Polymer matrix composites have sometimes been used for shielding electro magnetic interference (EMI). In order to make clarify the effects of the fiber volume fraction of the composites on creep phenomenon, the creep behavior during 3-point bending creep tests was investigated at various constant temperatures. The material used had stainless steel fiber conductive fillers and a metamorphic poly-phenylene ether matrix. Three fiber volume fractions were 0.73, 1.52 and 2.39%. The master curves of creep compliance were obtained by applying the Arrhenius reciprocation laws of time and temperature for all materials. The shapes of the 3 master curves were found to be the same, being shifted on the time axis and on the creep compliance axis. Therefore it was concluded that creep behavior of the materials depended mainly on the viscoelastic property of the matrix resin and also depended not only on time and temperature, but also on fiber volume fraction.

View full abstract

Fatigue degradation and life prediction for a plain woven glass fabric reinforced polyester under tension-torsion biaxial loading were investigated. A fatigue damage accumulation model based on the continuum damage mechanics theory was developed, where modulus decay ratios in tension and shear were used as indicators of damage variables (D). In the model, the damage variables are considered to be second-order tensors. Then, the maximum principal damage variable, D^* is introduced. According to the similarity to the principal maximum stress, D^* is obtained as the maximum eigenvalue of damage tensor [D^'_<ij>]. Under proportional tension-torsion loading, fatigue lives were satisfactorily predicted at any biaxial stress ratios using the present model in which the fatigue characteristics only under uniaxial tension and pure torsion loadings were required.

View full abstract

The fatigue reliability under elevated temperatures in advanced carbon fiber reinforced PA46 was estimated. The influence of mesoscopic structures on fatigue fracture under elevated temperature conditions was focused on. The main results are as follows; A compact high-bride fatigue test system was constructed in order to maintain a high-temperature condition and measure crack tip heating using a thermal camera. Carbon-fiber (CF)-reinforced PA46 showed better fatigue strength than other test pieces. Through observations with a microscope and thermal camera, it could be found that fatigue processes consist of three stages : crazing, meso-crack forming, meso-crack propagating. Meso-crack forming stages was longest in the fatigue life. The effects of matrix and reinforced fibers on meso-crack forming could be controlled based on the weakest link hyPothesis.

View full abstract

A survey of degradation in overhead traveling cranes was performed and fatigue cracks were often detected in highly stressed regions in cranes that have been used for fifteen years or more. Service stresses of an overhead traveling crane with lifting capacity of 50 kN were monitored and analyzed by the rain-flow method. A fatigue test of gusset welded joints made of SM490A steel was carried out under both constant loading and program loading determined based on the service stress monitoring. The fatigue life of the gusset welded joints is dominated by crack initiation rather than propagation. The crack growth rate of the welded joints under program loading is correlated with an effective stress intensity factor range, ΔK_rem, estimated based on linear accumulation of the stress intensity factor range which takes into account residual stress at each step in a block. The prediction of fatigue propagation life using ΔK_rem agrees with the experimental result in the range of error within 15%, when the crack length is less than 20 mm.

View full abstract

An arrest function of fatigue short crack growth from notch root at 200 and 550°C in air was investigated for Fe-20Cr alloy containing 1.1 weight percent of dispersed Y₂O₃ particles and for SUS403 (Fe-12Cr) and SUS304 (Fe-18Cr-8Ni) stainless steels containing 0.2 weight percent of dispersed Pb particles. The main conclusions obtained are as follows. The fatigue Iimits at the high temperatures were 40∼60% higher for the alloy containing dispersed Y₂O₃ particles and the stainless steels containing dispersed Pb perticles than for their base metals. Above the fatigue limit, the fatigue life was longer for the materials containing dispersed Y₂O₃ or Pb particles than for their base metals, because the growth rate of a fatigue short crack, which was initiated from the notch root, was low. Those improvements in high-temperature fatigue properties were explained, considering that the presence of Y₂O₃ or Pb particles strengthened the oxide film at the notch root or at the crack tip of a short crack. Accordingly, we concluded that Y₂O₃ and Pb dispersed particles show an arrest function of fatigue short crack growth at elevated temperatures.

View full abstract

A small crack growth law-oriented approach was applied to explain the cumulative fatigue damage in a SUS 304 stainless steel which shows cyclic hardening behavior and the effect of prior fatigue damage on the subsequent crack growth rate in the case of two-step strain amplitude. After switching of strain, the stress amplitude and crack growth rate increased or decreased depending on the combination of the first and second strain amplitudes. These changes were interpreted as being induced by the effect of the prefatigue damage or strain history. A modified small crack growth law, compensating the discrepancy between the crack growth rates for constant and two-step strain amplitude, was proposed. Finally the modified crack growth law was found suitable for understanding the Miner rule in the case of materials which exhibit hardening behavior.

View full abstract

Strain controlled low-cycle fatigue tests were performed under push-pull loading conditions at -140°C, -90°C and room temperature (RT) using commercial pure iron having ferrite grain size of 400μm. To clarify the fracture mechanisms, observation of the fracture surfaces was also carried out with special focus on the role of deformation twins in the generation of microcracks which lead to final fracture of the specimen. The transition of the fracture modes occurs from surface to internal fracture modes with an increase in the plastic strain range Δε_p at testing temperatures of -90°C and -140°C. In a large Δε_p regime crack initiation occurs inside the material, and this internal crack leads to final fracture. Deformation twins strongly affect the generation of internal cracks at -140°C and -90°C. Three internal crack initiation sites were observed: (i) the intersection of deformation twin and grain boundary, (ii) the intersection of several deformation twins and (iii) the intersection of deformation twin and inclusion. The applicability of the Manson-Coffin low-temperature low-cycle fatigue of commercial pure iron is also discussed in relation to the fracture mode transition behavior.

View full abstract

The effect of grain size on mechanical properties of a T6-treated AZ91-magnesium alloy was studied. In order to obtain samples free from shrinkage, oxides and gas defects, the alloy was produced using a continuous casting process with various casting temperatures, and a constant cooling rate. In the T6-treated AZ91 alloy, discontinuous precipitates were observed clearly along small grain boundaries, while continuous precipitates were observed in large grains. It was found that hardness in a grain depended on the form of precipitation and the magnesium alloy having a smaller grain size showed larger values of fracture toughness, ultimate tensile strength, 0.2% proof stress and elongation at fracture. Fracture toughness of the alloy having a finer grain size increased from 18.3 MPa·m^1/2 to 21.4 MPa·m^1/2 after T6 treatment. The strengthening mechanisms leading to these results are concluded to be the grain-size dependence of stress concentration at the grain boundary, the density of grain boundaries and the grain boundary precipitates which act as obstacles when cracks propagate.

View full abstract

In order to elucidate the mechanism of deformation and fracture of microcomponents, numerical simulations are conducted for nanoscopic wire and film of nickel without lattice defects using molecular dynamics on the basis of the EAM (embedded atom method) potential. Applying a periodic boundary, large as well as small materials are subjected to a tensile strain along the [001] direction of the FCC (face-centered cubic) lattice. Here, the traverse stresses, σ_xx and σ_yy, in the former are kept at zero during the tension. The yield is brought about by the crystallographic slips on the (111) planes and there is little difference in the yield stresses among the wire, film and bulk. The slips continue to take place on multiple (111) planes and the plastic deformation leads to ductile fracture. Then, the displacement in the traverse direction on the cell boundaries of bulk is fixed in order to investigate the effect of constraint. It shows brittle fracture due to cleavage cracking. this implies that the constraint, which may be introduced by local inhomogeneity of the material, brings about early crack nucleation and reduces the ductility of matcrials without lattice imperfection.

View full abstract

Cavitation erosion was analyzed from the propagation of stress waves in 3-layer damping materials (first layer : metal, second layer : plastic, third layer : metal). The metals used were SS400 steel, pure copper and pure aluminum. The thickness of the first layer T_i varied from 1mm to 20mm. The damping materials were exposed to vibratory cavitation. The cavitation impact load acting on the surface decreased as T_i decreased below the critical thickness T_c, which is about 3 mm irrespective of kind of metals. This is because the intensity of the compression wave is reduced by the reflected tension wave emitted from the boundary between the first layer and the damping sheet. Therefore, the critical thickness T_c also observed in terms of mass loss rate. The T_c for both impact load and mass loss rate is calculated by T_c=Δt·c/4, where Δt is the time interval of impact load and c is the longitudinal wave velocity. The mass loss rate m, when T_i is less than T_c, is given by m={1+α-4·α·T_i/(Δt·c)}²×m_o, where α is the reflection coefficient between the first layer and the damping sheet, and m_o is the constant mass loss rate when T_i is greater than T_c. The calculated values of T_c and m correspond well with the experimental results.

View full abstract

An atomic force microscope (AFM) equipped with a small three-point bending testing machine was used for in situ visualization of intergranular stress corrosion (SC) crack growth under a constant displacement. The tests were conducted on a high-strength 7075-T6 aluminum alloy in laboratory air. The AFM was capable of imaging the surface topography of a growing SC crack on the nanometer order. The AFM has extremely high spatial resolution, and it was capable of monitoring a very slowly growing SC crack: even when it grew on the order of 0.l nm/s, it grew continuously when observed on the order of microns. When the crack grew along the grain boundary inclined to the tensile stress direction, not only Mode I and II crack tip displacement, but also Mode III crack tip displacement was observed. However, the Mode I stress intensity derived from crack tip displacement was responsible for the crack growth. The tip of a growing SC crack in laboratory air was very sharp. However, when the environment was changed to a vacuum, the crack tip became blunt, and the crack retarded. When the environment was changed again to laboratory air, the crack growth restarted after a crack retardation period, and the tip became sharp again. We discussed the SC crack growth mechanisms based upon nanoscopic in situ visualization using AFM.

View full abstract

This paper is concerned with the large-deflection analysis of cracked beams by the boundary integral equation method (BIEM). In the analysis, a crack is simulated by an equivalent rotational spring, which connects two segments of the beam. The BIEM formulation is applied to each segment of the beam and the resulting nonlinear boundary integral equations are solved iteratively. Numerical computations for the cracked beams under the various boundary conditions are carried out and the influences of the crack depth and location on the center deflection and the stress distribution of the beams are investigated.

View full abstract

Some materials used to fabricate machines and structures have a strain rate dependence in their stress-strain characteristics at room temperature. In such materials, the deformation and strength depend on the loading velocity. We propose a noncontact evaluation technique using infrared thermography of macroscopic plastic deformation. To detect the heat generated during plastic deformation, an infrared thermal video system (TVS-2100) was employed. The relationship between the plastic deformation and temperature rise of the specimen surface was determined by analytical simulation of elasto/viscoplastic FE analysis coupled with transient heat conduction analysis. The experimental and numerical results are in good agreement, and the effect of the loading rate and the applicability of this evaluation technique by infrared thermography was demonstrated.

View full abstract

The elastic-plastic stress, strain and displacement fields at the tip of an arbitrary open notch meeting an interface for both plane stress and plane strain conditions are investigated analytically. The materials are assumed to be governed by the deformation theory of plasticity with linear strain hardening property. The governing eigenequation, which gives the order of stress singularities, is given in determinant form with traction-free boundary conditions and interface continuity conditions. Using the eigenfunction-expansion method, a solution is obtained for the magnitude of the notch tip singularities λ and the stress and the strain fields are computed for various open notch and material combinations. Mode I, Mode II and mixed-mode cases are discussed.

View full abstract

This paper deals with the two-dimensional thermoelastic contact problem of a rolling rigid cylinder of specified shape, which induces of friction and heat generation in the contact region, moving with constant velocity in an elastic half-space containing a subsurface crack. In the present temperature analysis, the speed of the moving heat source is assumed to be much greater than the ratio of the thermal diffusivity and the contact length. The problem is solved using complex-variable techniques and is reduced to singular integral equations which are solved numerically. Numerical results of stress intensity factors are obtained for a relatively short crack. The effects of the frictional coefficient, the sliding/rolling ratio, the crack depth and the crack angle on the stress intensity factors are considered.

View full abstract

When an eigenvalue λ, related to the singular stress field, is a single root, the corresponding stresses appear as γ^λ-1 as γ→0, where γ is distance measured from a singular point. However, if the eigenvalue λ is a multiple root, the corresponding stresses may be of type γ^λ-1 log γ as γ→0. In order to take the logarithmic stress singularity into account, this paper presents a new method for analysis of the singular stress field at the vertex of a composite wedge. A condition is derived for stress singularities of a form γ^λ-1(log γ)^m as γ→0, where m is zero or a positive integer.

View full abstract

This study deals with a row of equally spaced diamond-shaped inclusions with angular corners under various loading conditions. The problems are formulated as a system of singular integral equations with Cauchy-type or logarithmic-type singularities, where the unknows functions are the densities of body forces distributed in infinite plates having the same elastic constants as those of the matrix and inclusions. In order to analyze the problems accurately, unknown functions of the body force densities are expressed as a linear combination of two types of fundamental density functions and power series, where the fundamental density functions are chosen to represent the symmetric stress singularity of 1/γ^1-λ1 and the skew-symmetric stress singularity of 1/γ^1-λ2. Then, newly defined stress intensity factors of angular corners are systematically calculated for various shapes, spacings, elastic constants and numbers of diamond-shaped inclusions in a plate subjected to uniaxial tension, biaxial tension and in-plane shear. For all types of diamond-shaped inclusions, the stress intensity factor is shown to be linearly related to the reciprocal of the number of diamond-shaped inclusions.

View full abstract

In plastic deformation of porous materials, the relationship between distribution of voids and macroscopic mechanical properties is one of the most important issues. In the present paper, we use a modified anisotropic Gurson's yield function to evaluate the anisotropy due to the distribution of voids. Perforated sheets with regularly distributed holes were used as plane models of metals with voids and were examined under uniaxial tension. It is shown that the modified anisotropic Gurson's yield function is valid for evaluating the anistropy due to the distribution of voids. The aspect ratio, the inclined angle of the arrangement of holes and the diameter of holes are related mutually and are important in determining the anisotropy of models.

View full abstract

We have developed an optical equipment which possesses high detection sensitivity for measuring small optical retardation induced by stress by means of laser photoelasticity. A He-Ne laser is used as a light source to measure small stress in transparent materials. We explain the theory and process of the measurement of optical retardation in the materials. The magnitude of principal stress difference as well as the direction of the principal stress are obtained simultaneously and quantitatively using our equipment. In order to evaluate the performance of the optical retardation measurement equipment, the stress distribution of a pulled rectangular glass plate with notches at both sides was measured using the equipment. The results of stress distribution agreed with the analytical results. Stress at many points can be determined quickly using the equipment and scanning stress distribution measurement has been realized.

View full abstract

We previously applied on iterative scheme of domain decomposition algorithms for parallel computing to boundary element analysis in potential problems. 0ur aim of this work is to evaluate the efficiency of the scheme using a network of workstations, for different conditions of load balance in each domain. Application of complicated domain decomposition to a mold IC is considered as a example of industrial application.

View full abstract

Materials with ultrafine microstructures often display novel and improved properties compared to bulk. lt is known that composite materials containing nano fine particles do not obey the conventional rule of mixtures of mechanical properties. This paper presents how the mechanical properties for nanocomposite materials are estimated using the boundary element method (BEM). A model of a material, which is a spherical fine particle with transversely isotropic property embedded in an isotropic bulk materials, is considered for the present analysis. In particular, interfacial stresses originating from interfacial energy are considered as boundary conditions at the interface of particle and bulk. The model is analyzed by a multizone boundary element method, and the composite Young's modulus using the conventional rule of mixtures is compared with an apparent Young's modulus obtained from the analysis with surface stress on the interface. It was found that the apparent Young's modulus increases with decrease in particle size and the stress distribution in the particle is not uniform.

View full abstract

In particle or short-fiber reinforced, composites, cracking of the reinforcements is a significant damage mode because the cracked reinforcements lose load carrying capacity. This paper deals with elastic stress distributions and load carrying capacity of intact and cracked ellipsoidal inhomogeneities. Axisymmetric finite element analysis has been carried out on intact and cracked ellipsoidal inhomogeneities in an infinite body under uniaxial tension. For the istact inhomogeneity, as well known as Eshelby's (1957) soluton, the stress distribution is uniform in the inhomogeneity and nonuniform in the surrounding matrix. On the other hand, for the cracked inhomogeneity, the stess in the region near the crack surface is considerably released and the stress distribution becomes more complex. The average stress in the inhomogeneity represents its load carrying capacity, and the difference between the average stesses of the intact and cracked inhomogeneities indicates the loss of load carrying capacity due to cracking damage. The load carrying capacity of the cracked inhomogeneity is expressed in terms of the aveage stress of the intact inhomogeneity and some coefficients. The coefficients are given as functions of an aspect ratio for a variety of combinations of the elastic moduli of inhomogeneity and matrix. It is found that a cracked inhomogeneity with higher aspect ratio maintains higher load carrying capacity than one with low aspect ratio.

View full abstract

We presented an algorithm for shape and topology optimization of two-dimensional structures subjected to a plastic deformation. This algorithm is based on the generalized layout optimization method proposed by Bens φe and Kikuchi in 1988, where a prescribed amount of material is distributed in a given domain so that the work done by applied loads may be minimized. In the present algorithm, a given domain is assumed to be composed of microstructures with a regular triangular cavity, and the rotation angles and cavity sizes of the microstructure are taken as design variables. A material database of microstructures taking into consideration in the effect of plastification for various design variables is made to enable an elastoplastic analysis and a sensitivity analysis of porous material. The accuracy of the elastoplastic analysis using the database is confirmed through a numerical example.

View full abstract

In this paper, a new design method of robust structures using fuzzy sets is proposed. Robust structures have characteristics that are only slightly modified by any changes in external forces, boundary conditions, and productive errors. In our previous studies, the design method of robust structures using sensitivities were mainly investigated. However, sensitivities provide only local information when their values are evaluated. Therefore, it is not clear whether structures designed by sensitivities have robustness widely or not. This paper shows a design method of a structure that holds robustness widely. Because this method uses fuzzy sets, robust structures designed with this method are called fuzzy-robust structures. In the numerical examples, fuzzy-robust structures were designed. Then the effect of a change in the membership function or the α-level was examined. The effectiveness of this method is also discussed.

View full abstract

The neutron diffraction method is a useful nondestructive technique to measure the internal stress distributed within a ceramic component because the neutron penetration depth for ceramics is more than one millimeter. The diffraction from the Si₃N₄ (321) plane by monochromatic neutrons (wavelength λ=1.99Å) was used for stress measurement. The linear absorption coefficient for Si₃N₄ was 0.69 cm^-1, so that the internal stress distributed within specimens less than 5.4 mm thick could be measured by the neutron diffraction method. In order to generate a linear internal stress distribution, a bending moment was applied to a silicon nitride specimen, the thickness of which was 4mm. The incident beam angle and the widths of the incident and diffracted beam slits were adjusted to limit the diffracting area. The distribution of internal stress was measured by scanning the diffracting area from the tensile side to the compressive side. The stress in the diffracting area was determined from the change in the diffraction angle. The measured stress distribution was almost identical to the applied stress distribution. It is concluded that the internal stress of ceramic components can be measured by the neutron diffraction method.

View full abstract

We report the application of the neural network technique to the automatic characterization of defects detected by eddy current testing (ECT) of steam generator tubes of pressurized water reactors. Four parameters are determined to describe the trajectory of the ECT signal, and eight parameters, which come from two trajectories for two different AC current frequencies, are used as the input of the neural network. The orthogonal nature of these parameters is examined by comparing the reproducibility of the learned data by the neural networks using different sets of input parameters. The performance of the neural network is found to be improved by employing the probabilistic descent method for the back-propagation calculation, multiple networks for different defect types, and multiple teaching signals with small noises for one defect to make the network insensitive to noises. Finally, the generalization capability of the network is demonstrated.

View full abstract

Separation of an interarticular portion of human vertebra, or spondylolysis, is often observed in the fifth (L5) and the fourth (L4) lumbar vertebra. The present study investigates Spondylolysis from a mechanical point of view. For this purpose, finite element models of L5 and L4 are constructed by reproducing the shape of typical vertebral specimens of a 44-year-old man as accurately as possible. Then the upper and the lower surface of the vertebral disk are constrained, and constant loads are applied to the inferior facet faces of L5 and L4 in various directions. The dependence of the maximum principal stress in the interarticular portion on the load direction and the point of load application is discussed. The results show that the maximum principal stress in the interarticular portion in L5 is higher than that in L4 in most cases. This result agrees with the clinical observation that 80% of spondylolysis occurs in L5. It is also observed that the principal direction of maximum stress in the interarticular portion is Independent of the direction of the applied load, and is perpendicular to the surface of the separation observed clinically.

View full abstract

Extension, Flexion and axial rotation of spinal motion segments are analyzed to investigate the cause of spondylolysis which is observed frequently in the fifth (L5) and fourth (L4) lumbar vertebrae. A finite element model of a system of lower lumbar vertebrae including the related intervertebral discs and ligaments is constructed. Parts of the sacrum and ilium and the third lumbar vertebra (L3) are also added to the system to realize appropriate boundary conditions. The displacements of the sacrum are constrained, and uniform pressure is applied to the upper surface of L3 to take into account body weight and self-equilibrium force. In addition to these, a bending moment for extension or flexion, or a rotation moment for axial rotation is applied to L3. The results of the analyses show that stress concentration is observed at the interarticular portion of L5 and L4 in all motions. In particular, high stresses occur in L5 in the case of extension. The stresses in flexion are also marked in L5 and L4, but the values are significantly affected by the rigidities of the interspinous and supraspinous ligaments and the capsules.

View full abstract

When a pressure difference between the external auditory meatus and the tympanic cavity occurs due to a defect in the gas exchange function of the cavity, the tympamic membrane retracts toward the cavity and deforms. When this pressure difference is prolonged, plastic strain sometimes arises in the tympanic membrane, and the hearing threshold rises. This phenomenon is called the retraction pocket (RP). However, the generation mechanism of RP is unclear. Therefore, in this study, a finite-element method (FEM) is applied, and an attempt is made to clarify this mechanism by analyzing the relationship between the Tympanic displacement and the middle ear pressure. The results are as follows: RP is formed in the pars flaccida rather than the pars tensa, because the value of the yielding stress of the pars flaccida is smaller than that of the pars tensa; the strain in the pars flaccida is probably increased rapidly without the increase of the stress, when the negative pressure in the middle ear cavity is prolonged.

View full abstract

Bulk-wave generation in plates of aluminum, stainless steel, and carbon steel by a spiral elongated coil and a static magnetic field is analyzed experimentally and numerically. First, the field dependences of the wave amplitudes are investigated using the electromagnetic acoustic resonance (EMAR). The magnetic field is applied parallel to the sample surface. A theoretical model developed to explain the considerable difference in the field dependence between nonmagnetic and ferromagnetic metals is presented, which shows the dominant contribution of the magnetostrictive forces to bulk wave generation in ferromagnetic metal rather than the Lorentz force and the magnetization force, especially in the shear wave generation. The model is further developed by the FEM simulation of the two-dimensional body force profile within the metal. The simulation is Performed for a practical bulk-wave electromagnetic acoustic transducer (BW-EMAT), which has an elongated coil and a pair of permanent magnets of the coil. The calculation strictly predicts several important features of the BW-EMAT, and has significant implications for the practical use of the EMAT.

View full abstract