The elastic problem of thick-walled tubes of a functionally graded material(FGM) under internal pressure in the case of plane strain (εz=0) has been studied. The model components of the FGM are based on the combination of a matrix with three kinds of elastic moduli of 35, 70, and 210 GPa, and a particle with an elastic modulus of 360 GPa. The distribution profile is assumed by the expressions reported previously for a corundum/ plaster FGM. The elastic stress and strain distributions for the FGM became complicated because of radial elastic modulus gradations. The effects of distribution profiles on the deformation can be summarized by considering the variation of the normalized elastic modulus rate [(dE/dr)/E] in the radial direction. If a greater positive normalized elastic modulus rate is obtained, the circumferential stress at an inner plane of a tube is decreased, and then the tube begins to function as a compound tube.
By use of the thermoporoelastic theory previously proposed by the author, analysis is made of the thermal stresses which are induced in d fluid-saturated porous hollow cylinder subjected to a sudden increase in temperature and pressure on its inner surface. Since the problem formulated is axisymmetric, the displacement field is decoupled from the temperature and pore fluid pressure fields, which are still coupled with each other. Coupled diffusion equations for heat and fluid flows are solved by the Crank- Nicolson implicit method, because they involve nonlinear and integral terms. The primary focus of this study is on the influence of heat transportation by fluid flow through pores on the temperature and thermal stress distributions. This influence is quite marked in the case where the fluid diffusivity is much larger than the thermal diffusivity, suggesting the possible control of thermal stresses by active fluid injection.
In this paper the singular stress fields near a corner of jointed dissimilar materials are discussed based on the expression of the stress fields obtained recently by the authors. The stress fields are expressed as a sum of two terms: a symmetric term and a skew-symmetric term. The stress singularities for the two terms are different. The angle which makes the normal stressσθ or the shear stressτrθ the maximum is a function of the distance from the corner point, differing from the case of a crack.
In this paper the characteristics of the stress fields near a corner of jointed dissimilar materials under antiplane loads are studied. It is found that the singular stress fields due to the antiplane loads can be divided into two types: a symmetric type with the stress singularity of 1/r<1-λ1> and a skew-symmetric type with the stress singularity of 1/r<1-λ2>. If G2<G1, there exists only singularity of the skew-symmetric type, and if G2>G1, there exists only singularity of the symmetric type. An expression of stress field in the vicinity of the corner is presented. Based on this expression, the singular stress field is described always in terms of the constant KIII, λ1 for the symmetric type or the constant KIII, λ2 for the skew-symmetric type alone.
The fundamental deformation behaviour of rubber composite material, especially the nonuniform distribution of stress and strain, is studied. Tensile deformation of rubber composite material under the condition of plane strain is analysed numerically using the Mooney-Rivlin-type strain energy function. A planar model of in-homogeneous material is adopted as a model of the rubber composite material. Elliptic rubber inclusions with different elastic constants are assumed to be placed regularly in the matrix rubber. The problem of the incompressibility of rubber is treated, adopting the penalty function method in the finite-element analysis. The incremental load is applied and Newton-Raphson's method is used for the repeated numerical calculation of the nonlinear problem. It is found that the deformation of the composite model becomes similar to that under constant strain when the shape of the inhomogeneous region becomes slender in the stress direction. The distribution of stress and strain in the elliptic inclusion is nearly constant.
In large deformation analysis, the constitutive equation utilized will play an important role in finite-element solutions. Most of the finite-element codes employ hypoelasticity based on the Jaumann stress rate for largely deformed elastoplastic behavior of metal alloys. The present paper shows the finite-element formulation for large deformation analysis in the elastic range using the different alternate objective stress rate of Green, which has not been studied in detail in previous works from the viewpoint of numerical analysis. Three kinds of shear deformations of a two-dimensional block in plane strain conditions are taken as examples to demonstrate the effects of the constitutive equations employed, and also discussed are the effects of different kinds of the isoparametric shape functions and the Gaussian integration techniques projected on the finite-element solutions.
The present study is concerned with a method for estimating the impact load acting on a body of arbitrary shape. The authors have already shown that the impact load can be estimated from the impact response of the body by an inverse analysis. In practical application, the impact response data involve noise caused by the measurement system. Since the inverse analysis is sometimes ill-conditioned, such noise tends to be expanded and the estimate of the impact load is obscured by noise. In order to overcome this difficulty, we present a method to minimize the mean square error of the estimate by applying the Wiener filtering theory. We demonstrate the applicability of the present method by numerical simulation of the longitudinal impact of a rod.
Atmospheric corrosion of thin-film cobalt-based magnetic recording media was studied by surface analysis and by experimental deposition of dummy particles. Thin-film corrosion tends to be affected by adhering dust particles even in clean environments, and the incidence of corrosion depends on the size of dust particles. Surface corrosion is more conspicuous away from rather than directly under adhered dust. Corrosion in these areas involves water adsorption in the porous overcoat, cobalt dissolution from the magnetic layer, and migration of cobalt ions through the carbon overcoat. We propose the following thin-film corrosion model: atmospheric vapor condenses in the crevice between the carbon overcoat and adhered dust, this water penetrates and diffuses in the carbon overcoat, and a differential aeration cell is generated by the difference in dissolved oxygen concentration between the edge and center of the areas with diffused water. Furthermore, we propose a corrosion test method for thin-film recording media that uses uniform particles.
In a former report, the authors introduced a stochastic theory of propagation of elastic waves in porous solids, and proposed a pore response function method for nondestructive pore characterization. This study was carried out to examine the applicability of this method to quantitative detection of hydrogen attack in steels. Preparing some samples of high-carbon steel with different hydrogen exposure times, the effect of the texture damage on elastic waves was investigated. After clarifying the propagation mechanism of ultrasounds in hydrogen-attacked steel, we presented a nondestructive technique with a pore response function method for evaluating both the size and area fraction of microcracks produced by the hydrogen attack.
In-situ observation at high magnification of materials being damaged by aqueous corrosion and corrosion fatigue is significant in the understanding of these mechanisms. In this study, in-situ observation of Au, Fe, SUS304 steel and SNCM439 steel under potential control in 1%NaCl and 0.1%HNO3 solutions is performed by STM. The STM image can be observed for all materials both in anodic and cathodic potentials if the proper tunnel bias is applied. In cathodic potentials the STM image does not change with time, while in anodic potentials the STM image changes rapidly unless the material is in a passive condition.
Brittle fracture surfaces at room temperature (∼25°C) and liquid nitrogen temperature (-196°C) for molybdenum single crystal were observed with a scanning tunneling microscope (STM) and a scanning electron microscope (SEM). STM images obtained showed that fine cleavage steps were formed on the fracture surfaces. The images were analyzed by fractal geometry. The brittle fracture surface of molybdenum single crystal had self-similarity (i.e., fractal characteristics) up to the very high magnification where the scanning range of 40nm in X and Y directions corresponded to about 150 atoms of molybdenum. However, this characteristics was expected to break down when the scanning range in the X and Y directions was below 20nm. This was confirmed by STM images with the scanning range of 16nm, where the cleavage step was not observed.
The scanning tunneling microscope (STM) has an atomic-level resolution. Oxide film is always formed at elevated temperatures in air. Therefore, STM observation of surface films is useful for investigating the basic mechanisms of damage such as oxidation and creep at elevated temperatures. In this study, the oxide films on nickel and iron, and the sulfur-segregated surfaces on iron and SUS304 steel were observed with STM at room temperatures in air. The images of these films were obtained under a high bias voltage such as 2V, in comparison with ordinary voltage such as 20mV. This was justified by the fact that the films were semiconductors.
An experimental method for direct comparison of surface roughening after plastic deformation and deformation of grains is developed. The shape of the free surface after plastic deformation is measured with a stylus measuring instrument and the data is entered into a personal computer. A new experimental technique for creating a precise 3-dimensional pattern of the surface is developed in the present paper. The pattern of grain boundaries observed by a microscope is also recorded in the computer and both figures are superposed on the display. Some experiments are performed on the compressive deformation of polycrystalline iron and discussion is presented on the relationship between surface roughening and deformation of grains.
A new compaction criterion and constitutive equations are proposed for the ceramic granule compact. Compressible behaviour and dilatancy phenomena of the ceramic granule compact can be expressed by these basic equations. Experimental methods are also proposed to determine the material constants necessary for the abovementioned equations. Based on these equations, a numerical simulation system is developed to estimate the mechanical behaviour of a ceramic granule compact in cold isostatic pressing.
Quasi-static and dynamic penetration tests were conducted using thin metal sheets of extrasuper duralumin, corrosion-resistant aluminum alloy and cold-rolled carbon steel. The failure mode before crack initiation differed from that after the initiation. Therefore, formulae for estimating the energy-absorbing capacity were proposed for before and after crack initiation. Especially after crack initiation, we derived formulae of the relationship between energy-absorbing capacity by assuming that the shape of the petals was logarithmically spiral. By comparing the estimated values with the experimental ones, it was clarified that the proposed formulae were valid for the assessment of the energy- absorbing capacity of thin metal sheets under penetration.
The microdynamics of chevron-notched isotropic carbon were investigated by the source inversion processing of elastic waves. The geometric correction factor for stress intensity factor Y* determined by compliance measurement was found to agree well with that of the superpose method by Munz et al. The fracture toughness of this material was 1.3MPa·m1/2. The AE monitoring system (ADAS) developed by the authors was demonstrated to measure the surface displacement generated by mode-I cracking. Therefore, the source waves, crack volume and released energy were obtained by the time-domain deconvolution integral of the detected displacement by the theoretical Green's function of the media. The area produced by stable crack propagation was found to be about O.5-1.0mm2, and to agree fairly well with the characteristic features of the fracture surface. The stable crack growth velocity was estimated to be about 250m/s.
Tension tests were carried out on Si3N4/SUS 304 joints at room and high temperatures. Analyses were also conducted on the same specimens using the finite-element method to evaluate residual stresses and stress concentrations. The results can be summarized as follows: (1) The maximum tensile residual stress is axial stress σz which occurs at the same point, very close to the end of the joining interface, for every Cu thickness and temperature. (2) The maximum tensile stresses occurring in the ceramics/metal joints stressed under tension at room temperature are constant, independent of the interlayer thicknesses. The joint strength is dominated by residual stress and stress concentration. (3) The maximum tensile stresses in the ceramics/metal joints stressed under tension at high temperature decrease with the increase in temperature. Therefore, the joint strength is dominated by residual stress, stress concentration and the decrease in joint strength due to the increase in temperature.
Mixed-mode (mode I and mode II) interlaminar fracture toughness of uni-directional reinforced and woven carbon fiber-reinforced plastic (CFRP) is evaluated using DCB specimens. A new testing apparatus is made in order to change the ratio of mode I to mode II across a wide range. The fracture toughness value is evaluated based on beam theory, and the deformation of the specimen is analyzed by the finite element method (FEM) . The relationship between compliance and crack length obtained in the experiment agrees with the results obtained by FEM. The constant fracture toughness value, Gc, which is independent of crack length, is obtained. Due to the decrease of the GI/GII ratio, the fracture toughness value increases. It is shown that the role of mode II changes largely due to the change in the ratio of mode I to mode II, though the mode I component maintains a nearly constant value.
In the design environment, most of the variables and parameters such as load, strength, size and material constants have some randomness and/or in many cases designers cannot know and determine the exact values of these variables. In such a case, the concept of reliability should be included in the design object instead of the conventional safety factor. This paper proposes a problem of optimum shape design for a 2-dimensional continuum, taking the reliability concept into consideration. When size, strength and outer forces vary according to the normal distribution, a shape is determined such that it minimizes the average volume under the constraint of the failure probability. In the body with the obtained shape, the probability of failure is uniform.
The reliability of the rotating blades is the most important factor in improving the reliability of steam turbines. The peak stress of the blade root and the disc groove which support the large centrifugal force of the blade should be analyzed accurately in order to assure the mechanical strength for both steady and vibratory stresses. The three-dimensional finite-element method is applied to the peak stress analysis considering slippage at the contact surface between the blade root and the disc groove. Nonlinear calculation is usually carried out to deal with the slippage effect. In this paper the appropriate methods to calculate the accurate local peak stress of the blade root and the disc groove are discussed, including nonlinear and linear analysis.
A sensitivity analysis code for the elastic-plastic dynamic response of piping systems is developed in the context of implicit time integration schemes. The formulation of sensitivity analysis originates in the perturbation method and retains its advantage. Namely, the decomposed effective stiffness matrix after the iteration for each increment is efficiently used to compute the sensitivity of any parameter involved in the piping system. A residual heat remover of a PWR (pressurized water reactor) under an earthquake loading is analyzed based on the present method with the isoparametric elbow element proposed by Bathe, and satisfactory results are obtained.
In a vehicle design which realizes good crash properties, it is desirable to increase the maximum and the mean crash load of a vehicle structure. A sensitivity analysis may be used to examine a vehicle structure efficiently. However, no report regarding this usage has been presented because of the complex nature of a vehicle crash analysis. In this paper, a new method for a sensitivity analysis is presented, which deals directly with a buckling load. As this is the first report, sensitivity of a structure is investigated to increase dynamic buckling strength, where the dynamic buckling is regarded as higher-order static buckling with a mode similar to the dynamic mode. A new approach to dynamic buckling problems based on a study of dynamic properties is introduced and discussed.