[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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Although many papers have suggested that the growth rate of small cracks can not be determined by linear fracture mechanics, there are a few reports indicating that linear fracture mechanics can be approximately applied to determine small crack propagation. In the hard materials, it seems to be easy to satisfy the condition of small scale yielding. This means that the growth rate of small cracks in such materials can be approximately expressed as a function of the stress intensity factor. In this paper, using a heat-treated 0.54% C steel, the rotating bending fatigue tests of plain specimens with a small hole are carried out under a wide range of stress amplitudes, and the validity of Paris's rule is investigated.

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An influence function method has been developed to analyze stress intensity factors, K, for surface cracks, which are subjected to arbitrarily distributed surface stresses. The method has been applied to the analysis of a series of influence coefficients for flat plates with a semielliptical surface crack. It has been shown, based upon the data of the influence coefficients, how easily the K-values for a semielliptical surface crack in a flat plate, with arbitrary aspect ratio and depth to thickness ratio, can be evaluated against arbitrarily distributed surface stresses.

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Fatigue crack growth and crack closure in the near-threshold region were investigated under different loading frequencies for three types of steel. The results show that the loading frequency influences the near-threshold characteristics in fatigue crack growth, through the different contributions of the fretting oxide induced crack closure. This behaviour is attributed to condensation of moisture between crack faces, which is influenced by the loading frequency. The formation of the fretting oxide debris promoted by the condensation of moisture becomes marked at a higher frequency. However, it is an unstable and complicating phenomenon, since the condensation is also influenced by relative humidity, test temperature and sheet thickness. Therefore, it is concluded that non-oxide controlled crack growth characteristics should be sued for the life prediction of structures.

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The quantitative analysis of crack coalescence is one of the significant problems in low-cycle fatigue of a smooth specimen, since the coalescence of many distributed cracks reduces the fatigue life extremely. In this paper, an analytical procedure is proposed to estimate the crack propagation life for material with a considerable frequency of crack coalescences. Two-dimensional modeling is employed to describe the distribution of cracks and the conditions of crack coalescence. When the initial distribution of cracks is given, based on the modeling, the subsequent crack propagation life is calculated successively by using the growth law of long through-thickness cracks through an energy density parameter related to the J-integral range. The proposed method is applied to a practical problem in pure copper and the predicted results are in good agreement with the experimental data.

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The effects of grain size for improving in fatigue strength was studied on plain carbon steel, with a special emphasis on the critical behavior of the non-propagating crack. It is found from experimental analysis that the endurance limit of plain carbon steel follows the Hall-Petch type relationship in a wide range of ferrite grain sizes. However, the effect of grain refinement on the endurance limit is not so prominent, compared with results already popularly known. Furthermore, an improvement in the fatigue strength of low carbon steel by grain refinement can be associated with the restriction of the length of a critical non-propagating crack, and with an increase of the crack growth resistance of matrix ferrite.

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The crack propagation behavior in fretting fatigue was investigated in carbon steel S45C. Fretting fatigue tests were carried out under cyclic direct stresses of 100 and 140 MPa, and a contact pressure of p=80 MPa for stress ratios of R=0.33, 0, -0.33, and -1. The crack closure phenomenon was monitored by the elastic unloading compliance method with a strain gage attached just beneath the surface crack front, and it was also observed on a TV monitor. The relationship between crack length a and crack propagation rate da/dN in fretting- and unfretting-fatigue was affected by stress amplitude σ_a and R. The crack closure phenomenon occurred markedly in fretting fatigue due to the compressive force by fretting pads. The effective stress intensity factor range ΔK_eff, calculated by using the measured crack opening rate U, can represent the crack propagation behavior in fretting fatigue relatively well without the effect of R or σ_a.

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This paper is concerned with the advanced damage and decreased stiffness ratio of FRP (SMC) due to cyclic loading in connection with its fatigue strength. Experimental evidence showed that the damage originates from the initiation of infernal cracks, which are classified into four types, and that some of them grow into the surface cracks. Also, for the purpose of estimating the extent of the damage, the crack density representing both the size and number of cracks during cyclic loading was employed. It was found that the stiffness ratio at Stage II is closely connected with the crack density, in spite of the existence of stress level dependence.

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A unified constitutive model and a continuous damage law are coupled to simulate life analysis for various loading histories on 2(1/4) Cr-1 Mo steel at 600°C°C. The viscoplasticity model and the incremental life prediction law are employed, and their commensurate coupling produces both the inelastic stressstrain response and the damage accumulation, simultaneously and continuously. A life prediction based on stable stress-strain hysteresis or creep deformation analysis is made for creep, fatigue and creep-fatigue loading, and the calculated life shows good correspondence with the experimental data. Subsequently, the aging effect on life prediction is examined.

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For the Leak Before Break (LBB) assessment of the nuclear piping system, analysis of the surface crack behavior in the ductile materials is indispensable. In this paper, the surface crack problems are treated with the finite element method based on the power-law hardening constitutive relation and the deformation theory of plasticity. The normalized solutions for the global J-integral and the displacement are obtained under the fully plastic condition. The models analyzed are plates with a semielliptical surface crack subjected to uniform tension. The solutions are applied to estimate the J-integral resistance for stainless steel type 304 from the experimental relation between load and crack opening describes well the constraint around the tip of the surface crack. The general J-integral characteristics of the surface crack penetration should be clarified by performing also experimental studies.

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A numerical method using the J^^^-integral is presented to analyze the dynamic problems of rapidly propagating crack by conventional finite elements. In order to simulate the crack propagation, the node release technique is employed in which the modal force near the crack-tip is gradually reduced to zero according to a formula chosen. It is found from numerical tests that satisfactory results are obtained by a linear relaxation formula. The dynamic stress intensity factor is determined by evaluating one of the three types of path(area)-independent formulae of J^^^-integral. It is shown that the computed results for elastodynamic problems are in good agreement with the analytical solutions and other available numerical results. It is also shown that the dynamic stress intensity factor of the viscoelastic problems can be determined by the formula of J^^^-integral which is independent of stress-strain relation.

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A Numerical experiment to examine the fitness of four methods of evaluating the K_IC based on the J-integral has been made. The methods examined were Irwin's method, the minimum value method, the two parameter Weibull distribution function method, and the three parameter Weibull distribution function method. The fracture toughness data consisted of about 20 K_IC data and about 150 J_c data obtained under the four test conditions. The main results obtained are as follows: (1) The scatter of evaluated K_IC by means of Irwin's method showed a smaller value than that of the other methods; (2) Suitable K_IC values are obtained by means of the minimum value and also Irwin's methods; (3) For the case of the minimum value method, the minimum required specimen number condition to obtain the appropriate K_IC is given by the following equation: Σβ≧2.5: (4) The two parameter Weibull distribution function method is appropriate to correlate the fracture toughness from a smaller specimen with that from a larger specimen, but it has some problems yet to be solved as a elastic-plastic fracture methodology to evaluate K_IC.

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Static and dynamic fracture toughness tests were carried out using the convenient J_IC test method proposed in the previous paper, to investigate the effect of temperature and strain rate on J_IC. It was found that in the upper shelf region, J_IC increases with a decrease in temperature and that J_Id is lower than, or almost equal to, J_IC. The reason why J _Id is lower than J _IC is considered to be that plain strain ductility at the crack tip is reduced with an increase in yield strength under high strain rate conditions.

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In this study, the dynamic fracture by a tensile pulse wave was investigated with emphasis on the dynamic stress intensity factor, which was determined by a photoelastic coating method using a high speed camera. The specimens used were anisotropic plates made of unidirectional glass fiber reinforced plastic with a crack paralleled in the fiber direction. Three kinds of FRP plates with a varied volume fraction of fiber were experimented on, and in each case, the tensile stress initiated the crack propagation and the stress intensity factor were analysed. The influence of anisotropic on the fracture criterion was made clear by comparing these results with those for an isotropic plate.

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Fracture tests of low alloy steels based on the J-integral have been performed on compact tension and three point-bend specimens to investigate the effect of specimen size on the tearing modulus. Also, tearing instability tests have been conducted to clarify the effects of specimen size, fracture toughness and cyclic load on the tearing instability characteristic. It has been shown that the specimen size has little effect on the tearing modulus up to ω=⃥2, but it has a notable effect on the critical value of the J-integral at the instability point (J_inst). For very tough material, J_inst depends on the testing compliance, but for not so tough material, it has little effect on J_inst. The value of J_inst under cyclic loading is found to be consistent with J_inst under the monotonic load condition.

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This paper describes the crack growth analysis of circumferential surface cracks in pipes, by the Line spring Method. The COA criterion is used for the crack growth analysis; and not only the radial crack growth but also the circumferential crack growth after penetration can be analyzed. The calculation is carried out for two circumferential cracks. One shows the maximum load after the penetration, and the other shows the maximum load before the penetration. Under the load controlled system, the former satisfies Leak-Before-Break while the later does not.

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Two types of high-modulus carbon fibers were coated with various thicknesses of silicon carbide by the chemical vapor deposition method, and the tensile-test of the mono-filament was carried out. Two failure modes of fibers, i.e., a fracture followed by a cracking of a coated layer (bonding-mode), and a fracture including a debonding of a coated layer (debonding-mode), were observed by SEM, depending on the type of fiber. The tensile strength of fiber in the debonding -mode decreased with increasing thickness of a coated layer, whereas the fiber in the bonding-mode did not reduce its strength up to a certain thickness of a coated layer. And, the strength in the bonding-mode was higher than that in the debonding-mode. Aluminum matrix composites using the SiC-coated carbon fibers were produced by the squeeze casting method. The tensile strength of the composites increased with increasing thickness of the coated layer. In this case, also, the composites using the fiber in the debonding-mode were not so strong compared with that of the fiber in bonding-mode.

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The optimization technique of the buckling eignvalue of FRP laminated plates is presented on the basis of the Hessian matrix. The fiber orientation is taken as design variable. The buckling eigenvalue problem is dealt with by the finite element method. The rates of change of the eigenvalue up to the second order are evaluated with the aid of the second order perturbation technique in order to constitute the Hessian matrix which is sued to determine the changes of the design variables needed to maximize the eigenvalue approximately. The procedure of such approximation is to be repeated until the eigenvalue converges to the largest value. The numerical analyses with respect to Born/Epoxy laminated plates under axial compression show that optimization, that is, maximization of the lowest buckling eigenvalue is made possible by use of the Hessian matrix.

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To investigate the cross effect of SUS304 stainless steel, combined tension and torsion stress tests were carried out at various temperatures. The torsional stress-strain relation subsequent to tensile prestrain and the tensile or compressive stress-strain relation subsequent to torsional prestrain were investigated experimentally. In these stress-strain relations, latent hardenings were observed. The flow stress in one stress direction subsequent to preloading in another direction was higher than that by the monotonic loading condition. This latent hardening phenomenon depended on temperature and the prestrain value. It was observed clearly at room temperature, but not at 600°C. This hardening magnitude was increased with prestrain.

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Room temperature uniaxial constant-loading tensile tests on HT80 steel showed significant creep. At stress levels around the yield strength, the creep was primary; but accelerating creep was found near the ultimate strength. Room temperature constant-loading fracture toughness tests on HT80 steel also showed significant creep. Time-dependent crack growth was found under the fracture toughness tests. The crack growth resistance under constant loading was lower than that under the conventional displacement-controlled fracture toughness test. The critical J-value at the stabilized load above which unstable fracture occurred, was 1.26 times the J_IC-value. The stable crack under constant loading seemed to propagate by repeating the blunting and re-nucleation processes. The crack growth rate at room temperature under constant loading was found to be controlled by the modified J-integral, J^^..

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The present paper deals with the problem of the duration of impact between two bars. The two bars collide with each other longitudinally, and one is plastically deformed, while the other remains elastic during the collision. In order to estimate the duration of contact between two bars impacting each other, the variations of stress at the impact ends are examined theoretically and experimentally. The theoretical predictions based on the strain-rate dependent theory for plastic wave propagation showed that as the length of a specimen, the impact velocity and the magnitude of the dependence of strain-rate increase, the duration of contact increases regardless of the rising mode of an impact velocity and its rise time. The experimental verification was made by measuring the time variation of elastic-plastic stress at the impact end of a lead specimen in collision with an elastic stress bar made of carbon steel. The theoretical predictions almost agreed with the experimental observations.

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When applying the results of a model study to practical problems, it is convenient to find the relation of similarity of both. we experience the impact behavior as shown in a contact of cam and cam straddle, more space after punctuation arc spot welding, etc. This paper studies a mechanically similar manner of linear elastic to the collision of two round bars or a round bar and periphery of circular disk, which are of various dimensions and material constants. It is important that the similarity of stress and the motion equation are valid for the dimensions, material constants of the member, and the time. Supposing the similarity of dynamic manner, we could investigate the response to the use of the material which is stable to the impact, in stead of, for example, titanium of firing caused by heat storage resulting from a rapid deformation, and valuable materials which need not be subjected to an unknown impact load.

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In this paper, we consider the interaction of an edge dislocation with a circular hole and a circular inclusion in an infinite body. They are placed in the order of the circular hole, the edge dislocation and the circular inclusion. The analysis is developed by Airy's stress function and by applying the bipolar coordinates. The method of perturbation is adopted for the determination of the unknown coefficients involved in the solution. The forces on the dislocation and the unstable equilibrium positions of the dislocation are calculated and shown in figures.

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The large deflections of a fiber-reinforced curved beam is analysed in the framework of the theory of ideal fiber-reinforced composite materials. The curved beam is reinforced in its axial direction and simply supported at its both ends. It is subjected to uniform pressure at the lateral surface as well as at the ends. for a linear elastic response, solutions are obtained and the results are shown in figures.

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The torsional impact response of a thick-walled cylinder with a circumferential edge crack is considered. Laplace and Hankel transforms are used to reduce the elasto-dyanmic problem to a pair of dual integral equations. The dual integral equations are solved by using an integral transform technique, and the result is expressed in terms of a singular integral equation of the first kind. The kernel of the integral equation is improved in order that the calculation may be made easy. A numerical Laplace inversion routine is used to recover the time dependence of the solution. The dynamic singular stress field is determined, and the numerical results on the dynamic stress intensity factor are obtained to show the influence of inertia, geometry, and their interactions.

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In this paper, an attempt is made to analyze the impulsive stresses in a glass plate subjected to the transverse impact of steel balls. For the case of a mass striking an infinite glass plate, the contact force variation is obtained by solving a nonlinear integral equation given by the use of Lagrange's classical plate theory and Hertz's law of contact. By regarding this force as an impact load for the case of the central impact of a steel ball on a supported circular glass plate, the impulsive stresses in a glass plate are analyzed using the three-dimensional dynamic theory of elasticity. Finally, it is shown that these analytical results are in good agreement with the experimental results.

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The power spectra of the internal forces in a vehicle running on an uneven road are investigated based on finite element modeling. The modeling is two-dimensional to treat the random excitations through the front and rear wheels. The damping of the suspensions and tyres is of a non-proportional type. The equation of motion is uncoupled by use of the complex eigenvalue problem. An attempt is made to apply the filtered Poisson process (FPP) to the simulation of a random road surface. The power spectrum generated by the FPP can simulate well the one published as ISO DP 8608 when the parameters are selected properly. The power spectrum of a spring reaction is evaluated regarding the effect of vehicle speed, the sensitivity of the spring constant and the deviation caused by uncertain damping, together with that of the bending moment in the model beam.

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In this paper, 'The body force doublet method' is proposed. This method of analysis uses the distributed body force doublet in order to satisfy given boundary conditions. For application, single-edge-cracked specimens under three- or four-point bending are analyzed, and the stress intensity factors and the crack opening displacements are evaluated. Their approximate expressions, effective for practical application, are shown.

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This paper is concerned with the elastic-plastic analysis of an infinite plate with a crack under von Mises' and Tresca's yield condition. The method of analysis is by the body force method extended to the elastic-plastic problems. In this method, the solutions are obtained by superposing the stress fields of a pair of point forces in an infinite plate with a crack, so as to satisfy the constitutive equation of plasticity. The main results obtained are as follows: (1) If Tresca's yield condition is used for the strain hardening material under the plane stress state, the plastic zone near a crack tip is found to be "⇽"shape, i.e. one part satisfying the yield condition |σ₁-σ_z|=σ_s extends slenderly along the x axis, and another part satisfying the yield condition |σ₁-σ₂|=σ_s extends at about 60°° to the x axis. When the rate of strain hardening is very low, the plastic zone becomes a line shape similar to when the Dugdale model is applied to a non-hardening material. (2) If the stress intensity factors under the combinations of several loads and crack lengths are equal, the distributions of elastic-plastic stress and strain are nearly the same, independently of crack length, within the range where the plastic zone is smaller than half of the respective crack lengths.

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Effects of the contact condition at the first ridge on mechanical behaviors of threaded portions are analysed by an axi-symmetric FEM which can deal with the three-body elastic contact problem including the effects of friction on two contact surfaces between threads of bolt and nut, and between fastened plate and nut. Since the cross sectional shape of nut at the first ridge changes circumferentially due to the lead angle of thread, it causes non-symmetric mechanical behaviors concerning the ratio of flank load and the stress concentration at the root of bolt. Four types of models are used to estimate these non-symmetric properties approximately. Each of them represents the cross section rotated one fourth revolution around the axis. The analytical object is composed of bolt, nut and fastened plate, and only the shape of nut is changed. The analysis is done by the axi-symmetric FEM ignoring the effects of lead angle. The ratio of flank load and the stress concentration at each cross section show some remarkable effects of the non-symmetric behaviors.

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The Finite Element Method is applied to an analysis of the stress distribution in the spherulite under uniaxial tension. The spherulite model of our calculation consists of three parts: the nucleus in the center, the crystalline macrofibrills and the amorphous region. The crystalline region, which has a higher Young's Modulus than the amorphous region, grows in the radial direction, dividing into branches. Our result predicts that in the sector near the tensile axis, the crystalline deformation is dominant; and in the sector perpendicular to the tensile axis, the amorphous deformation is dominant; and that the stress concentration which occurs in the junctions of crystalline macrofibrils and the center region, in which the distribution density of the junctions is fairly high, would be the nucleus of interspherulite fracture. Since a rupture was observed in our experiment of tensile loaded POM spherulites.

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This paper describes a method for the determination of fringe orders and directions of principal stress in the whole region of a 2-dimensional photoelastic model by image processing. The fringes (isochromatics, isoclinics) are extracted as points where the first derivative of the density distribution is zero. The determination of fringe orders or directions of principal stress between extracted fringes uses the values and pattern of extracted fringes. The results determined can be put out as a shaded image to monitor TV, and be utilized effectively to stress analysis by the finite element method, etc.

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The dimensional invariance related to the reliability index is examined. The reliability indices β, based on the Advanced First-Order Second Moment (AFOSM) method by use of the stochastic finite element method, are evaluated in the cases of bars and square plates under tension, whose Young's modulus is regarded as spatial homogeneous stochastic process. The reliability index for the bars is obtained by means of the analytical formulation for continuum. The numerical results show that the reliability indices for several autocorrelation fucitons converge well with the increase of the finite element division, and the index for the bar agrees well with the analytical one. This suggests strongly that the dimensional invariance holds for the reliability index evaluated by the finite element discretization and AFOSM method.

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This paper is concerned with the reliability index of the second moment method for structural reliability analysis. The first problem considered is whether μ/σ_z is invariant, where μ_z and σ²_z are the mean and variance of the function z=G(X₁, X₂, …, X_n) and G&lE;0 represents the failure state. X_i(i=1n) are the failure governing random variables. It is shown that μ_z/σ_z is not invariant. This result reveals the essential cause as being that β_FOSM lacks invariance, where β_FOSM is the reliability index of the FOSM (First-Order-Second-Moment) method. Hence, if invariance is the indispensable requirement for the measure of reliability, μ_z/σ_z is not adequate as a basis of such measure. The second problem considered is the relation between β_AFOSM and μ_z/σ_z, where β_AFOSM is the reliability index of the AFOSM (Advanced-First-Order-Second-Moment) method. Since β_AFOSM is invariant whereas μ_z/σ_z is not invariant, β_AFOSM may not be a good approximation of μ_z/σ_z . Numerical comparison of μ_z/σ_z, β_FOSM, β_AFOSM< reveals that β_AFOSM is not a better approximation of μ_z/σ_z than β_FOSM is. Hence, β_AFOSM is better interpretted as the shortest distance from the origin to the surface G(X₁, X₂, …X_n)=0 in the space of the normalized variables than as an approximate value of μ_z/σ_z.

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In this paper, the transient thermal stresses in a finite slab cooled by a linear moving coolant on the surface are measured by a strain gage. At the same time, the transient thermal stresses in an infinite slab heated by a linear moving heat source on the surface are theoretically analyzed. The industrial applicability of the theoretical results for an infinite slab is investigated by comparison of the theoretical results with the experimental ones.

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