We investigated AE (acoustic emission) properties in a fatigue test of an aluminum alloy and the frequency analysis of measured waveforms, The results obtained are as follows. (1) The occurrence patterns of an AE event in the fatigue process have three stages according to the fatigue damage; therefore we can monitor the progress of the fatigue damage of the specimen by monitoring the AE event count. (2) In the case of the pulsating fatigue test, a high AE event count was obtained near the pulsating load to be maximum. (3) The AE energy rate at the first load showed a maximum and increased with increasing stress amplitude. (4)A good correlation between the AE energy at the first load and number of cycles to failure was obtained enabling us to predict the lifetime of the specimen from the AE energy rate at the first load.

抄録全体を表示

In order to investigate the effect of surface roughness on the fatigue strength, fatigue tests for maraging steel which was subjected to aging treatment and was free of residual stress, were carried out. As an artificial surface roughness, extremely shallow periodical notches with notch radii P=0 and 200μm, and notch depths a=12, 22, 40 and 65μm were introduced. In addition to the effect of surface roughness, the effect of nonmetallic inclusions was also considered. The equivalent defect size (√(area_R)) which controls the fatigue limit of specimen containing the surface roughness is defined by the function of depth and Pitch of the roughness. The lower bound of the fatigue limit for various levels of roughness can be predicted by substituting the equivalent maximum defect size (√(area_maxt)) to the √(area) parameter model. The value of √(area_maxt) can be evaluated by considering the interaction effect between the maximum inclusion size and roughness. The predicted values were examined by experiments and it is shown that the method of prediction is available for practical applications.

抄録全体を表示

Nitriding heat-treatment is often used to improve the fatigue strength of steel automobile parts. However, there is no established method for predicting fatigue strength. We thus conducted rotational bending fatigue experiments using different nitriding heat-treatment conditions, materials and notch shapes as variables, with the objective of establishing a means for predicting fatigue strength in nitriding heat-treated material. As a result, we have found that it is possible to predict the fatigue strength of nitriding heat-treated material using the √(area) Parameter Model fatigue-strength prediction formula for high-strength steel proposed, when consideration is given to post-nitriding heat-treatment hardness, residual stress, surface layers of nitriding compounds, inclusions and other factors.

抄録全体を表示

Fatigue strength in high-humidity environment has been studied for aluminum alloys such as a high-strength aluminum alloy (A7075-T6), an annealed alloy (A7575-O) and a corrosion-resistant alloy (A5052-H34). Pits are easily initiated in the early stage at humidity exceeding the critical humidity. The critical humidity is 60 pct RH for high-strength aluminum alloys (A7075-T6 and A7075-O) and 70 pct RH for a corrosion-resistant alloy (A5052-H34). Fatigue strength in a high-humidity environment is reduced markedly for all aluminum alloys and is almost the same as that in ion-exchanged water. Thus, a transition of environmental fatigue strength was found with increasing humidity. The reduction ratio of fatigue strengh for A7075-T6 is higher than that for A7075-O and A5052-H34 due to the initiation of large corrosion pits and a high notch sensitivity. The results were compared with those for structural steels, and it was found that the critical humidity is lower and the reduction ratio higher for aluminum alloys than for structural steels.

抄録全体を表示

In-situ observation on fatigue crack growth behavior in single crystal Ni-base superalloy CMSX-4 in vacuum at 700°C and 800°C was studied with a special view of mesoscopic fracture analysis. Especially, influences of γ' precipitates on high temperature fracture mode of CMSX-4 were examined. The main results obtained were as follows; (1) There was an obvious difference between fatigue crack propagation (FCP) behaviors at 700°C and at 800°C. Since FCP at 700°C was in a brittle manner, FCP diagram could not be obtained. FCP at 800°C was stable and FCP property was controlled with Paris' law. ΔK_th was not found in FCP diagram at 80O°C, since roughness-induced closure could not be occured in this material and oxide-induced closure could not be occured in vacuum. (2) Fracture mode at 700°C and 800°C was consisted of two types as FCP and slip plane decohesion (SPD). A change from FCP to SPD at 700°C was earlier than that at 800°C, since strength ofγ' precipitates at 800°C was higher than that at 700°C. with a mesoscopic view, fracture mode of CMSX-4 was mainly dominated by strength of γ' precipitates.

抄録全体を表示

The present authors previously proposed the "U (crack opening ratio)-estimation" method for predicting a fatigue life of a two-dimensional crack propagating in residual stress fields based on the superposition principle. This method was extended for application to three-dimensional cracks. In this study the method was applied to a quarter-elliptical corner fatigue crack in residual stress fields in SUS304L-clad SPV32SR steel. Experiments were conducted for three cases; (1) propagation of a corner crack in an SPV32SR specimen in the absence of initial residual stresses, (2) propagation of a corner crack from the SUS304L side to the SPV32SR side of the clad steel, and (3) propagation of a corner crack from the SPV32SR side to the SUS304L side. The prediction method was applied to those three cases assuming that those cracks remained a quarter ellipses. The predicted results were compared with the experimental ones. Good agreement was observed between the predicted and experimental results for case (l). A slight (difference was observed between the predicted and experimental results for case (2), while much better agreement was found between the predicted and experimental results for case (3). Generally the predicted lives were in good agreement with experimental ones and it was confirmed that the present fatigue life prediction method was valid.

抄録全体を表示

A model for fracture of ceramics has been proposed. Based on the model, an evaluation method of fracture strength for ceramics has been developed. It is well known that fracture strength for ceramics has a large scatter because of included defects. Generally it is thought that the scatter of fracture strength depends on the defect size alone. However, we experience that fracture toughness has a scatter even if it is measured by the IF method. The crack introduced by the IF method is very small; thus, it cannot be considered that defects are included in the crack surface. Therefore we have established a model for fracture of ceramics by introducing the probability distribution of fracture toughness. Fracture probability for an arbitrarily shaped specimen can be evaluated by using this model.

抄録全体を表示

The effects of crack size, compliance and fracture toughness on plastic collapse behavior and ultimate strength of a statically indeterminate pipe system have been analyzed. The main results obtained are as follows. 1) For the case of J_IC≥500kN/m and dJ/da≥100MN/m², these parameters have little effect on plastic collapse behavior and ultimate strength. 2) The J-integral value at the onset of plastic collapse depends on crack size, pipe span length, compliance and loading system. However the highest value is lower than 8 MN/m. 3) The plastic rotation angle of plastic hinge at the onset of collapse is lower than 0.2 [rad]. This value is important when considering the LBB (leak before break) behavior of a piping system. 4) The analysis method adopted in this paper is useful and convenient for evaluation of the tearing instability behavior of a piping system.

抄録全体を表示

The stress intensity factor of a semi infinite crack in an infinite plate under a transient and quasistatic thermal stress field generated by a point heat source acting on a crack surface was analyzed. A typical stress around a point heat source is severe compression in the vicinity of the point of heat, then it turns positive value as the distance from the point of heat increases and finally it becomes zero at infinity. Therefore, in the crack surface heating, a wide contact region of crack surface was found just around the heat source, but the crack surfaces gradually open each other due to the positive stress away from the point of heat and the singular stress appears at the crack tip. A crack tip stress intensity factor was obtained as a function of heating time and/or velocity of the moving heat source and the degree of heat dissipation from surfaces of thin plate to the atmosphere. Two types of different technique for determining an extent of the contact length and stress intensity factor were proposed. It was found that the SIF is significantly influenced by the degree of heat dissipation from the plate surfaces in the case of crack face heating with moving heat source.

抄録全体を表示

Constitutive models to describe creep deformation and creep damage of unidirectional fiber-reinforced metal matrix composites are developed from a phenomenological point of view. The constitutive modeling is based on the well-established thermodynamic formalism for internal state variable theories. A damage-coupled kinematic hardening model for transversely isotropic materials is first formulated in the invariant form on the basis of the Malinin-Khadjinsky model. An isotropic hardening model is then developed by assuming a particular representation of the kinematic hardening variable. The extent of the composite damage is described in terms of a scalar variable, and a directional effect of the damage on the composite behavior is characterized by a fourth-rank anisotropic tensor. The evolution equation of the creep damage is analogous to the model developed by Kachanov and Rabotnov, but it includes a coupling between creep deformation and creep damage. The analytical expression of the creep rupture time is derived in a particular case of the isotropic hardening model, and for a constant stress creep it depends on both the time and degree of damage at the moment of hardening saturation.

抄録全体を表示

This paper is concerned with the dynamic treatment of thermal stresses in a ceramic-metal composite hollow circular cylinder with an interlayer of functionally graded material (FGM) subjected to uniform impulsive electromagnetic radiation. Heating is assumed to be a step function in time, and to diminish exponentially with distance from the inner surface of the cylinder, assuming negligible heat conduction. In treating problems, the nature of the stress-wave buildup in the absorption cylinder is studied for the case of a temperature-dependent solid, i. e., material properties vary with temperature. The nonhomogeneous hollow circular cylinder is approximated as a laminated hollow circular cylinder consisting of different, homogeneous and isotropic cylindrical layers which are perfectly bonded to each neighboring layer. The numerical procedure employs the characteristic method based on the integration of the governing equations along the characteristics. Numerical examples are presented for several cases, which show the significance of the temperature-dependent properties of the material and the effects of the thickness and composition distribution of the FGM interlayer on the magnitude of the dynamic thermal stresses.

抄録全体を表示

A thermal stress problem of a layered half-space heated by a prescribed temperature on a circular region of the interface is solved in the framework of the three-dimensional thermoelastic theory. The stress singularities at the point of temperature discontinuity are investigated. It is found that the shear stress on the interface shows a stress singularity of logarithmic type at the point of temperature discontinuity. The behavior of the singularity are compared with those for two-dimensional analysis. The results show that the type and the strength of singularity of the debonding and shear stress are the same as those for two-dimensional analysis.

抄録全体を表示

This paper deals with stress separation in thermoelastic stress analysis. The problem of stress separation can be divided into two : (1) an inverse problem to estimate the unknown boundary values from the sum of the principal stresses inside the body, and (2) a direct problem to compute the stress components inside the body based on the estimated boundary values. This two-step method can be applied to a wide range of practical problems if both the inverse and direct problems are solved by FEM or BEM. In this study, the problem is formulated by BEM. It is shown that careful treatment of the inverse problem is essential for attaining an accurate result of the stress separation. The combination of the truncated singular value decomposition and Hansen's L-curve method is found to be effective for that purpose.

抄録全体を表示

In the present paper, a formulation of the boundary element method (BEM) for elastic problems based on the non singular boundary integral representation is suggested. In the conventional analysis by BEM, the accuracy of the numerical results is generally geverned by the numerical treatment of the singular kernel included in the integral equations. In the present procedure, a reference field equation for a uniform gradient of the displacement is superposed on the original one to regularize the singularity of the integrand. In addition, the same technique is applied to the integral equations for inner points in order to obtain highly accurate results in the area very close to the boundary. Numerical calculation for fundamental problems is carried out to confirm the validity of the present formulation, and it is shown that unknown nodal values along the boundary are more accurate than those obtained by general formulation and the accuracy of the displacement and stress even in the vicinity of the boundary has been clearly improved.

抄録全体を表示

Details of the singularities of residual stress at the interface edge remain unclear. This paper presents a theoretical solution of the residual stress, and discusses paradoxical problems which are related to the logarithmic singularity, based on Goursat's stress function. It is found that logarithmic singularity occurs not only in some special cases. but also in the case in which material properties and geometrical shape are near to such special cases. This paper also shows that the residual stress cannot be described only by Dunders' parameters.

抄録全体を表示

This paper deals with a row of equally spaced equal ellipsoidal inclusions in an infinite body subjected to tension. Based on the concepts of the body force method, the problems are formulated as a system of singular integral equations with cauchy-type or logarithmic-type singularities, where the densities of body forces distributed in the γ and z-directions of infinite bodies having the same elastic constants of the matrix and inclusions are unknown functions. In order to satisfy the boundary conditions along the inclusions, eight kinds of fundamental density functions proposed in our previous paper are used. In the analysis, the number, shape and distance of inclusions are varied systematically; then, the magnitude and position of the maximum stress are examined. For any fixed shape and size of inclusions, the maximum stress is shown to be linear with the reciprocal of the squared number of inclusions.

抄録全体を表示

Genetic Algorithm (GA) is known to be excellent in searching globally optimal solutions, though it requires a large population size of the chromosomes and a large number of generation. Some studies are carried out in order to reduce the population size and the number of generations to obtain the solutions. For the purpose, the Authors try to discuss size optimization of truss structures whose members are discretized in the cross-sectional areas by using GA. The structure is coded by a binary string consisting of substrings, which represent the cross-sectional areas of the members and whose numbers are equal to the numbers of the members. Two types of GA models are adopted. In one type the operations are applied to all the substrings simulataneously during a generation. On the other hand, in the other the operations are applied to each substring, one at time. Various kinds of GA strategies, such as simple GA, elite strategy, hybrid strategy combained GA with fully stressed design, etc, are tested in numerical experiments. From their results the authors confirm that the hybrid strategy are most efficient and stable for searching globally optimal solutions.

抄録全体を表示

Grain boundary grooving in crystalline aluminum is simulated by computer molecular dynamics, and impurity effects are investigated. We use a Morse potential that includes equilibrium spacing, γ_A1, and potential well depth, |u_A1| to characterize aluminum-aluminum interaction. We also use a two-body interatomic potential that includes equilibrium spacing, γ_m, and potential well depth, |u_min| to characterize aluminum-impurity interaction. The simulations show that when γ_m is smaller than γ_A1 and when |u_min| is close to |u_A1| (with the relative difference smaller than 20%), grain boundary grooving is prevented. This effect is explained by a decrease in the ratio of grain boundary diffusion to surface diffusion. Diffusion coefficients obtained by these simulations show that impurities at the grain boundaries which satisfy the above conditions (e. g., copper) strengthen surface diffusion without strengthening grain boundary diffusion.

抄録全体を表示

We studied a new and simplified method for estimation of residual stress in a welded structure using inherent strain. Using this method, welding residual stresses can be calculated by elastic analysis with the finite element method. Residual stress at the welded joint of a pipe penetrating a thick plate is obtained by this method, and compared with the results from direct measurement. Inherent strain at the welded pipe joint is assumed to be a simple distribution, and residual stress is elastically calculated. Inherent strain distribution is inferred from that of welded joints with simple shapes. The residual stress distribution from inherent strain analysis agrees well with the results of direct measurement of a mock-up specimen. The proposed method is useful to estimate simply welding residual stress in various three-dimensional structures.

抄録全体を表示

In the previous report, the authors proposed a simple adaptive mesh refinement technique for the elastoplastic finite-element analysis, where an element is refined when the absorbed energy density reaches a prescribed value during the loading procedure. Though the method gave accurate solutions when the prescribed value for mesh refinement was not very large, the solutions were not accurate when the prescribed value was so large that the material yielded. In the present report, several algorithms are added to the original one to overcome this problem. The additional algorithms are summarized as follows, (1) If interpolated stresses of the elements newly created by refinement exceed the yield surface, the stress components are reduced to the yield surface. (2) When the material yields, the method proposed by Owen and Hinton is employed in order to prevent the stresses drifting away from the initial yield surface. (3) The effective stress at a node is employed as a criterion for mesh refinement in addition to the absorbed energy density. Through numerical examples, it is shown that the modified algorithm gives accurate solutions. Secondly, the algorithm is applied to large-deformation problems. Consequently, it is shown that a more precise solution is obtained effectively by the present method compared with the solution obtained by the initial mesh subdivision.

抄録全体を表示

Generally inelastic properties of metal materials depend on the deformation rate. However, past studies on viscoplastic constitutive equations do not successfully describe actual flow stress properties and stress relaxation properties as well as temporal hardening during reloading after relaxation. We have experimentally and theoretically examined the effect of strain ageing on resistance in plastic flow using SCM435 steel which consists of complex specifications and hence shows considerable strain ageing. In this study, using 99.98% pure copper which is considered a pure material and hence shows little strain ageing, it is shown that strain rate depenence can be assessed quantitatively during loading when strain increases, Compared with the behavior of materials which show considerable strain ageing, it is shown that the concept of the viscoplastic constitutive model with consideration of strain ageing is applicable to problem of inelastic behavior.

抄録全体を表示

The propagation of ultrasonic waves in ceramics-dispersed aluminum alloy composite was investigated for the purpose of developing a nondestructive method for inspecting ceramic particles. From the results of ultrasonic tests, the following important laws of ultrasonic waves were revealed. Propagation time decreases with an increase in SiC volume fraction; and (2) pulse width increases with an increase in SiC particle size. We studied these phenomena based on a probabilistic method, and derived formulas for the relationship between ultrasonic waves (propagation time and pulse width) and dispersed particles (volume fraction and mean size).

抄録全体を表示

To maximize efficiently the buckling strength of fiber-reinforced composite laminate Plates with stiffeners under a weight constraint, a two-level optimization technique is proposed for determining the plate and stiffener dimensions, and stacking sequence of composite plates. In the first level, the lamination parameters and the dimensions of the plate and stiffener are taken as the design variables, and the buckling load is maximized by sequential linear programming and the finite element sensitivity analysis. Then, the stacking sequence which gives the optimum lamination parameters is sought using the genetic algorithm in the second level. Numerical results of design examples are also described, and the efficiency of the suggested technique is discussed.

抄録全体を表示

Inelastic material behavior is usually expressed in terms of constitutive equations. However, it is not easy to estimate parameters included in these equations from the experimental data. This paper describes a model using neural networks for the estimation of material transient responses. To overcome the difficulty for the neural networks to learn the material response curve, two neural networks are independently used to learn the two curves that can be decomposed from the original stress-strain curve based on the internal stress concept. Numerical examples show that the present neural network approach can map the stress-strain curve with good precision.

抄録全体を表示

The shape fixity and the shape recoverability of a shape memory polymer film of polyurethane series were investigated by means of thermomechanical cycling tests. The main results are summarized as follows. (1) In the case of small maximum strain, the deformation properties do not vary under cyclic loading. (2) The shape fixity of the material does not vary under cyclic deformation. (3) In the case of large maximum strain, the shape recovery temperature rises and the irrecoverable strain increases under cyclic deformation. Mechanical training before practical use is effective in order to obtain stable shape recoverability. (4) Thermal stress resulting from cooling under constant maximum strain appears in the case of small strain. (5) The deformation properties of the materials with different glass transition temperatures show almost the same tendency.

抄録全体を表示