Low cycle fatigue tests were carried out over a wide range of plastic strain levels including an extremely low cycle fatigue (ELCF) regime on ferritic ductile cast iron under push-pull loading conditions. To evaluate the internally accumulated fatigue damage, the measumement of the density of the materials was also taken after a given number of strain cycles. Results show that ( 1 ) in the ELCF regime, the generation and the coalescence of microvoids, which originated from debonding of the graphite-matrix interface inside the material, lead to the final fracture of the specimen ; ( 2 ) in this case, the local density of the specimen, at the region where fatigue damage is concentrated, decreases gradually with strain cycling and its most drastic change occurs at the final stage of fatigue. This drastic decrease in density at that stage is caused by the rapid coalescence of microvoids inside the material.

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A study was made to investigate the static fatigue behavior with alumina-alumina and alumina-stainless steel butt joints. The static fatigue test was performed under a constant bending moment at constant temperatures ranging from 580 to 888 K. For the specimen bonded at 903 K, static fatigue life, t_f, is related to the applied stress o and test temperature T, regardless of the adherent materials, as follows ; 1/t_f =(1/t₀)(1/σ)^m0 exp {-(Q₀-αln σ)/RT}, where t₀, m₀ and α are material constant, R the gas constant and Q₀ is activation energy of the fatigue process. While for the specimen bonded at higher temperature of 1073 K, t_f is expressed as following form ; 1/t_f =(1/t^, ₀)σ^m exp {-Q/RT}, where t^, ₀ and m are material constant. A good agreement between the calculated and the observed value of fatigue life of the joint is obtained. The SEM observation of the fracture surface was also made and a possible mechanism for static fatigue process in the bonded zone was discussed.

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Low-temperature fatigue cracking behaviour can be analysed successfully by a four-parameter Weibull function. The characteristic fatigue stress intensity factor, Ke, and threshold stress intensity, K_th, can be strictly determined. The Ke values at low temperature show Ke= γσ_y√(d), where σ_y, d and γ are yield stress, grain diameter and proportional constant, respectively. Also derived from the four-parameter Weibull function is K_th=K_e/2^k, where k is one of the parameters in the function. Namely, K_th= γ(σ_y√(d)/2^k). This is confirmed and explains the higher K_e and K_th at lower temperature, because of the increased σ_y at low temperatures.

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Rotating bending and torsional fatigue tests were carried out on plain specimens and small-hole specimens of annealed cast carbon steel. The specimens are regarded as isotropic. The fatigue limit ratio between rotating bending and torsional fatigue (τ_w/σ_w) in the isotropic carbon steel is different from that in a rolled round bar of carbon steel. We should use isotropic materials in fatigue tests under combined stresses. In torsional fatigue, crack propagation on plain specimens is of the shear type in the initial stage and tension type in the latter stage. Cracks of the shear type were generated in both the circumferential and the axial directions. The crack growth laws in bending and torsion are expressed by dl/dN= C₁τⁿ_al. The dependency of 1/N_f on the stress amplitude τ_a in the torsional fatigue is nearly the same as the dependency of the crack growth rate on τ_a.

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In order to study the effect of environment and multiaxial stress states on subcritical crack growth behavior of machinable ceramics containing mica flakes in a glass matrix, dynamic fatigue tests (constant stressing rate tests) were conducted by four point bending of plates (uniaxial tension), concentric ring loading of disks (equibiaxial tension), and diametral compression of disks (tension-compression) under different environments of air, water, and vacuum. The subcritical crack growth parameters, n, were determined from the stressing rate dependence of the fracture toughness value in both air and water environments. In any case of multiaxial stress states, the n value obtained in a water environment was obviously higher than that in an air environment. Furthermore, multiaxial stress effects were exhibited with a somewhat higher n value under equibiaxial tensile stress state in an air environment, and with a significantly lower n value under a tension-compression stress state in both air and water environments. These tendencies can be explained by the facts that in the biaxial stress state crack branching and formation of multiple cracks occurs easily with the absence of a strong orientation dependence for crack growth and with the existence of mica grains dispersed in the matrix, whereas in the tension-compression stress state the strong orientation dependence of the stress intensity at the crack tip prevents the crack from going around the mica grains.

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In this paper, the singular stress field around a crack meeting a bimaterial interface is analyzed. A uniform expression is presented for the singular field. The intensity of the singular stress field is defined in terms of two real constants K_I and K_II. By using the body force method, K_I and K_II are calculated. In the numerical analysis, the singularities of stress field are characterized by introducing the proper basic density functions of body force doublets. The use of the basic density functions enables one to obtain accurate solutions.

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In this paper, the hypersingular integral equation method (HIEM) is applied to calculate the stress intensity factors of curved crack problems. The stress fields induced by two kinds of displacement discontinuity are used as fundamental solutions. Then, the problem is formulated as a system of integral equations with the singularity of the form of r^-2. In the numerical calculation, two kinds of unknown functions are approximated by the products of the fundamental density functions and polynomials. The calculation shows that the present method gives rapidly converging numerical results for curved cracks under various geometrical conditions. In addition, a method of evaluation of the stress intensity factors for arbitrary shaped curved cracks is proposed using the approximate replacement to a simple straight crack.

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The present paper discusses an inverse problem of two-dimensional piezothermoelasticity in an orthotropic infinite plate exhibiting crystal class mm2. An unknown thermal loading is inferred when the electric potential difference between the plate surfaces is given, and the mechanical and electric boundary conditions are prescribed. The method of solution proposed by the authors is applied to analyze the inverse problem. The numerical calculations are carried out for a cadmium selenide solid. The thermal loading, elastic displacements, stresses, electric field intensities and electric displacement on the surfaces are illustrated graphically.

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For the purpose of developing dynamic plate theory with mechanical and thermo-elastic coupling, the most general higher-order theory is proposed by using the power series expansion of displacements and thermal quantities along the plate thickness. Lower-order theories with truncation of the coefficients for displacements and thermal quantities in the present paper are shown for the cases of bending behavior as well as stretching behavior.

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In an analytical method for axisymmetrical elastic problems in finite deformations which depend on the displacement functions, for compressible materials, by considering the possibility of omission of the displacement functions and the relationship between the appearance of body forces and the additional stresses which could not be reported in our previous paper, we clarify the structure of the analytical method of axisymmetrical surface force problems when torsional deformations do not exist, and formulate it. Also, from the above consideration, we denote that the axisymmetrical surface force problems can be classified into three systems of additional stress. Furthermore, the analytical method of axisymmetrical problems is examined by a basic example.

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One of the difficult problems in the study of the constitutive equation for cyclic plasticity is the prediction of ratchetting behavior which is induced by the superposition of a cyclic secondary load to a constant primary load in the biaxial case, or by the mean stress in a uniaxial case. This paper shows the constitutive equation in which the memorization of the back stress was considered for ratchetting behavior, especially for biaxial ratchetting behavior. To verify the applicability of the constitutive equation to ratchetting behavior, biaxial ratchetting tests were carried out using type 304stainless steel at room temperature. As a result, it was found that the simulations based on the constitutive equation had good agreement with the tests.

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In order to investigate the relationship between surface roughening and deformation of respective grains, three-dimensional observation of surface roughness of polycrystalline iron during uniaxial compression was carried out. The pattern of grain boundaries and contour map of surface roughness were superimposed on the display of personal computer. The three-dimensional surface roughness data were also high-pass-filtered to get detailed figures. Fractal dimension was introduced to characterize the change in three-dimensional surface roughening during plastic deformation. Box - counting method was adopted to measure the fractal dimension. It was found that three -dimensional surface roughness showed fractal characteristics. The fractal dimension was almost constant with the increase of the applied strain.

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The behaviors of creep rupture and main crack initiation lives at 700°C with doubly notched thin plate specimens (1mm thick) having various notch root radii were investigated, using a commercial austenitic steel, SUS310S. It was found that specimens with concentration factor, α, ranging from 1. 3 to 3. 0 showed slight notch strengthening with the value of the rupture time ratio, RTR, of 1. 3 to 2. 0, while those with a larger than 3. 0 represented notch weakening having RTR of 0. 5 to 1. 0. These strengthening and weakening behaviors were explained by comparing the distributions of equivalent stress under steady-state creep conditions across notched sections with those of smooth ones. Furthermore, a relationship between the main crack initiation life, t_i, defined as when a crack length reaches 100 μm, and the mean notch opening displacement rate, V_m, that is, t_iVⁿ_m=C (n and C are constants), was obtained, regardless of notch root radius, which was analogous to the Monkman - Grant relationship.

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This paper deals with a method for predicting the three-dimensional distribution of internal defects from the two-dimensional observation of intercepted defects on an inspection plane by an inverse analysis. At first, the fundamental relationships between them were analytically derived on the basis of a statistical model where multiple penny-shaped cracks in an infinite body were intercepted by an inspection plane. The validity of the method was confirmed by a numerical simulation. It was, moreover, applied to small creep fatigue cracks distributing in a smooth specimen of a Type 304 stainless steel, and the volumetric density and the distribution of crack size were successfully evaluated.

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The torsional rigidities of heated weld-bonded box section members have been studied using the theoritical method. This method, based on elastic theory in thin plates, was developed from the stress analysis method of the same kind of box section members which we presented previously. Computations were carried out to investigate the influence of softened adhesives in higher temperatures on the rigidities. As a result, it was found that the torsional rigidities are not usually dependent upon spot pitches, however, they are lowered when the temperature increased to 50°C or above.

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This study is concerned with an application of the boundary element method (BEM) to 2-dimensional residual stress evaluation based on the theory of inherent starin. Inherent strain was regarded as an initial strain in BEM analysis. The applicability of BEM in residual stress evaluation based on the theory of inherent strain was discussed by comparing results of BEM analysis and experiments.

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A design sensitivity calculation technique based on the implicit differentiation method is fomulated for isoparametric boundary elements for three-dimensional(3D) shape optimization problems. The practical sensitivity equations for boundary displacements and stresses are derived, and efficiency and accuracy of the technique are compared with the semi-analytic method by implementing the sensitivity analysis of typical and basic shape design problems numerically. The sensitivity calculation technique is then applied to the minimum weight design problems of 3D bodies under stress constraints, such as the shape optimization of the ellipsoidal cavity in a cube and the connecting rod, where the Taylor series approximation, based on the boundary element sensitivity analysis at current design point, is adopted for the efficient implementation of the optimization.

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The sample point error analysis and the related adaptive boundary refinement, proposed by one of the present authors, are extended to problems with subregion partition, which is often required for higher accuracy and for dissimilar materials. Interface between the two subregions should be discretized to raise accuracy of the entire calculation by the boundary integral equations for each subregion combined by using the compatibility and equilibrium conditions. A parameter regularizing the difference between influene of the boundary error is introduced. Two examples are tested to verify the availability of the proposed method.

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This paper concerns a new boundary element analysis approach for the object divided into subregions. In this approach, the matrix equation for each subregion derived from the boundary integral equation is transformed to an equation similar to the stiffness equation of finite element method (FEM). The global equation is made by superposition of these matrix equations, which is simple and general, while the existing approach depends on the initial boundary. The present approach is derived theoretically and is compared with the existing ones in order to indicate its features. Then, it will be applied to analysis of two-dimensional elastic and potential problems.

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The effects of humidity on the mechanical behavior of polyamide-6 (PA) and cellulose-acetate (CA) were investigated. The specimens were exposed in a chamber at constant temperature and humidity for 4∼16 days, and measurements were made on the variations of weight and tensile properties. In addition infrared analysis was conducted using the attenuated total reflectance attachment with a KRS-5 prism. The result are as follows : ( 1 ) In PA, there exists a linear relationship between the weight change and the maximum stress ; i. e. it becomes heavier and more ductile when it is exposed to high humidity and temperature. The main factor of the weight change is moisture absorption and/or desiccation, and this relationship also holds for the specimens under dry-wet cyclic exposure. ( 2 ) The variation of yield stress of CA is correlated well with its weight change but not with the exposed humidity. IR spectroscopy shows that the weight change of CA occurrs by dispersion of plasticizer as well as by absorption of water. Due to dispersion of plasticizer, the variation of the yield stress under dry-wet cyclic exposure is nonreversible.

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Mechanical properties or fracture behavior of bonded dissimilar materials is affected by residual stress near the interface. As ceramics are too hard to work flexibly, almost the shapes of interface are finished flat. In this work, a properly subdivided FEM model for evaluations effective thermal residual stress, was applied to thermal elastoplastic analysis. The effect of the thickness of the metal applied for the joint on stress distribution near the edge of the interface, was investigated using the FEM method. The FEM analysis results were applied to develop an advanced bonding method. Actually, the bonding strength was increased using our proposed method.

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This paper proposes a numerical simulation method for nonlinear stress-strain relationships of short fiber-reinforced plastics in which interfacial debonding between fibers and matrix occurs. The criterion of the interfacial debonding is expressed in terms of the released elastic strain energy. Nonlinear finite element analysis is carried out and the simulation results agree well with experimental results. The influences of the critical energy release rate for interfacial debonding, aspect ratio of fibers and fiber orientation on the stress-strain relationships are also discussed. We concluded that the proposed simulation method is useful for the prediction of nonlinear stress-strain relationships for short fiber-reinforced plastics.

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The stresses and deformations in an isotropic laminated plate (steel plate adhered to acrylic plate with different thicknesses) due to impact were analyzed. In-plane stresses were obtained with good accuracy by the plate bending theory. To obtain the out-of-plane stresses, in-plane stresses were substituted into the three-dimensional dynamic theory of elasticity and a numerical integration method was employed. This method is called an approximated three-dimensional theory in this paper. When the plate was impacted on the acrylic side with a long steel bar, an almost circular delamination was created on the interfacial boundary layer of the laminated plate. In the experimental record of the impact force history, a clear signal could be recognized at the creation of the delamination. Employing the data of the experimental impact load history as input for the numerical calculation, out-of-plane stresses on the interfacial layer were obtained. The size of the circular delamination were considered in terms of the resultant shear stress on the boundary layer.

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Generally, stress concentration occurs near the edge of the interface bonding two materials with different mechanical and thermal properties. Investigation of the residual stresses in such dissimilar materials is important for application of the bonded structure of ceramics-metal to new structural materials. In this paper, the residual stresses in an axisymmetric joint of ceramics and metal with several apex angles were analyzed using the universal finite-element code called Marc. The finite-element analysis was carried out for conditions in which the temperature of the joint was varied from 1 273 K to 300 K, and after that a uniform axial tensile stress of 20 MPa was applied to the ceramics side of the bonded structure. Moreover, the influence of residual stresses on the metal processing ratio was inspected and the effectual metal shape on the stress relaxation was investigated.

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Advanced fiber-reinforced laminated composite materials have been used for structural members because of their high specific strength and stiffness. This paper deals with the problem of dynamic stability of laminated cylindrical shells subjected to axial step loading. First, the axially symmetric motion of the shell under the loading is determined. Subsequently, certain perturbations are superimposed on this motion, and their behavior in time is investigated. The symmetric state of motion of the shell is called stable if the perturbations remain bounded. The solutions for the prebuckling motion and the perturbated motion are obtained by the use of Galerkin's method. The inevitability of dynamically unstable behaviors is proved analytically and the effects of various factors such as compressive load ratio, number of layers, dynamic unstable mode and dimension of cylinder are clarified.

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A practical step-by-step calculation algorithm for the dynamic displacement and stress sensitivities with respect to shape design parameters is developed based on the implicit differentiation method and the Newmark method for isoparametric finite elements for two-dimensional shape optimization problems. The method is applied to the minimum weight design of a hole shape in a plate and a connecting rod subject to the dynamic stress constraints under impact loading, and the validity of the shape optimization technique is confirmed numerically.

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Spall phenomena between laminas in short circular cylinders made of carbon-fiber reinforced plastics (CFRP) are investigated experimentally. The CFRP cylinders are impacted by circular thin steel plates which are fired by a light gas gun. The effects of initially existing defects in the specimens on the interlaminar spall fracture are examined by the measurement of stress waves, ultrasonic nondestructive inspections, and Finite Element Method (FEM) simulations.

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In this paper, axial crushing tests on thin-walled circular cylinders of aluminium were conducted on an impact testing machine accelerated by air pressure, which is installed in the authors' laboratory at Tokyo Institute of Technology, and measurements for the variations of the loads applied to the specimens and the crushing deformations with time during impact were made with considerable accuracy. The dynamic load-deformation diagrams were extracted from these measurements to estimate the energy absorption abilities of the specimens, thus determining the effects of dimension or material of the specimens and crushing mode. The above results were compared with those of static crushing tests and the differences were discussed.

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In reliability-based design and testing, reliability R(= 1 - P) and confidence level C( = 1 - γ) are usually used in a combined manner. In the previous paper, an attempt was made to use the mean reliability as a one-dimensional measure. In the present paper, this idea was applied to determination of the design allowable stress S_a given that strength follows a two-parameter Weibull distribution W(α, β). The factor k in the expressions of S_a= β^^^/k^1/a for the case when only the scale Parameter β is unknown and S_a= β^^^/k^1/a for the case when both the shape and scale parameters are unknown was determined based on this idea. The variations of k with the sample size n were obtained by a theoretical analysis and Monte-Carlo simulation.

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Recently FEM code is used in a wide engineering area. Two prototypes of expert system for generation of single-purposed FEM code are developed with the knowledge-based technique. This system is a part of FEM instruction system. These systems cover two dimensional structural problems, and can generate more than 50 kinds of FEM code. One expert system is written by Prolog and uses the frame-system for the knowledge representation scheme. Another expert systems is written by C⁺⁺ and uses the Object-oriented knowledge frameworks. The merits and demerits of two systems are compared and discussed.

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In this study, the hr-adaptive meshing method for boundary element (BEM) analysis is developed and implemented into the expert system for BEM analysis developed by the authors in a previous paper. The adaptive meshes obtained by this system are made to be learm by using a multilayered neural network with an automatic reduction of association units. It is found that it becomes possible to generate adaptive meshes for unlearning problems by using the neural network. This method is expected to be very useful particularly for the BEM analyses of problems with the shape slightly changing in the optimal deisgn.

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