The mechanism of fatigue failure in the ultra-high cycle regime was studied using a martensitic stainless steel. The effect of internal hydrogen trapped by nonmetallic inclusions on high cycle fatigue behaviour has been discussed by Murakami et al. In order to investigate more in detail the mechanism of the elimination of the conventional fatigue limit and the influence of hydrogen trapped by inclusion, specimens with various artificial small defects were prepared. The fatigue failure of a specimen which contained an artificial hole occurred at N_f ≅10⁸ from a nonmetallic inclusion which is smaller in size than the artificial small surface hole. In the case of the specimen containing two artificial holes connected with a fatigue crack, non-propagation crack was observed at the edge of the artificial hole, and the fatigue limit was in good agreement with the value predicted by the √area parameter model. In the case of the specimen whose fatigue crack originating at an inclusion, the crack continued propagation, and finally the specimen failed. Considering that nonmetallic inclusions trap hydrogen, it is presumed that the elimination of the conventional fatigue limit due to fatigue crack originated at nonmetallic inclusion is caused by synergetic effect of cyclic stress and hydrogen trapped by inclusions.

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Thin films of single crystal silicon are expected to many applications for semiconductor devices and MEMS well. It is important to evaluate the mechanical properties of the thin film. Especially, it is essential to clarify the strength and fracture properties of the thin film with stress concentrations. In this study, MEAM molecular dynamics simulation is conducted to investigate the effect of notch depth on the deformation and fracture behavior. The loading direction is [110] and the notch is in the [001] direction on (110) plane. In a smooth specimen, Young's modulus was 153.1 GPa which is less than that of the bulk material. In notched specimens, the stress at crack propagation decreases with increasing notch depth. When the notch depth is shorter than 1 nm, the stress intensity factor at the crack propagation decrease with decreasing notch depth. However, when the notch depth is longer than 1 nm, the stress intensity factor is nearly constant irrespective of notch depth.

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The R-curve method for predicting the fatigue thresholds of notched components was applied to the fatigue thresholds of notched components under combined loading of cyclic torsion and tension-compression. The prediction was compared with the experimental data obtained from thin-walled tubular specimen with a hole under the combination of cyclic torsion and axial loading. The effect of the hole size on the fatigue thresholds was calculated from the R-curve method for three cases : cyclic torsional stress, cyclic torsional stress with superposition of a in-phase cyclic tension-compression stress with the same amplitude, cyclic axial tension-compression stress. The stress ratio was R =-1. The experimental data agreed well with the prediction both for crack initiation and fracture. The measured length of nonpropagating cracks had some scatter and the maximum length agreed fairly well with the predicted line. The nonpropagating crack length normalized with the hole radius at the threshold stress for fracture was predicted fairly constant without respect to the hole size, while it varied slightly with the loading condition. The effect of the in-phase combination of axial and torsional stress loadings on the fatigue threshold was predicted by assuming the crack direction perpendicular to the maximum principal stress. A good agreement between predicted and experimental results was obtained.

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This is the experimental stress analysis that detects cyclic biaxial stress based on shape of grown grains occurred at the hole edge of copper foil with circular holes. In view of actual application, both the minimization of hole diameter and the effect of moduli of elasticity of the material were inves-tigated in this report. It became clear that the minimum hole diameter and the measurement region were 0.2 mm and about 15 mm2 respectively, since the difference of the grown grain shape didn't appear remarkably on 0.1 mm hole diameter, even if the biaxial stress changed. And it was demonstrated that shape of grown grains was influenced by a shear strain that occurred to a copper foil. The constitutive equation for obtaining the biaxial stress was corrected in the form considering the effect of the moduli of elasticity of measured material on this method.

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The effect of stress ratio (R=σ_min/σ_max) on subsurface fatigue fracture of high-speed tool steel, SKH51, was investigated using smooth specimen subjected to axial reversed loading in air at room temperature. From the experimental results, interior inclusion induced fracture occurred in each stress ratio. The number of cycles to the transition from surface fracture mode to subsurface fracture mode depended on R. A granular-bright-facet (GBF) area formed around the nonmetallic inclusion inside the fish-eye zone on the fracture surface of the specimen in long-life fatigue regime, N_f>106, at R=-1.3, -1 and 0. From the detail observation of GBF area by three-dimensional SEM, the roughness of GBF area depended on the stress intensity factor range, Δk_{inc, s,} at the inclusion of facture origin.

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In this study, a linearization approach is used to develop an implicit integration scheme for the high-temperature inelastic constitutive models based on non-linear kinematic hardening. A non-unified model is first considered in which inelastic strain rate is divided into the transient and steady parts driven, respectively, by effective stress and applied stress. By discretizing the constitutive relations using the backward Euler method, and by linearizing the resulting discretized relations, a tensor equation is derived to iteratively achieve the implicit integration of constitutive variables. The implicit integration scheme developed is shown to be applicable to a unified constitutive model in which back stress evolves due to static and dynamic recoveries in addition to strain hardening. The integration scheme is then programmed as a subroutine in a finite element code and applied to a lead-free solder joint analysis. It is thus demonstrated that the integration scheme affords the quadratic convergence of iteration even for considerably large increments, and that the non-unified and unified models give almost the same results as each other in the joint analysis.

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In this study, the theoretical analysis of a transient piezothermoelastic problem is developed for a piezoelectric strip with a parallel crack under static electric loading and thermal shock loading conditions. The crack faces are supposed to be insulated thermally and electrically. By using both the Laplace transform and the Fourier transform, the thermal and electromechanical problems are reduced to a system of singular integral equations, respectively, which are solved numerically. Some numerical results for the temperature change, the stress and electric displacement distributions, and the electric displacement intensity factors in a transient state are shown in figures.

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The thermoelectromechanical fracture problem for a symmetrical functionally graded piezoelectric slab containing a center crack parallel to the free boundaries is considered in this study. It is assumed that thermoelectroelastic properties of the medium vary continuously in the thickness direction, and that the slab is under thermomechanical loadings. The crack faces are supposed to be insulated thermally and electrically. By using the Fourier transform, the thermal and electromechanical problems are reduced to singular integral equations, respectively, which are solved numerically. Numerical calculations are carried out, and detailed results are presented to illustrate the influence of the crack length and the material nonhomogeneity on the temperature-stress distributions and the stress intensity factor.

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In this paper, a quantitative prediction method for the gatigue limit reliability of a notched metal with different sorts of inhomogenities is proposed. The fatigue limit of the notched specimen cosists of the microcrack nonpropagation limit σ_ω1, small crack nonpropagation limit σ_ωd and macrocrack nonpropagation limit σ_ω2. The reliabilities of σ_ω1 and σ_ωd are predicted by the stress-strength model which consists of “statistical characteristics of hardness in a small region” and “statistical characteristics of a mechanical condition in the small region containing the defect under a cyclic stress”. The σ_ω2 is predicted by the Linear Fracture Mechanics. The present method was applied to the aluminum cast alloy JIS AC4B-T6 where eutectic Si, Fe compuound and porosity are contained. The fatigue strength at 10⁷ tress cycles was defined as the fatigue limit. Then rotating bending fatigue tests of notched specimens with constant notch depth t =0.5 mm and various notch root radii ρ=2, 1, 0.3, 0.1 mm were carried out. The validity of the present method was examined by comparing predicted results with experimental results.

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The purpose of this study is to absolutely evaluate the fracture strength of ceramic films deposited on the hard substrate by considering the residual stress on films, and to clarify the effect of the deposition condition on the fracture strength of ceramic films. TiN films were deposited onto two kinds of WC-Co substrates with different hardness using dc magnetron sputtering using various bias voltages V_B. Sphere indentation tests were carried out to obtain the micro fracture strength σ_{f, m} for ring crack initiation on TiN films using sphere indenters of varying diameter 2R. The main conclusions are the following. (1) σ_{f, m} depend on 2R, but are independent of the hardness of a substrate. Based on the probabilistic theory assuming a Weibull distribution, the relationship between the mean value of σ_{f, m} and 2R can be predicted. (2) σ_{f, m} increase as V_B is increased. (3) Based on the probabilistic theory, the residual stress σ_R on TiN films and the fracture strength σ_f of TiN films under uniform tensile stress condition for various V_B were estimated from the distribution characteristics of σ_{f, m}. Variation tendency in σ_R is consistent with that by X-ray stress measurement. But, variation tendency in σ_f is opposite to that in σ_{f, m}.

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We have analysed the stability of carbon (C)-hydrogen (H) complex in compressively stressed silicon (Si) single crystal with ab initio calculation. First, the relationship between tensile stress and the strain of Si crystal along [100], [110] or [111] direction was obtained. It was found that the calculated Young's modules of Si crystal agreed with experimental results within 10% difference. Next, the stable structure of C-H complex in strain-free Si crystal was investigated with calculating the total energy of several structures. It was found that the most stable structure consists of the substitutional C atom and interstitial H atom located at the nearest bond center (B) site. Binding energy of C and H atom was about 1.25 eV. Finally, we have studied the stability of C-H complex under uniaxial [110] compressive stress. Here, [110] direction is in (110) Si plane and perpendicular to (110) Si plane. Total energies were compared between complex A (C-H complex is in (110) plane) and complex B (C-H complex is in (110) plane). It was found that complex B is more stable than A from 0.25 to 15 GPa compressive stress.

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A method of coupled magneto-superelastic analysis by the sequential approach is proposed for shape memory alloy (SMA) helical spring actuators controlled by magnetic force. The commercial finite element software ANSYS/Emag is used for the magnetic field analysis, while the one-dimensional finite element program developed by the authors is used for the analysis of superelastic behaviors of SMA helical springs. The validity of the proposed method is verified by applying the method to the analysis of actuator models utilizing SMA composite or ferromagnetic SMA helical springs and comparing the calculated results with the experimental results.

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This study deals with the optimization problem of material distributions for maximizing the applied internal pressure of thick-walled functionally graded material (FGM) circular pipes subjected to thermomechanical loading. For the FGM circular pipes without cracks, the ultimate internal pressure is determined using the tensile strength criterion. For the FGM circular pipes with cracks, radial edge cracks emanating from the inner surface of pipe are supposed and the ultimate internal pressure is determined using the fracture toughness criterion. The optimum material distribution for maximizing the applied internal pressure is obtained using a nonlinear mathematical programming method. Numerical results for a thick-walled TiC/Al₂O₃ FGM circular pipe reveal that the load carrying capacity can be improved markedly by choosing an optimum material distribution profile.

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Ultrafine-grains (UFG) are induced by warm or cold-rolling process under severe plastic deformation. A transition of microscopic structure of a metal caused by behaviors of dislocations is closely connected with a macroscopic deformation. The information of dislocation field and deformation field is desirable to be coupled in the case of simulation of UFG formation. Profiles of formation process of UFG, e.g., aspects of subdivision and induced grain sizes, are expected to be numerically predicted. In this paper, a numerical method for estimating generation of subdivisions is proposed, and also the induced grain size is introduced into a strain rate sensivity model as an argument. A dislocation-crystal plasticity FE simulation based on the geometrically necessary crystal defects proposed in the previous paper and on the above model is carried out for large deformation of an FCC polycrystal under severe compression plane strain. Distributions of crystal defects and crystal orientations in a specimen are visualized and the influence of strain on separation of grains and one of strain rate sensitivity on refinement of grains under cold-rolling process are discussed in detail.

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In this paper. the eccentricity factor in the progressive crushing of tubes subjected to axial compression is studied by using finite element method. It is found that the eccentricity factor is dependent on the thickness-to-radius ratio t/R_o : the eccentricity factor for a thick tube is higher than that for a thin tube. The reason why the eccentricity factor is a function of t/R_o and always takes the value of m>0.5 is related to the fact that the relations between the stress and strain are different in the outside portion and inside portion of the fold produced in the crushing. Based on the consideration of this fact, an approximation equation is proposed to evaluate change of the eccentricity factor with the tube thickness.

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In this study, partially debonded spherical particles in a particulate composite are analyzed by three-dimensional finite element method to investigate the effect of debondings on the composite stiffness, and the way to replace a debonded particle with an equivalent inclusion is examined. The variation in Young's modulus and Poisson's ratio of a composite with the increase of the debonded angle was evaluated for different particle arrangements and particle volume fractions, which in turn compared with the results derived from the equivalent inclusion method. Consequently, it was found that by replacing a debonded particle with an equivalent othotropic one, the macroscopic behavior of the damaged composite could be reproduced so long as the interaction between neighboring particles is negligible.

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This paper proposes a numerical simulation for the interlaminar damage propagation in FRP laminates under transverse loading, using finite element method. First, we conduct drop-weight impact tests on CFRP cross-ply laminates. A ply crack was generated at the center of the lowermost ply, and then a peanut-shaped interlaminar delamination was propagated in 90/0 ply interface. Based on these experimental observations, we present a numerical simulation for the interlaminar damage propagation, using cohenive zone model to address the energy-based criterion for damage propagation. Moreover, this simulation addresses interlaminar delamination with high accuracy by locating a fine mesh region near the damage process zone, while its computational efficiency is kept by the use of an automatic mesh generation. The simulated results of interlaminar delamination agree well with the experimental results. Moreover, we showed that the computational cost for the simulation is dramatically reduced by the proposed method.

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This paper proposes a numerical simulation based on finite element analysis (FEA) for the tensile damage and final failure in FRP cross-ply larninates. The simulation addresses multiple damage (transverse cracks, interlaminar delaminations, fiber breaks) simultaneously for the tensile failure process of cross-ply laminates. Transverse cracks are addressed as initial damage in 90°ply of cross-ply laminates by the cohenive zone model (CZM). The interlaminar delamination at the tips of those transverse cracks are addressed by aligning cohesive elements between 0°ply and 90°ply. Moreover, the fiber breaks in 0°ply are addressed by truss elements, which represent fibers in 0°ply. The simulated results of tensile damage evolution agree well with the experimental results. Especially, our simulation could reproduce a staggered pattern of transverse cracks and interlaminar delarninations at the tips of transverse cracks in [90/0] s cross-ply laminates.

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The fiber reinforced plastics (FRP) have been utilized in various structural applications of automobile and aerospace industries because they have excellent features of high specific strength and stiffness ratios. The FRP composites are fabricated typically by stacking orthotropic layers, each of which is composed of reinforcing fibers and matrix materials. In this work, an optimum design approach is proposed to optimize buckling performance of laminated composite plates by using the lamination parameters. The approach consists of two parts : the first part is to optimize the lamination parameters by a gradient method, and the second part is to obtain the optimum lay-up design for the maximum buckling loads by minimizing the errors between the opimum parameters and the parameters for all possible discrete lay-up designs. The feedback concept is considered to ensure the global solutions. It is shown in numerical examples that the proposed approach is quite effective in determining the lay-up design for the best buckling performance.

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It is necessary to get a fundamental data of FW-RP cylinder setting up a side circular hole for a design of mechanical structures transmitting torsion load. In this study, the torsion rigidities near the side hole were measured by static torsion tests. Moreover, the properties of FW-RP cylinder with a side circular hole were calculated by an original FEM program that is able to simulate FW-RP cylinder. As a result, the following conclusions were obtained. (1) The partial torsion rigidity of FW-RP cylinder with a side circular hole is remarkably dropped down near the side hole. Especially, the reduction of the partial torsion rigidity is very large in thick FW-RP cylinder. (2) When the diameter of a side circular hole is large, the distribution of the stress around the hole is much influenced by the fiber orientation angle θ. (3) The maximum value of normalized stress around the side circular hole occurred at inside of the FW-RP cylinder in a thin cylinder (t=1mm). The reason of the movement of the maximum normalized stress is based on the difference of the rotation displacement about z-axis along thickness.

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Optical Coherence Tomography (OCT) has been developed as a cross-sectional imaging method of microstructural biological tissue with high resolution 1-10μm. Recently, multifunctional OCT system has been promising, e. g. estimator of bio-mechanical characteristics. In this study, presented is Optical Coherence Straingraphy (OCS) on the basis of OCT system, which can diagnose tissue strain distribution. This is composed of the recursive cross-correlation method, resulting in deformation vector distribution with high resolution. In addition, the spatial vector interpolation by local least square method is introduced to remove erroneous vectors and smooth the vector distribution. This was experimentally applied to compressed homogeneous and heterogeneous samples. Conse-quently, the deformation vector distribution has less erroneous vectors and better agreement with the estimation of ideal uniform compression than that obtained by the conventional direct cross-correlation method. Therefore, the proposed method was validated with the smooth strain map having the same high resolution 20μm as that defined by optical system.

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