Petch relationship between grain size and the fatigue strength of nitrided pure iron and titanium. This effect does not influence the fatigue strength of nitrided pure iron because the grains do not grow during nitriding. In contrast, the above effect results in the inferior fatigue strength of nitrided pure titanium because grain growth inevitably occurs during nitriding at high temperatures.

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Mod 9Cr-1Mo steel is potential material for the steam generator in fast breeder reactors. One of the features of creep-fatigue life property of the steel is the significant life reduction in compressive hold tests. In order to study the possible reason for the life reduction in compressive hold tests, microcrack propagation behavior under high-temperature fatigue in vacuum and in air were periodically observed. Microcracks propagated in the direction normal to the stress axis in vacuum while they propagated in the maximum shear direction in air. It was found that the crack propagation rate in air was faster than that in vacuum until the crack grew to 1 mm length due to an oxidation effect which was insignificant for cracks larger than 1 mm. Acceleration of the crack propagation rate in the compressive hold test was considered to be caused by accumulation of tensile strain in the center of the specimen due to an imbalance in the strain distribution between the tension and compression sides.

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The estimation of fatigue crack propagation rate in structural materials and the clarification of crack propagation mechanism are extremely important. In this study, a low cycle fatigue test and a fatigue crack propagation test were carried out. Then the relation between the low cycle fatigue strength and the fatigue crack propagation rate was investigated, and the factors affecting the fatigue crack propagation rate were clarified on the basis of a damage accumulation model. Consequently it was found that Young's modulus E, the fracture ductility ε_f and the yield strain ε_N considerably affected the fatigue crack propagation rate da/dN, and the following equation involving the effective stress intensity factor range ΔK_eff was proposed in order to consider the effect of material properties on the fatigue crack propagation rate. da/dN=506(ΔK_eff/E)^2.79/(ε_f√(ε_y))

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Dynamic crack propagation in epoxy specimens was studied using the method of caustics in combination with a Cranz Schardin high speed camera. Single-edge-cracked tensile speciments were fractured under pin-loading conditions so that cracks could undergo acceleration, deceleration and re acceleration stages in one fracture process. Dynamic stress intensity factor K_ID, crack velocity a and acceleration a were evaluated in the course of crack propagation to examine the effects of a and a on K_ID, Results showed that a and a strongly affected the values of K_ID, and for a constant a the decelerating crack had a larger value of K_ID than the accelerating or re-accelerating crack. Also, it was found that K_ID could be expressed as two parametric functions of a and a for epoxy specimens.

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As TiNi shape-memory fiber-reinforced epoxy matrix composite with given prestrain ε_r in TiNi fiber at room temperature is subjected to temperature increase so as to exceed austenitic finish temperature (A_f), the TiNi fiber shrinks to its original length by ε_r resulting in compressive stress in the epoxy matrix. This compressive stress in the matrix was utilized to reduce K₁ of a sidenotched TiNi/epoxy composite, which was confirmed by the present experiment with two types of temperature control : Joule heating by passing an electric current through TiNi fibers and subjecting the composite to a constant temperature using an iso temperature furnace. An analytical model baged on Eshelby's model is developed in order to compute the averaged matrix compressive stress which is then coupled with Tada's formula for computing ΔK₁. The experimental trend that |ΔK₁|increases with prestrain ε_r was in good agreement with the predictions based on the present model.

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In this paper, interference between an eliptical inclusion and a crack is considered. The problem is analyzed by the body force method, using a solution for point force doublets outside an elliptical inclusion in an infinite plate as a fundamental solution. Numerical results for the crack tip stress intensity factors are presented for a case of two interface cracks and for a case of a kinked crack, a portion of which lies on the interface. Based on the numerical results, the effects of the geometry and elastic properties of the inclusion on the stress intensity factors are investigated.

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Delamination crack growth behavior under mode II cyclic loading was investigated with interlayer toughened CFRP laminate, T800H/3900-2. Fatigue tests under a constant maximum energy release rate showed that the growth rate was independent of the crack length. Then, load-shedding tests were carried out under various stress ratios. The fatigue crack growth resistance of the interlayer toughened CFRP was higher that of conventional CF/epoxy. The crack growth rate under various stress ratios was well correlated to the stress intensity range near the threshold region. Thus, the mechanism of fatigue fracture under mode II loading is completely different from that under mode I loading. Fractographic observation showed that the fatigue fracture occurred on the principal shear stress plane. Large hackle patterns which are typical for static mode II tests were not observed under fatigue loading. A mesoscopic fracture model was proposed to explain the microscopic observation and the stress ratio dependence.

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The propagation behavior of a two dimensional continuous crack is simulated in a SiC particle dispersed Al₂O₃ matrix composite along with the related variation of fracture resistance with crack extension, Stress intensity factors at the tip of a multiply deflected crack during propagation are calculated numerically by the body force method, in which stresses around the crack tip are represented by superposing externally applied stresses on residual ones. The fracture resistance decreases with crack extension when the crack approaches a particle, but increases up to 2.5 times the matrix toughness when it leaves the particle. The residual compressive stress in the radial direction, resulting from the thermal expansion mismatch between SiC particle and Al₂O₃ matrix, enhances the apparent fracture resistance of interface. The crack propagation behavior is simulated for both cases of weak and strong interface under the assumption of no failure in dispersed particles. It is found that the influence of interfacial toughness is minimal for the entire fracture resistance. By averaging the simulated fracture resistance curves, a rising R-curve behavior is observed.

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In the present paper, a formulation to analyze a singular stress field with a crack is discussed. In this formulation, function distributions of the displacement and traction defined on the boundary are expressed by superposition with singular term and regular one. The former is a main term of the function expressed with stress intensity factors K_I (mode I) and K_II (mode II), and the latter is a discrete function approximated with non-comforming polynomials and nodal values. Employing the singular functions, the stress intensity factors are determined directly with high accuracy, and the distrlbutions of the displacement and stress in the vicinity of the crack tip are obtained precisely. Through the computation of typical models with a crack which is a mode I, mode II or mixed modes of I and II, the validity of the present formulation is confirmed.

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Fracture characteristics of a TIG welded joint and heat treated materials of JNI, which is an austenitic stainless steel recently developed for use as a cryogenic structural material, were evaluated by the small punch (SP) testing method using miniaturized specimens at 77K and 4K. The area under the load displacement curve up to the maximum load was defined as the SP energy to evaluate the fracture characteristics of the materials in SP tests. Fracture surfaces were also observed using a scanning clectron microscope. Solution-treated JN1 steel showed ductile fracture behavior with a high SP energy value even at 4K, and serrations were observed in the load displacement curve at 4K, In the weld metal and fusion line specimens of the TIG welded joint, serrations also occured at 4K, but the fusion line specimens showed lower SP energy values than weld metal and solutiontremed JN1 steel at 77K and 4K. In addition, the fusion line specimens showed pop in behavior at 4K, that is, cracks approximately 0.1-1mm long were induced by pop-in in the vicinity of the fusion line at the early stage of loading. JN1 materials heat treated under the conditions of 650°C 5h and 700°C In showed similar fracture characteristics to those of the fusion line specimens at 77 K and 4 K except for the cracks induced by pop-in. With increasing temperature and time of heat treatment, SP energy decreased significantly, and the fracture surface showed a transition from ductile dimple to brittle intergranular cracking. It is shown that the SP testing method is useful for evaluating the fracture behaviors of microscopic regions such as the fusion lines of welds at cryogenic temperatures.

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In order to evaluate the shape of a 3-D surface crack nondestructively, the ICFPD (Induced Current Focusing Potential Drop) technique was used. Using an ICFPD probe, the values of surface potential drops were measured around the surface crack introduced on a SUS 304 stainless steel plate and the size and shape of the crack were estimated based on the calibration curve plotted with the measurement results for 2-D cracks. The evaluation showed that by this technique the maximum depth of the 3-D crack could be exactly estimated but the depths near the end of the crack were underestimated. From the measurement of the distribution of the surface potential drop, it was shown that the flowing path of the induced current bypassed the end of the crack on the surface if the depth of the crack at the measured points was longer than the length to the end of crack. The difference between the true and the entimated crack depth was shown to be linearly proportional to the ratio of crack depth to surface crack length. An estimation based on this relation suggested that the shape of a 3-D crack could be estimated by the ICFPD technique.

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In this paper, the authors propose a simple method for estimating the stress field near a notch in a plate under in-plane loading with strain gages. In this method, the stress field near a notch in practical and complicated structures can be estimated easily by determining the unknown coefficients of stress field expressions in a plate containing a free circular hole that is inscribed in the notch root. For this purpose, experimentally, it is necessary to measure only strain values at four points on a cireular are whose radius is 1.5 times the notch root radius using rosette-type strain gages. And the unknown coeffeients are determined from these strain values with the method of least squares. We have demonstrated this method using eight types of the notched plates that differ in shape. Consequently, estimation crrors were significant in a few cases that the shapes of the notched plates are extreme. However, we obtained estimation results accurate enough for practically except the cases above mentioned.

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The higher accuracy and better quality are required on the gear drives ; It's necessary to study the strength and profile of ellipsoidal gear which is predicted to be used widely in the future. However the stress distribution and deformation of the gear's teeth when a load is applied to the boundary have not been extensively analyzed. In this study, the stress distribution and deformation of an ellipsoidal gear's teeth is analyzed using a complex stress function. This method makes it possible to obtain any ellipsoidal gear's profile using a mapping function and to analyze the stress distribution and deformation correctly and easily using above mentioned stress function.

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The caustic pattern which is formed by a concentrated load Pacting on a half-plane of an optically anisotropic material is theoretically considered, and the method for determining the value of P by caustics for birefringent as well as optically inert materials is shown. The effect of initial curve γ 0 on the evaluation of P is experimentally studied. It is shown that the value of P determined from the caustic pattern is influenced by the radius of γ₀ when the size of γ₀ is less than half the thickness of the specimen. The reason for this phenomenon is discussed from two points of view, namely, the existence of three-dimensional stress and the effect of load distributed by the shape of a loading pin. Moreover, a technique for mesuring the optical constants in caustics is proposed. These constants can be easily determined from the caustic pattern obtained by the concentrated load, and the validity of the technique is experimentally ascertained for several polymers such as Plexiglas, epoxy restn and polycarbonate. The absolute photoelastic coefficients and the photoelastic sensitivity of these materials are also obtained by this method.

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In this study, an analytical method of development of thermoelastic problems for a medium with Kassir's nonhomogeneous material properties is developed. For the isothermal problems of such a nonhomogeneous body, an analytical method of development has already been given by Kassir under the assumption that the shear modulus of elasticity G changes arbitrarily with the variable z of the axial coordinate according to the relation G (z)=G_vZ^m. However, no analytical procedure for the thermoelastic field has been established up to date. In this study, we introduce the thermoelastic displacement potential function, and an analytical method of development of the axisymmetric thermoelastic field is proposed. Assuming that G, thermal conductivity λ and coefficient of linear thermal expansion α vary with the variable z of the dimensionless axial coordinate according to the relations G (z)=G₀z^m, λ (z)=λ₀z¹, α (z)=α₀zⁿ, the axisymmetric temperature solution in a steady state for a semi-infinite body is obtained, and the associated thermal stress components are evaluated theoretically. Numerical calculations are carried out for several cases taking into account the variation in nonhomogeneous material properties, and the numerical results are shown graphically.

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We proposed a method for predicting the noise insulation levels of a resin composite cover, where the damping effect of resin is considered using viscoplastic constitutive equations and the sound pressure level is calculated by acoustic-structure coupling analysis. In this method, several parameters in the constitutive equations must be determined by experiments. It is, however, inconvenient to determine the parameters for every possible resin composite material. In this paper we introduce the homogenization method, which makes it possible to obtain the mean material constants of a composlte material and to consider the effect of shape or size of a reinforcement. First the formulation of the dynamic analysis of a resin composite material with the homogenization method is introduced, where the resin is assumed to be viscoelastic and the reinforcement is assumed to be elastic. Second the formulation is applied to the acoustic structure coupling analysis. Finally it is demonstrated that the shape or size of the reinforcement has a great influence on the noise insulation of a resin composite cover through several numerical examples.

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In this paper we propose a simplified evaluation method of the fundamental natural frequencies or the compressive buckling loads for simply supported symmetrically laminated plates with bending twisting coupling. A highly accurate approximate solution for the fundamental frequencies or the bucking loads is obtained using the second-order perturbation method. The fundamental frequencles or the buckling loads for any kind of symmetric laminates can be evaluated from the contours on the lamination parameter planes obtained using the perturbation method. The present evaluation method may be an effective design tool for evaluating vibration and buckling of general symmetric laminates.

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The present study treats the nonlinear deformation characteristics of air-dome membrane structures under uniform internal pressure. An incremental load method based on a linear approximation or a mathematical programming method is applied to analyze the nonlinear deformation behaviors by minimizing the potential energy. As examples of numerical analyses, the deformation characteristics are shown for elliptic membrane structures with various kinds of ffber orientation angles, for a membrane structure with an initial deflection, and for cable-reinforced membrane structures.

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This paper is concerned with an analytical formulation and a numerical solution of thermal stress and deformation for moderately thick shells of revolution made of functionally graded material (FGM) subjected to thermal loading due to fluid. The temperature distribution through the thickness is experessed using a curve of high order, and the temperature field in the shell is determined using the equations of heat conduction and heat transfer. the equations of equilibrium and the relations between strains and displacements are derived from the Reissner-Naghdi shell theory. The fundamental equations derived are numerically solved using the finite difference method. As numerical examples, functionally graded cylindrical shells composed of SUS 304 and ZrO₂ subjected to thermal ioads due to fluid are analyzed. The results show that the present method gives correct temparature distributions and that the temperature distributions, stress distributions and deformations vary significantly depending on the compositional distribution profiles in FGM.

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A new press working with increasing thickness for automobile parts was examined using elastoplastic FEM analysis. The main results were obtained as follows : (1) The mechanism of increasing thickness was determined by the FEM analysis parameter of strain. That is, the effect of increasing thickness of plate was achieved by increasing the strain in the circumferential direction. (2) The suitable press working of a piston-pulley was examined by means of FEM analysis. It was suggested the forming with increasing thickness. (3) A close agreement between the results of a proof test and analytical results was obtained. So it was confirmed that the new press working with increasing thickness of plate was useful in practice.

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This study is concerned with the structural reliability analysis of a symmetric laminated plate subject to buckling load. The material constants and the orientation angle of each layer as well as applied loads are treated as independent random variables. Since the buckling load is obtained as a minimum eigenvalue through Galerkin's method, buckling failure can be modeled as a series system consisting of eigen modes. When a design point obtained by FORM corresponds to the reversed laminated sequence is also considered as the design point. The failure probability bounds of this series system are determined by Ditlevsen's bounds. Compared with the reliability obtained from Monte Carlo simulation, the reliability obtained from FORM is shown to be useful for evaluating the reliability of designs qualitatively. Then, a reliablity-maximized design is compared to a buckling-load-maximized design.

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In the present paper, a theoretical mechanism analysis based on plastic hinge models with perfect plastic material properties is suggested in order to estimate the energy absorption abilities of thinwalled circular and square tubes by axial crushing under impact loading. The relationship between dynamic bending moments and angular velocites measured in our previous research is employed insted of the constitutive equations obtained from tensile tests because the main energy absorption in thin walled tubes occurs by bending work at plastic hinges. On the basis of this procedure, the time histories of the deformation and the absorbed energy can be calculated with good agreement with the experimental results. Crushing periods could not be determined theoretically in the case of square tubes, but were determined in the case of circular tubes. In addition, the axial crushing of thin walled tubes was carried out successfully taking into account the dynamic relationship between perfect plastic bending moments and bending angular velocities in plastic hinges.

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Local rules for forming structures are very important for optimum design and artificial life techniques. In this paper, a new local rule is proposed by analogy of a bone remodeling system and a neural network system. From the local rule, it is shown that the previously presented local rules. such as the cellular automata model and the fully stressed design criterion, can be derived. Moreover, the local rule can be changed adaptively to global rules of optional problems. Finally, maximum eiffness design problems of a beam model, which is one of the examples of structural forming, are analyzed by the proposed local rule.

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The behavior of an end notched flexure specimen subjected to impact loading was investigated by transient finite element analyses. The aim of this study is to confirm the validity of the estimation method for mode II interlaminar fracture toughness of composite materials under a high loading rate. which was proposed by the authors. Finite element results showed that flexural vibration of the specimen could be effectively suppressed when gentle gradient ramped incident stress waves were applied. while marked vibration tended to be excited when sharp-gradient step incident stress waves were applied. In the former case, a simple formula based on the static compliance method can be applied to estimate dynamic energy release rate as the authors suggested previously. Furthermore, the estimation error in the proposed method can be reduced by evaluating energy release rate from the surface strain of the specimen.

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An impontant theme from the point of view of assuring safety in impact load proof design is to know the amount of absorbed energy due to the deformation of a structure when subjested to an impact load. In order to be able to measure the buckling load and deformation time response of a thin walled membrane while minimizing possible influences from reflected wave disturbances, an experimental impact buckling apparatus was developed utilizing a sensing block devised in our research laboratory. In this paper, the special features of the apparatus will be discussed. As well, the experimental results of the generated impact force on a thin-walled membrane while undergoing buckling in the apparatus will be presented.

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In the previous paper we proposed a method for shape and topology optimization (i. e., layout optimization) of mechanical structures using a genetic algorithm. The method is contrived in such a way that the behaviors of layouts are investigated to obtain the optimum layouts by selecting the layouts of mechanical structures as individuals, and by adopting removal and addition parameters of elements as chromosomes. It was demonstrated by various experiments that the method is efficient and stable in convergence of the solutions. In this paper good convergence of the method is confirmed by schema theorem and experiments. Several strategies are also proposed, such as scaling, elite strategy, and our method for limiting the search domain. For the purpose of improving the convergence. the effects of these strategies on efficiency and stability of convergence are discussed together with the results of experiments.

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A formulation is presented for interval analysis of eigenvalue problems on the basis of the finite element sensitivity analysis and the representation of uncertainties involved in a structural system by a convex model. The first-order approximation is employed to express the response change due to the uncertainties that are assumed to be confined in a convex hull by means of the sensitivity analysis. The maximum and minimum of responses, by which the interval is bracketed, are searched on the convex boundary by the Lagrange multiplier method. The validity of the present formulation is demonstrated by a numerical example of the axial buckling load of an elastic straight column in the case of uncertain Young's modulus.

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Since Si substrates do not contain any dislocations, it is important to estimate not only the critical stress of dislocation movement, but also the critical stress of dislocation generation, taking into account the fabrication process of semiconductor devices. Therefore, we propose a method for evaluating the critical strength for dislocation generation near the surface of Si substrate using a ball press test. An electromagnetic actuator was used to press a sapphire ball against a {100} oriented Si substrate at high temperature under vacuum. Several press tests were performed using a variety of load conditions. After each press test, the Si substrate was dipped into an etching solution containing fluoric acid, which allowed all dislocations to be observed. The stress conditions under the ball were estimated, thereby revealing the relationship between the stress and dislocation generation. Dislocations are located on the {111} planes in the <110> directions, and are generated around the center of the ball indenter, where the highest resolved shear stress is on the Si surface. The method appears to be useful not only for measuring the critical strength of dislocation generation, but also for observing the influence of surface treatment on the Si substrate.

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Topology of any one-dimensional complex can be represented by zero and one-dimensional homology groups which are isomorphic to the direct sum of additive groups. In this paper, a method of constraint on the topology of a frame treated as a one-dimensional complex is proposed using homology groups. As a numerical example, the total strain energy of the frame is minimized under the constraint of topology and constant weight. Useless members are eliminated from a ground structure by use of genetic algorithm. Any number of additive groups can be freely set up as a topological constraint because of the generalized inverse matrices, and a rule of code in the genetic algorithm is designed so that all strings generated in the process of optimization could satisfy the topological constraint. As a result, it is found that loops in the topology of the optimum structure adjoin each other. The proposed method is also applied to the topological optimization of a square board fixed on a rigid wall and loaded vertically on points distant from the wall.

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We describe an optimal shape design for a column subjected to follower force. One end of the column is clamped and the other free. The follower force acts on the free end of the column. The elcenvalue of the column is expressed as a function of the follower force. To calculate the eigenvalue, the finite element method is used. The optimization problem is formulated as the shape optimization problem for the same column mass with a reference uniform column and reduced to a discrete optimization problem. The column is divided into several parts with the same length. In each part of the column, the transverse shape change is expressed as a piecewise linear function. To solve the optimization problem, constructive algorithms (CAs) are used. The solution is compared with those reported in literature.

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Recently, in order to improve power plant thermal efficiency, steam turbine blades have been used under the severe mechanical strength condition. In particular the blade root supports large centrifugal and vibrational forces. The optimum design for the root and groove shape of turbine blades must be performed in order to improve the mechanical reliability of turbine. In this paper, an effective method for solving this type of problem using sensitivity analysis based on FEM with linear programming is discussed. The optimal figure of the blade root and groove of which all peak stresses are reduced is obtained with this program system.

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This study was carried out in order to develop a new design procedure for screw threads based on the fatigue strength, in which the influence of mean stress was also considered. The Haigh diagram, which is a simple and useful tool for the design of screw threads, was newly constructed using the prediction procedure of S-N curves developed by us for different initial fastening loads. It was found that the design values recommended in the JIS application series may cause fatigue failure in the bolt and nut fastener especially for during use of high strength level screw threads.

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