The arrest function in fatigue crack propagation at 25 and 550°C in air was investigated for Fe-20Cr alloys containing dispersed oxide particles of Al₂O₃, ZrO₂ and Y₂O₃ in weight percents from 0.3 to 1.2. The P_max-constant ΔK-decreasing test, where the minimum load, P_min, increased with increasing crack length while the maximum load, P_max, remained constant, was employed in order to avoid crack closure. The main conclusions obtained are as follows : (1) Fatigue crack propagation properties including the threshold were independent of the material at room temperature under the closure-free condition. The fatigue threshold values at 550°C were higher than those at room temperature for all materials. In addition, the threshold values at 500°C were about 50% higher for Y₂O₃-dispersed alloys than those for the base alloy and Al₂O₃-or ZrO₂-dispersed alloys. (2) The increase of threshold level at 550°C for Y₂O₃-dispersed alloys was explained in terms of suppression of the spalling of the oxide layer formed at the fatigue crack tip by dispersed Y₂O₃ particles. Accordingly, we concluded that Y₂O₃ dispersion could show the arrest function of the fatigue crack propagation at elevated temperatures.

抄録全体を表示

Rotating bending fatigue tests were conducted on finished (material A) and electropolished (material EA) austempered ductile iron. The S-N curve of material A represented combined fracture, whose starting points of fracture is on the surface layer for a small number of cycles and is an internal defect for a very high number of cycles. On material EA, the structural sensitivity is lower than on material A, and is of a similar degree to 90% pearlite ductile iron. In the horizontal transitional part, the crack does not propagate to a significant extent. Due to the difference of matrix structure, the fatigue strength of the bainitic matrix is 25% higher than that of the 90% pearlite matrix structure.

抄録全体を表示

This paper discusses the improvement of the fatigue limit of martensitic-ferritic dual phase steel (S20C) with high-cycle fatigue tests under both rotating bending and reversed torsion. Control of the morphology of the martensite second phase brings about higher fatigue strength through the restriction of the coalescence of small cracks than that by the conventional increase in the micro-Vickers hardness of the martensite second phase where the crack coalescence inevitably causes propagation of a fatal crack. This restriction can be attained by controlling the microstructure with ferrite encapsulated by martensite. The level of the fatigue limit depends on the degree of restriction of small crack propagation with this microstructure.

抄録全体を表示

The fatigue crack growth behavior of a high-strength aluminum alloy, 7075-T651, was examined in deaerated and aerated 3.5% NaCl solution, in addition to in air. The relation between stress intensity range and crack growth rate in the same environment depended on stress ratio. For long cracks, the crack growth rate was always higher at higher stress ratio, while it was sometimes lower at higher stress ratio for short surface cracks. The effect of oxygen concentration was not observed for either long cracks or short surface cracks. The growth rate of long cracks in corrosion fatigue was almost the same as that in air when the stress intensity range was smaller than 7 MPam^1/2. In this region, however, the growth rate of short surface cracks in the solution was higher than that in air, and the growth rate of short surface cracks was higher than that of long cracks in the same environment. Although the difference in crack closure was considered to be responsible for the acceleration of short surface crack growth in air, the acceleration of short surface crack growth rate in corrosion fatigue could not be explained solely by the difference in crack closure. The difference in chemical environment within the crack should be considered.

抄録全体を表示

To clarify the effect of thermal-barrier spraying on the fatigue behavior of materials, low-cycle fatigue tests of thermal-barrier coated SUS304 stainless steel were performed under strain controlled condition at 973 K. Triangular strain waveform and tension or compression hold trapezoidal strain waveform were adopted. From the experimental results, decrease of fatigue of thermal-barrier-coated SUS304 was observed under triangular strain wave as compared with that of the base metal. On the other hand, fatigue life of the coated one under trapezoidal wave was same as that of the uncoated one. Difference of fatigue life between coated and uncoated material was explained by crack initiation behavior, which was obtained by A. C. potential method and observation by optical microscope. The number of cycles to crack initiation of coated specimen under triangular wave is smaller than that of uncoated one. On the other hand, crack initiation life of coated specimen is similar to uncoated one under trapezoidal waveform. Cracks initiated at the aluminum particles precipitated at the boundary between base metal and bonding layer. Also, it was discussed from the experimental results that the crack initiation and fatigue life were affected by the flaws on the ceramics layer formed during the spraying process.

抄録全体を表示

Rotating bending fatigue tests were carried out on the circumferential notched specimen of three kinds of carbon steel with nearly equal ferrite grain sizes of 20μm but different volume fractions of pearlite. Fatigue tests were also carried out for 0.15% carbon steel with ferrite grain size of 50μm and 0.45% carbon steel with ferrite grain size of 5μm. The effect of microstructures on the fatigue strengths σ_ws (the limit stress for slip band), σ_w1 (the limit stress for macrocrack initiation), and σ_w2 (the limit stress for fracture) is discussed by comparing these values between three kinds of carbon steel. The main results are as follows : (1) Since σ_ws is localized to a highly specified region, variation of K_tσ_ws is small irrespective of notch root radius. (2) If the grain size of ferrite is nearly equal, variation of σ_w1 is small irrespective of pearlite volume fraction under the certain amount of pearlite. (3) In medium carbon steel, σ_w2 increases with increasing grain size of ferrite. (4) Notch root radius at the branch point ρ_o varies with the grain size of ferrite.

抄録全体を表示

Tensile fatigue tests were performed on smooth and notched specimens of a ferritic (FDI) and an austempered ductile iron (ADI). Stress to number of cycles to failure curves were measured, and a notch factor, K_f, was determined from the static strength to fatigue limit region at 5×10⁶ cycles. In FDI, K_f is much lower than the form factor, α_e, of the notches, but in ADI, it was close to α_e. The notch sensitivity factor η in AID is 0.6-0.9 while it is 0.2 in FDI, indicating very high notch sensitivity of ADI. The results were discussed in terms of the over sressed depth (δ) concept estimated from the stress distribution at the notch root and the fatigue strength of smooth specimens. The δ is 0.1-0.2 mm in ADI while it is 0.5-1 mm in FDI. Macroscopic and SEM fractography were also performed, and the high notch sensitivity was attributed to the brittle and defect-sensitive ausferrite matrix.

抄録全体を表示

In order to clarify the physical basis of scatter in the fatigue life of age-hardened Al alloy 6061-T6, rotating bending fatigue tests of smooth specimens were carried out at several stress levels. Fifteen or seventeen specimens were fatigued at each stress level and the initiation and propagation behaviors of major cracks were examined for all specimens. The statistical treatment of each set of lifetime data was performed by assuming the Weibull distribution. The lifetimes studied here were the fatigue life, the initiation life of 30μm crack and the crack propagation lives from 30μm up to 100μm, from 100μm up to 1 mm and from 1 mm to fracture, respectively. The scatter characteristics of these lifetimes were investigated based on the CV vs σ_a relation (CV : coefficient of variation, σ_a : stress amplitude).

抄録全体を表示

An investigation was carried out in order to develop a sphere indentation method for evaluating microfracture strength of surface layers of structural ceramics. Indentation tests on Si₃N₄ plates using a Si₃N₄ spherical indenter as well as conventional bending tests were carried out. The relationship between the ring crack initiation strength evaluated from indentation tests and bending fracture strength was examined using the concept of effective area based on the Weibull distribution. The indenter size dependence of the ring crack initiation strength was also examined using the same concept.

抄録全体を表示

Tensile tests of Ti-6Al-4V and S45C plates with a hole were performed, and the characteristics of deformation near the hole were investigated through finite-element analysis. To clarify the crack initiation conditions, the relationship between the deformed shape of the notch and the equivalent plastic strain at the crack initiation is calculated. The main results are summarized as follows. (1) The critical equivalent plastic strain of a notch root for a crack initiation is determined by the value of notch root radius ρ alone, independent of notch depth. (2) Fact (1) can be explained through similarity of relative equivalent plastic strain distributions. (3) The deformation parameter h/d (h : height of circular hole after deformation, d : diameter of the hole) calculated by FEM is in good agreement with the experimental result in the range where no crack is initiated inside a plate in Ti-6Al-4V or no large deformation of the inhomogeneous structure including small cracks occurs in S45C.

抄録全体を表示

A high-pressure phase diagram at 5.4 GPa of Al-rich Al-Fe alloy has been determined by using the quenching method. The solid solubility of Fe in Al was increased to 0.25 at% and the eutectic temperature was raised to 850°C. Variations of some material properties for the obtained supersaturated solid solutions as a function of Fe content have been determined. The shear modulus and Vickers hardness of the solid solutions increased at the rates of 0.65 GPa/at%Fe and 119.73/at%Fe, respectively. Variations of material properties were also measured in the process of isochronal aging.

抄録全体を表示

This paper presents a unified analysis of isotropic out-of-plane shear problems containing two circular elantic inclusions and shows the analytical solutions. The applied forces considered in this paper are longitudinal shear stress at infinity, concentrated force, screw dislocation, dipole force, dipole dislocation and distributed forces, at arbitrary positions. The analysis is based on the complex variable method using the Mobius transformations by Honein and Herrman. Several numerical examples are given by graphical representations.

抄録全体を表示

Several numerical examples are illustrated by many graphical representations using the theoretical results of closed-form analytical solutions for an anisotropic semi-infinite thermoelastic body under a concentrated steady-state heat source with various surface boundaries such as free, fixed and two sliding conditions. Previous results for the two-dimensional problem are also included as particular cases of our solutions.

抄録全体を表示

An axisymmetric contact problem between a rigid punch and an elastic layer bonded on an elastic half-space is solved. Coulomb-type friction is assumed on the contact surface. Using Boussinesq's stress functions for the layer and the half-space, the problem is reduced to solving simultaneous integral equations with respect to the tractions and displacements on the contact surface. The integral equations are discretized into a system of algebraic equations to obtain the numerical solution of the problem. Contact stresses and displacements have been computed for the case of rigid flat-ended punch. The stick-slip boundaries have also been determined for various combinations of parameters, namely, thickness of the layer, rigidity of the layer, and coefficient of friction.

抄録全体を表示

Stacking of linear inhomogeneous layer elements approximates well a gradient inhomogeneous layer, and makes calculations easy. Each of the elements varies in thickness depending on the acoustic impedance. A formulation which makes use of the transfer matrix method reduces the number of calculations required. The variation of acoustic impedance within the layer is an important parameter for the characterization of the inhomogeneous structure of the material. To characterize the structure with a small number of calculations, the stacking method is developed and applied to the calculation. The most effective method of numerical estimation for a solid with monotonic variation of impedance, typical of the gradient functional materials, is investigated while changing the thickness of the stacked elements. The numerical example for an actual functionally gradient material, SiC-TiC layer, is shown with the gradient parameter p and adjustment parameter k.

抄録全体を表示

This paper discusses the stress concentration problem of an elastic solid with an elastic circular-cylindrical inclusion under tension. A method of solution is developed for the problem mentioned above by using fundamental solutions of axisymmetric problems of elasticity. The fundamental solutions are defined as solutions for the problem of an elastic solid subjected to axisymmetric body forces acting along a circle. Performing numerical calculations, influence of the length of the elastic circular-cylindrical inclusion on the stress distribution around the inclusion and on the central section is investigated. The influence of the share moduli of elasticity on the stress distribution is also shown.

抄録全体を表示

Young's moduli of VCR magnetic tapes 11 μm thick are ultrasonically measured along and transverse to the longitudinal direction. A tape sample is supported in a coupling fluid and a wide-band, line-focus PVDF transducer generates and receives the leaky Lamb wave signal (S₀ mode) together with the specular reflection. The signals are digitized and stored in the hard disc for subsequent processing. Young's modulus can be calculated from the phase velocity of the Lamb wave extrapolated to the zero-frequency limit. The influence is determined for the velocity dispersion due to the finite thickness/wavelength ratio as well as the acoustic coupling to the fluid. Three signal processing methods, i.e., direct timing, spectrum, and cepstrum methods, are compared with each other as well as with the static loading test. The VCR tapes were found to possess a large elastic anisotropy ; Young's modulus in the transverse direction was found to be 50% larger than that in the longitudinal direction.

抄録全体を表示

Significant interest lies in the influence of rate of change of strain rate on elongation. Uniaxial ductility of rate-dependent sheet materials which have the ability of strain hardening as well as strain rate sensitivity is examined. The analytical results combined with conventional instability theories predict the significance of the rate of change of strain rate during uniaxial deformation in estimating the ductility. In various uniaxial tensile tests used by a computer-aided Instron machine, the effect of accelerated strain rate is recognized about limit strain. Furthermore, the instability strain is evaluated by using standard deviation of axial strain distribution, and the influence of accelerated strain rate on instability strain is confirmed. In the present work it was clarified that limit strain and instability strain could be increased by using a higher value for the strain rate path.

抄録全体を表示

The flow stress in tension after torsional prestrain, for example, is higher than that in merely tensional loading. This phenomenon is called the cross effect, cross hardening or latent hardening. In the present study the cross effect was investigated with torsion-tension combined loading tests. It was observed that the cross effect as well as Bauschinger effect is a kind of delayed phenomenon due to change of stress path. We proposed a hypothesis that unconstrained Orowan loops around pile-ups produced during the preloading work as forest dislocations against moving dislocations on the cross-slip planes during the subsequent loading. Since the density of forest dislocations determines the flow stress, the free Orowan loops cause the cross effect for cross loading and also cause the Bauschinger effect for reverse loading. The above explanation for the cross effect was confirmed by the tension test after torsional and reverse torsional prestrains.

抄録全体を表示

According to many studies of measuring a residual stress of a worked material performed by the authors and others, the three-dimensional stress state must be considered in the subsurface layer. This paper presents a new measurement method of three-dimensional stress using polychromatic X-rays. In this method, three-dimensional stress is evaluated from strain distributions which are measured for six independent directions using the X-ray energy dispersive method. The residual stress distribution in a subsurface layer of the carbon steel after grinding was measured by applying this method. As a result, its three-dimensional stress was estimated using an exponential approximation curve for strain distribution.

抄録全体を表示

Measurement of stress based on the magnetoelastic interaction effect in ferromagnetic materials relies on a change in permeability due to the stress. A method for quantitatively evaluating the stress-permeability relation was proposed by combining measured magnetic flux density near a specimen with nonlinear analysis of the magnetic field by the finite-element method. First, the flux density near the specimen was measured in a low magnetic field independent of the detecting system. Next, the permeability was modeled by a linear function of the flux density. Also, the permeability was modeled to relate it with the stress in a linear form. Finally, the stress-permeability relation was determined qantitatively by comparing the measurement with the nonlinear analysis of the magnetic field based on the model of permeability.

抄録全体を表示

Adequate kinetic equations are required to express phase transformations which include reverse processes of heating as well as cooling during welding, heat treatment and so on. Governing equations which depend on temperature, elastic strain and distribution of grain diameter of a material are proposed in order to enable the representation of the diffusional transformation during the continuous process between heating and cooling. Diagrams of time-temperature-transformation (T-T-T) and time-temperature-austenitization (T-T-A) are calculated for eutectoid steel. Based on the kinetic theory proposed in this paper, a method of the welding process is presented using a viscoplastic constitutive equation, which may cover the region of molten and solidified states. Numerical results show the practical modes of temperature, structure change and induced stress/strain in the process.

抄録全体を表示

Functionally gradient materials (FGM) have been developed as ultrahigh-heat-resistant materials in aircraft, space engineering and nuclear fields. In the heat-resistant FGM which contain particles (ceramics) in the matrix (metal), the matrix will be subjected to plastic deformation, particles will be debonded, and finally cracks will be generated. The constitutive equations of FGM which take into account the damage process and change in temperature are necessary in order to solve these phenomena. In this paper, the constitutive equations of particle-reinforced composites with consideration of the damage process and change in temperature are estimated by the equivalent inclusion method in terms of elastoplasticity. The stress-strain relations and the coefficients of linear thermal expansion of the composites (Al-PSZ&Ti-PSZ) are calculated in ultrahigh temperature.

抄録全体を表示

The present authors proposed the adjoint boundary integral method for identifying heat source or force in a domain from values of boundary integrals involving an adjoint operator. The integrals can be evaluated if boundary values are available over the entire boundary of the domain. In the present study a Poisson field source is identified from noisy and discrete boundary observations by applying the method. Adaptive Gauss quadrature enables evaluation of the boundary integrals from observations at discrete points, which deviate from the Gauss points for approximate evaluation of boundary integrals. Numerical simulations are carried out for identifying location and intensity of a concentrated source in a two-dimensional domain. Effects of errors in boundary observations, deviation of locations of observation points from the Gauss points, the order of Gauss quadrature, and the location of source on the accuracy of the identification are discussed. It is shown that the location and intensity of the source can be estimated reasonably from noisy and discrete observations by applying the adjoint boundary integral method with the adaptive Gauss quadrature.

抄録全体を表示

In this paper, static and/or dynamic plate theories are formulated based on the most generalized successive higher-order approximation method. Typical successive approximation theories are deduced by introducing some specializing assumptions into the present formulation method. These are generalized theories of several previously proposed kinds of consistent theories. At the same time, using these formulations, the theoretical characteristics of the theories and their interrelations will be shown. From these study, successive approximation plate theories, higher-order plate theories based on the asymptotic method and consistent plate theories are situated globally.

抄録全体を表示

The transformation-induced plasticity (TRIP) steel possesses such favorable mechanical properties as high work-hardening characteristics, tenacity and strength. The strain-induced martensitic transformation, however, strongly depends on the temperature and strain rate imposed, and an appropriate improvement of mechanical properties is realized under quite restricted circumstances. To obtain the required mechanical properties, a constitutive model which can suitably predict the deformation behavior including transformation under a wide range of deformation conditions has been developed by employing Olson and Cohen's model for strain induced martensitic transformation kinetics. Then, deformation behavior of SUS304 (18-8) cylinder has been simulated under different environmental temperature with different strain rates by thermocoupled finite element method accounting for the latent heat induced by martensitic transformation. The possibility of the improvement of such mechanical properties of the material as strength, ductility and toughness has been individually discussed.

抄録全体を表示

The basic deformation properties of pseudoelasticity and shape memory effect due to martensitic transformation and R-phase transformation were investigated experimentally for TiNi shape memory alloy (SMA) wire subjected to various maximum strain. The properties of transformation temperature were discussed based on differential scanning calorimetry. The main results were summarized as follows. (1) The deformation properties of pseudoelasticity and shape memory effect were determined based on a stress-temperature phase diagram in which stress and temperature were variables. The stress-temperature phase diagram was useful for design of SMA elements. (2) If maximum strain was in the martensitic transformation region, both reverse transformation stress and reverse transformation temperature were almost constant. If maximum strain was in the upswing region after completion of the martensitic transformation, reverse transformation stress decreased and reverse transformation temperature increased with an increase in maximum strain, resulting in movement of the reverse transformation lines downward and to the right.

抄録全体を表示

In the previous report, the systematic analysis of the strength and deformation and the reinforcing effect of rings were discussed, utilizing the method introduced by this author, and it was found that when the distance between rings is very small, the probability of constructing an optimum structure exists. In this report, the method for constructing such structure above has been developed, and using this structure, several case studies have been performed. Consequently, it is found that the strength of the cylindrical shell made of plastic can be reinforced by steel wire ring to over one hundred times that in the case of no rings, and that the time required for calculation is within several tens of seconds, even with the use of a pocket calculater.

抄録全体を表示

Mechanical properties of shape memory polymer vary at glass transition temperature T_g. In the present study, the influence of loading temperature upon shape recoverability, shape fixity and the cyclic properties of shape memory polymers of the polyurethane series has been investigated experimentally for uniaxial tension under thermomechanical cycling. The results are summarized as follows. (1) Deformation resistance of the materials increases with an increase in the number of cycles. (2) In the case of the loading temperature at T_g + 20 K, residual strain which appears after cooling and unloading recovers at temperatures in the vicinity of T_g. In the case of loading temperatures below T_g, strain recovers at temperatures in the vicinity of the midpoint glass transition temperature T_mg. (3) In the case of loading temperatures above T_g, recovery stress which appears upon cooling under constant maximum strain is about twice as large as the stress in the loading process. The recovery stress occurs at temperatures in the vicinity of T_mg. (4) Excellent shape fixity can be obtained if the material is cooled under maximum strain and unloaded.

抄録全体を表示

Molecular dynamics simulation is used to investigate the atomic mechanism of frictional sliding between a pin and a solid film consisting of several atomic layers on the flat surface of the substrate. The strength of the interaction of atoms in the film or between the pin and the film or the film and the substrate are modeled using virtual potentials determined by the minimum of potentials and cut-off distance. As a result, the conditions for minimizing wear are identified as follows. (1) A smaller load is needed. (2) The strength of the interaction of atoms between the pin and the film must be lower than the strength of the interaction of atoms of the film. (3) The film must have sufficiently strong interaction of atoms to prevent atomic defects in the film caused by the penetration of the pin. Results of the simulation suggest the possibility that a type of atomic wear takes place when atoms jump from the surface of the film upon compression.

抄録全体を表示

FBR uses liquid sodium as a primary coolant and is characterized by operation at high temperatures (380°C thru 550°C) and under atmospheric pressure. Therefore, the main vessels are essentially required to cope with thermal stresses in their structural design. To reduce such stresses, it is necessary to make the vessel walls as thin as possible. This makes it essential to study the stability of reactor structures against buckling under earthquake loadings. The purpose of the present study is to establish a rational seismic buckling design guideline by making a comprehensive survey of buckling data which will be of definite importance in securing the seismic stability of FBR main vessels. The study started in FY 1987 has been progressing toward completion in FY 1993. In this 1st report, we describe the conceptual background and the outline of the seismic buckling design guideline.

抄録全体を表示

Although plastic shear-bending buckling of the cylindrical part of FBR main vessels under horizontal earthquake loading is one of the most critical problems in structural design, the evaluation method of buckling strength is not specified in related standards in Japan. The purpose of this paper is to describe the results of static buckling tests of cylindrical shells and to present formulae for determining the allowable buckling strength of FBR main vessels. Shear buckling and bending buckling are evaluated under the assumption that both buckling modes interact slightly with each other. Shape imperfection within the plate thickness regarding shear buckling, and one-half the plate thickness regarding bending buckling is considered in the formulae originally.

抄録全体を表示

The mechano-compatibility required for an artificial joint is accord between the artificial bone and the natural bone structure. The purpose of this research is to develop an evaluation system of stress transmission style from the artificial bone to the natural bone. In this work, a formulation quantitatively expressing the stress transmission style of cancellous bone is derived by considering the structural optimization. The usefulness was verified by the quantitative measurement of the soft X-ray image of a normal human proximal tibia. Finally, applying the formulation to the models of existing knee prostheses, the mechano-compatibility of these prostheses was evaluated.

抄録全体を表示