Stress analysis is conducted for a traversely reinforced composite with a Model 1 crack in plane strain condition using the boundary element method (BEM). The crack is introduced in the matrix at the center of the layered composite plate. The results obtained are summarized as follows. (1) When the normalized crack length, a^*=a/d, (a : crack length, d : thickness of matrix layer) is much smaller than unity, the high stress region near the crack tip is confined to the matrix. The magnitude of stress intensity factor (SIF), K, is close to K₀ for a homogeneous body composed of a single matrix material. The crack is called a small crack for composite materials. (2) As a^* becomes longer, the normalized SIF, K^*=K/K₀, decreases due to the constraint by the adjacent fibers. K^* shows a minimum near a^*=1 and increases when a^* increases. (3) K^* tends to be saturated as a^* exceeds 5. The energy release rate calculated by the asymptotic magnitude of K^* coincides with that of the homogeneous orthotropic body where the elastic constants are determined by a mixture of those in the matrix and fibers. The crack is called a large crack for composite materials.

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The effects of damage field on the stress singularity at a crack tip are investigated. For a radial damage distribution represented by a power function γ^m of radius γ with its center at the crack-tip, an analytical solution of asymptotic elastic stress fields is obtained for a mode III crack. The asymptotic distribution of the stress field is found to depend on the power law index m of the given damage distribution, and the well known elastic singularity disappears when the power law index m of the distribution exceeds a value of 3/4. For more general damage distributions, numerical analyses by means of the finite element method were performed, and the effects of the geometry shape of two local damage fields on the stress distribution around the crack were elucidated. It was shown that, though the preceding damage field behind the crack-tip significantly influences the stress field in front of a growing crack, the analytical solution for the circular damage field gives essentially the same stress singularity as that for more general damage distribution. The results provide important insights into some fundamental aspects of the local approach to fracture based on continuum damage mechanics.

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Fatigue strength is influenced by type of loading and the shape of a cross section of the specimen. In this work, the effect of the loading type on the crack growth rate was investigated in cases of push-pull, rotating bending and plane bending loadings. The crack growth rates were almost uniquenly determined by the term ε^n'_pl regardless of the type of loading, although the crack growth rate was different for each type of loading with the same nominal stress level and crack length. The validity of the term ε^n'_pl for evaluating crack growth rates for three different types of loading was confirmed using three kinds of metals.

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The crack arrest capability of a tear strap in a pressurized precracked fuselage was studied through instrumental axial rupture tests of small-scale models of an idealized fuselage. Upon pressurization, rapid crack propagation was initiated at an axis through crack along the stringer and immediately kinked due to the mixed mode I and II states caused by the one-sided opening of the crack flap. The diagonally running crack turned at the tear straps. Dynamic finite element analysis of the rupturing cylinder showed that the crack kinked and also ran straight in the presence of a mixed mode state according to a modified two-parameter crack kinking criterion.

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Effects of ambient temperatures ranging from -50 to 50°C on microstructure and fatigue failure were examined for using injection-molded PP sheets. A three-layer structure was formed in the specimen by contact with a cooled mold wall during injection. Higher molecular orientation and hardness were observed in the intermediate layer, which was for med by strong shear under rapid cooling, while lower molecular orientation and a high degree of crystallinity were measured in the core region. These differences in structures and properties between the two layers may cause higher stress under loading, and as a result brittle cracks may be easily formed in the intermediate layer, exerting a large influences on the fatigue lifetime. At temperatures above room temperature, an increase in tan δ (a decrease in E') became more evident on increasing the level of the stress amplitude, and a fracture mode changed from brittle to ductile. At a low temperature of 10°C and under high stress amplitude, an increase in tan δ was also observed. In this case, however, the tan δ increase may be attributed to internal friction due to inner structural changes under cyclic loading. At much lower temperatures, the value of tan δ clearly decreased, indicating more elastic behavior which was quite different from typical cases.

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The fatigue limits of single-, dual-and tri-phase steels were estimated using the crack-growth and arrest simulation, which was based on a model for continuous distribution of dislocations ahead of a small fatigue crack tip. The effects of microstructural parameters such as grain size, hardness and volume fraction of the three phases on the fatigue limit were systematically analyzed using this simulation. The simplified estimation equation for fatigue limit was derived from the simulation results as follows : δ_w=HV₁·(k₁+k₂/√(D₁))+Σ_{j=2, 3}(HV_j-HV₁)·V_j·(k_3j+k_4j/√(D_j)), where HV_j is the hardness, D_j is the grain size, and V_j is the volume fraction of the j-th phase. Microstructural design for high fatigue strength steels is discussed with reference to this equation. Also, a more accurate simulation method is outlined including the effects of cyclic hardening and softening behavior of materials.

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Static tensile strength of fatigued material, that is, residual strength was investigated and it is tried to make relationship between tensile fracture mechanisms and fatigue mechanisms clear. It is recognized that materials clearly fatigued because of degradation of the materials'stiffness and occurrence of AE waves in fatigue process. Generally, fatigue damages lead deccrease of tensile strength on some high-polymers and metals. However the residual strength of fatigue-damaged SMC didn't degraded in spite of increasing of fatigue cycles. This is why the static tensile fracture mechnism is different from fatigue mechanism. Thereby the tensile fracture mechanism is also investigated by observation of fatigue crack and fracture surface. As a result, in fatigue process, fatigue crack progress across specimen. Simultaneously, longitudinal debonding occurred around and in chopped strands. And it is showed that residual strength depend on longitudinal damages and that the fracture toughness of fatigue-damaged SMC is larger than that of virgin one. This means that fatigue-damaged SMC needs more energy to be fractured and that the longitudinal damages prevent transverse crack from progressing in static tensile process. Therefore the residual strength didn't degrade in spite of the fatigue damage

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Structural Ceramics have excellent wear- and heat-resistant characteristics. However, in some applications, they are exposed to very high temperatures and high heat-flux environments, and are also subjected to repeated thermal loadings that change rapidly with time. Such repeated thermal loadings are causes of material fracture. Therefore, it is important to investigate the thermal shock fatigue characteristics of these ceramics. We present results of thermal shock fatigue strength tests for a machinable structural ceramic called MACOR, composed of SiO₂-MgO-Al₂O₃. The thermal shock fatigue experiments were done using a CO₂ laser, and the crack generation of the specimens was measured from AE (acoustic emission) signals. Consequently, the thermal shock fatigue strength of MACOR was estimated from the critical power density, and it was proposed that the thermal shock strength of ceramics should be cstimated using a minimum value P_LF [W/mm²] of the critical power density below which ceramic fracture is independent of the spot diameter. Finally, the P_LF of 0.2 W/mm² was obtained from thermal shock fatigue experiments using MACOR.

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The coupled mode of phase transformation, temperature variation and induced stress is simulated using the molecular dynamics method. A two-dimensional array of atoms with fcc structure is employed for simplicity, and a Morse-type two-body potential is introduced to evaluate the atomic force. A non dimensional description of the fundamental equation is derived to account for the general material properties. Calculations are made for the region with temperature gradient in the x-direction, while periodic boundary condition is imposed in both directions. Characteristics of the material and parameters such as melting temperature, specific heat and thermal conductivity are first evaluated followed by steady and non-steady state heat conduction problems coupled with induced stresses. Here, emphasis is placed on the effect of the phase transformation depending on the temperature range below or above the melting point. Simulated results of temperature and stresses indicate the practical feature of macroscopic distribution, especially when compared with the solution of the heat conduction equation plotted in non-dimensional form using a Fourier number.

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We describe the influence of a vacuum and water absorption on the tensile fracture behavior of four different types of aramid fibers : Kevlar 29, Kevlar 49, and Kevlar 149, manufactured by DuPont, and Technora from Teijinn Co. Ltd. Single fiber tensile tests were conducted in air and in vacuum (4-5×10^-3Pa) for fibers preconditioned in air, in water at 80°C for two months, and in vacuum for about four hours. The mechanical properties of aramid fibers, longitudinal elastic modulus and fracture strength decreased as a result of water absorption. Fiber fracture was associated more with fiber splitting when preconditioned in water. When tested in a vacuum, the air-preconditioned fiber strength decreased from that in air. Moreover, the strength of the fibers preconditioned in vacuum decreased more when tested in air compared with that of the air-preconditioned fibers tested in a vacuum. The fiber surfaces were closely examined using an atomic force microscope (AFM). The AFM's nanometric resolution showed fiber surface damage caused by water absorption, high temperature, and preconditioning in a vacuum. The degradation and fracture mechanisms of the fibers were discussed.

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The compressive strengths and failure mechanisms of laminated carbon matrix composites reinforced with unidirectional carbon fibers are investigated for various off-axis angles (θ) of the fiber with respect to the loading axis. The compressive strength and elastic modulus decrease with increasing θ. The failure mechanism involves delamination due to fiber buckling at θ=0°(mode I), shear sliding along the fiber direction at 10°≤θ≤45°(mode II), and shear sliding along the direction perpendicular to the fiber fiber at 55°≤θ≤90°(mode III). Although the compressive strength is dependent on θ, the calculated shear strength in the sliding direction has a constant value of 23 MPa for mode II and 7.5MPa for mode III. The sliding along the fiber direction is accompanied by some fiber buckling due to fiber misalignment, resulting in a higher shear strength for mode II than for mode III.

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This paper deals with numerical solutions of singular integral equations in stress concentration problems for longitudinal shear loading. The body force method is used to formulate the problems as a system of singular integral equations with Cauchy-type singularities, where unknowns are the densities of body forces distributed in the longitudinal direction of an infinite body. In order to satisfy the boundary conditions along elliptical boundaries, four kinds of fundamental density functions are newly introduced. Then the body force densities are approximated by a linear combination of those fundamental density functions and polynomials. The calculations are carried out for several arrangements of elliptical holes, and it is found that the present method yields rapidly converging numerical results. The body force densities and stress distributions along the boundaries are shown in figures to demonstrate the accuracy of the present solutions.

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In this paper, one-dimensional transient heat conduction and thermal bending problems under the axisymmetic condition are developed theoretically for a nonhomogeneous circular plate due to a uniform heat supply from its surfaces. We constructed the approach for multiobjective optimization of nonhomogeneous material composition for a functionally graded circular plate, taking into account the thermal stress relaxation and improvement of the heat resistance. We have dealt with this multiobjective optimization using the weighting method. Using both these analytical solutions for the temperature field and the associated thermoelastic one, and the approach for multiobjective optimization of a nonhomogeneous material composition, numerical calculations of optimal material composition are carried out for a titanium alloy/zirconium oxide functionally graded circular plate subjected to a uniform heat supply from one side of its surface. The tradeoff relationship between the maximum absolute stress ratio and the maximum heat flux is elucidated. It is shown that this approach is useful for the multiobjective optimal design of nonhomogeneous material composition, taking into account both securement of strength against thermal stress and improvement of heat resistance against thermal loads.

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Stresses and stiffnesses of weld-bonded box section cantilever beams with a concentrated load at the free end have been studied analytically. The influences of spot-welded spaces, Young's modulus and thickness of the adhesives on the stress distributions and the bending rigidities were investigated considering the shearing forces of the beam. Maximum shear stresses in the adhesives occur at the inner edge of the adhesive layers and decrease with the decrease of Young's modulus and increase of thickness of the adhesives. The analytical results were consistent with the results obtained using the finite-element method.

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The backscatter of elastic waves from an inhomogeneous layer inserted between two homogeneous half spaces, was investigated by introducing the equivalent homogeneous layer model. It was observed that discontinuation of the acoustic impedance at the layer interfaces mainly affects the backscatter intensity. The continuous variation of impedance inside the inserted layer has little affect especially in the high-frequency domain. Consequently, the layer system can be approximated by a suitable homogeneous layer system. Development of a new equivalent homogeneous layer model is the main subject of this paper. The model introduced will simplify the evaluation of backscatter intensity of a gradient inhomogeneous material. The simple modeling method is discussed and the model is evaluated and compared with the results for impulsive reflecting energy calculated by the model and by the exact analysis.

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In this paper, the dynamic stability analysis of an elastic rod subjected to conservative and nonconservative forces by using the integral equation method is presented. The stability analysis of multiple-parameter elastic system is taken consideration with two types damping terms, which are the internal and the exteral damping effects. The numerical method derived from the boundary integral equation in the sub-domain, which was proposed for dynamic stability analysis of oneparameter system subjected to nonconservative force, is applied to the analysis of multiple-parameter systems. The models in numerical examples are cantilever-type elastic rods under the combined action of these forces. The obtained numerical results proved the effectiveness of our method and showed mechanical characteristics for these multiple-parameter systems.

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This paper presents a progressive type of variational principle for topological shape optimization. The principle makes it possible to derive a finite element method (FEM) for searching structural shapes with minimal weight and high strength in the optimization operation. The principle is based on that of minimum potential energy. In the formulation of this principle, we first assume both a search domain and an optimum one. The search domain includes the optimum one. Then, the search domain is locally eliminated according to the information of minimum strain energy in the domain. When a maximum equivalent stress in the search domain reaches the yield stress as a constraint condition, it is evaluated that the search domain agrees with the optimum one. Therefore, we obtain the variational principle for the topological optimization. Next, we derive the FEM based on this principle. Finally, we obtain the optimum topological shapes of two-dimensional finite element models using the FEM. Hence, the validity of the variational principle is confirmed from the numerical solutions of the models.

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A new X-ray approach was proposed for determining the distribution of stress on the surface of a material using an imaging plate (IP). In this method, the computerized tomography (CT) was applied to stresses averaged over a linearly irradiated area obtained using a diffraction ring detected using an IP. Projections of residual stresses were reconstructed using the CT approach and the distribution of stress σ_x (x, y) was obtained. As long as the mean stresses were measured from the linearly irradiated area, the distribution of stress can be determined using the ordinary X-ray technique for polycrystalline aggregates by the present method without the ordinary X-ray approach for a single crystal using a four-circle goniometry.

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Adhesion strength evaluation of molding compounds is a critical issue in both structural design and material selection of plastic IC packages. We previously proposed a new adhesion test method that can separate residual stress from adhesion strength, and confirmed that the measured true adhesion strength can be applied to the quantitative prediction of interface delamination in a dry package. This paper describes the influence of moisture absorption of a molding compounds on true adhesion strength. When the average moisture content of an adhesive specimen is the same under the two kinds of moisture conditions (55°C/80%RH, 85°C/85%RH), the drop in true adhesion strength of the specimen that has absorbed moisture at 50°C is about 40 percent of that at 85°C. We also evaluate interface delamination in a moisturizing package by considering both the adhesion strength drop and the swelling of the molding compound caused by moisture absorption. The predicted temperature at which the delamination occurs agrees well with the experimental results for moisture-absorbed packages.

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An analytical model to relate the microstructure of an injection-molded short-fiber composite to the overall stiffness of the composite is developed. An injection-molded short-fiber composite generally has a three-layer structure ; namely, the fiber orientation distribution in both surface layers is different from that in the middle layer. First, the mechanical properties of each layer are predicted using Eshelby's equivalent inclusion method. Second, the overall stiffness of the injectionmolded composite is estimated using a laminated plate theory. Good agreement between the theoretical results based on the present model and the experimental ones in an E-glass/polyester composite is obtained. Additionally, the effects of the fiber-aspect ratio, thickness ratio of surface-to-middle layers, cut-off angle for fiber misorientation, and two- and three-dimensional distribution types are investigated by parametric study.

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Nanoindentation measurement for a tungsten (001) single crystal was conducted using an AFM ultramicro-hardness tester. For an electrolytically polished tungsten (001) single crystal, sudden penetration of around 50 nm occurred at 1.3mN using a three-sided pyramidal indenter with an apical angle of 115°, and at 0.24 mN using an indenter of 60°. However, this behavior did not occur for the mechanically polished surface, where a heavily deformed layer of no more than 6 μm in thickness existed. These findings lead to the idea that such phenomena arise from the same source as upper/lower yield behavior in tensile tests for bcc crystals, i.e., relatively high Peiels potentials and low dislocation mobility. Also, the radius of curvature of the indenter, which is of great importance in nanoindentation, was estimated from both load-penetration depth curves and AFM topographic images.

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In order to support the ultrasonic inspection technique of defects, a method that combines the expert system with the neural network is proposed. Knowledge of selection of the NDT inspection and ultrasonic testing methods is expressed by the rule base in detail. Recognition knowledge of the detected ultrasonic wave signals is difficult to extract, so the recognition of the waveform reflected from defects is carried out using the neural network. Setting of the initial values is performed using a knowledge base. Many learning iterations of the neural network may be required, and sometimes the neural network may not converge. We aim to construct a flaw detection system based on a newly developed knowledge base. The usefulness of the system is verified by the improvement of the learning speed of this system. The results of learned data in terms of detected waveform are added to the case-base as object-oriented knowledge expressions, which may be used to support the nonprofessional user.

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The electric field in a conductive material with multiple circular cracks is analyzed by means of the crack-flow modification method (CFMM). It is found that the increase in the potential difference of the cracked meterial normalized by the potential difference without cracks, ΔV/V₀, is in proportion to the product of the effective volumetric crack density and the mean of the cubed crack radius, n_eff[γ³]_m. This universal relationship is also valid for multiple semi-circular cracks on the surface of the material and gives the physical meaning of damage detected by the direct current electrical potential method.

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Recently, joints that combine a shrink fit with anaerobic adhesives have been used in order to increase joint strength. In this study we deal with the stress analysis and strength evaluation of bonded shrink fitted joints subjected to torsions. The stress distributions in the adhesive layer of bonded shrink fitted joints are analyzed by using the axisymmetric theory of elasticity when an external torsion is applied to the upper end of the shaft. The effects of the outer diameter and the siffness of rings on the interface stress distributions are analyzed by the numerical calculations. Using the interface stress distributions, the joint strength was predicted. In addition, the value of the joint strength was obtained experimentally. It is seen that a rupture in the adhesive layer is initiated from the upper edge of the interfaces when a torsion is applied to the upper end of the shaft. The numerical results show more conservative values than the experimental results. It is found that the joint strength of bonded shrink-fitted joints is greater than that of shrink-fitted joints.

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The free mesh method (FMM) is simple, accurate and suitable for parallel computing. In this method, the total stiffness matrix is obtained by assembling the temporary local element matrices, so the method does not require connectivity between nodes and elements as input information. This paper describes FMM using Delaunay tesselation at the stage of creating a set of temporary local elements around each node during application to a 3-dimensional problem. In the analysis of a steady thermal conduction problem, the result of the present method was virtually the same as that of the usual FEM using global Delaunay tesselation. The CPU times with FMM and FEM were compared for problems of varing sizes. Results showed that the present method was highly efficient for dealing with larger problems.

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A mathematical model is proposed for various types of arteries in the active state on the loading process with an incorporation of an activation parameter, an inelastic strain and stiffening of elasticity. In this model the total stress, which is defined between the unloaded configuration and the current configuration, is divided into a passive stress and an active stress. The passive part is expressed by an incompressible isotropic strain energy density function. The active part is expressed as a fiber stress of smooth muscles whose stress-strain relationship is determined by an isometric force-length relationship with a use of the activation parameter. The inelastic strain corresponds to the deformation between the reference configuration and the unloaded configuration. Stiffening of the elastic property is considered mainly for elastic-type arteries. The simulation results for pressure-diameter relationship of arteries in the passive and active states show that the model describes the relationships in the physiological pressure range well. The stress analyses of the arterial walls show that the circumferential stress changes to a compressive one in the active state with a strong contraction and lower transmural pressure of the vessel.

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