The liability of recycled plastic product was estimated using the fatigue fracture toughness. The role of fibers at the crack tip was simulated by FEM using EWS. The main results obtained were as follows. (1)The fatigue fracture toughness K_FC of recycled reinforced PA·MXD6 was lower than that of virgin material, whereas K_FC of 'Ecomaterial' and virgin material were almost the same. (2)Crack propagation behaviors of recycled material was remarkably different from that of virgin material. In recycled material, fatigue crack propagation perpendicular to the loading direction was turned into unstable crack growth parallel to the loading direction. (3)Crack length of fracture mode transition decreased with increase of recycled frequency. The criterion of unstable fracture could be estimated by clarification of relationship between transition crack length and fatigue fracture toughness. (4)Crack deflection behavior was examined by FEM simulation. Crack deflection in recycled material resulted from inhomogeneous strain distribution in the process zone.

View full abstract

The aim of this work is to investigate the effects of notch depth on crack initiation, early crack growth behavior and fatigue life of pure titanium implanted the nitrogen ions by the dynamic mixing. Three point bending fatigue tests were carried out using an electro hydraulic fatigue testing machine with a frequency of 20Hz and a stress ratio of R=0 at room temperature. The fatigue crack initiation and the early crack growth were observed by using an optical microscope adopting and a plastic replica method. The main results obtained are summarized as follows. (1)The fatigue life of N-ion-implanted pure titanium decreased with increasing notch depth, which depended on magnitude of stress concentration factor. (2)For N-ion-implanted pure titanium, there were few slip bands on the surface of the notch root. Several intergranular or transgranular micro cracks at the notch root surface coalesced into each other at the early fatigue stage. The crack initiation life of which the fatigue crack grew to 0.1mm was well represented by K_tΔσ regardless of the notch depth. (3)At early fatigue crack growth process, there were two types of fatigue crack growth. It was a tendency to appear the second type of crack growth as the cyclic stress increased. (4)The two types of crack growth were affected by the microstructure. The crack length at which the early fatigue crack started to propagate successively agreed well with the distance from the notch root at a constant value of σ/Δσ=1.5 which was estimated from nondimensional elastic stress distribution, σ/Δσ.

View full abstract

The evaluation of fracture of a crack in an adhesive joint is important in relation to the structural integrity of adhesive joints and composite materials. It has been reported that the fracture toughness of a crack in a ductile adhesive joint depends on the bond thickness. Although this is an important problem of adhesive structures, the mechanism of the dependence has not yet been elucidated. In this study, the J-integral and the near-tip stress of a crack in an adhesive joint are investigated by elastic-plastic analysis. It is found that the stress field ahead of a crack tip under the mode I load does not depend on the shape of the specimen. In the case of a very thin adhesive layer, a decrease of the bond thickness causes increased stress ahead of the crack tip which results in a decrease of fracture toughness.

View full abstract

The fracture behavior and crack propagation process of carbon fiber fabric mono-lamina composites are examined by in situ SEM, and acoustic emission (AE) signals are measured simultaneously. As a result, crack propagation and patterns of AE event count rate strongly depend on fabric structures. It is shown that the position of a notch relative to a fabric texture unit has an influence on the occurrence of an initial crack and crack propagating paths, particularly for the materials with plain or twill texture. The crack propagates along one direction in a texture unit, and the fiber strands in the unit like a frame preventing a crack from propagating beyond the unit. However, when the deformation becomes large the crack occurring within the texture unit continues to propagate by circumventing the cross section of strands. There exist two fracture forms for a carbon strand, separation of the strand and fiber breakage. Corresponding to these two forms, the spectrum peaks achieved by the power spectrum analysis of AE signals are about 150kHz and 400kHz, respectively.

View full abstract

This paper describes surface crack detection by the alternating current potential drop (ACPD) method. A. C. potentials were measured for SUS304 and SS400 cracked specimens from 50Hz to 5kHz. The variation of potential with crack depth using sinusoidal a. c. for SUS304 did not depend on the frequency, but it did for SS400. The potential ratio measured using sinusoidal a. c. precisely agreed with that using rectangular a. c. in which a. c. potentials at many frequencies could be obtained by one measurement utilizing fast Fourier transformation. The increase in potential with increasing crack depth in experiments agreed with that calculated by the finite element method taking account of the skin effect.

View full abstract

The non-isothermal filling of a powder/binder mixture in metal injection molding is simulated by the viscoplastic and the heat conduction finite element methods. For the molding of products with a nonuniform thickness distribution, a simplified three-dimensional scheme is proposed. The computing time for the simplified three-dimensional scheme is of the same order as that for two-dimensional problems. Complex overlapping between the surfaces of the mixture due to the occurrence of jetting during the molding is dealt with by the use of a remeshing scheme. Material flow in metal injection molding into a rectangular die with a linear thickness distribution is simulated. The jetting behaviro is considerably influenced by the thickness distribution of the die.

View full abstract

The quantitative estimation of the failure stress of a lithium niobate (LN) single crystal due to thermal shock was investigated. Cylindrical test specimens of 50mm diameter and 10mm thickness were obtained from a LN bulk single crystal by pulling in the c-axis direction. A test specimen was heated to about 470K in an oil bath, and then thermal shock was applied to it by pouring room temperature silicon oil; cracking occurred during cooling. The surface temperature was measured by using thermocouples. A temperature profile of the test specimen was obtained from an axisymmetric finite element program for unsteady heat conduction analysis, using the measured surface temperature as the boundary condition. Then the thermal stress was calculated from a three-dimensional finite element program for thermal stress analysis, using the temperature profile of the test specimen obtained from heat conduction analysis. The program for thermal stress analysis takes into account the anisotropy of a LN single crystal, such as of elastic constants and thermal expansion coefficients, as well as their temperature dependence. Four-point bending tests of a LN single crystal were carried out to examine the relationship between thermal shock cracking and failure due to mechanical load.

View full abstract

The impact transition characteristics and fracture behavior were investigated in friction-welded joints with softened structure and grain refinement by normalization treatment. After manufacture using 0.46%C hot-rolled bar steel, friction-welded joints tested in this study were kept at 850°C for 1 h in a vacuum furnace and then cooled with N₂ gas. The impact specimens were machined from the central portion of the bar joint, and a V-notch was made along the weld interface of the joint. The impact tests were carried out at various temperatures using the instrumented Charpy impact machine. The testing temperature was controlled using liquid nitrogen and heating oil. The main results obtained are as follows. (1)The absorption energy of the normalized joint was very much larger in all regions than the absorbed energy of the as-welded joint and was restored to the same level as that of the hot-rolled base metal, with the exception of the upper shelf energy. (2)The energy transition temperature was about 24°C in the normalized joint, which agreed closely with the temperature estimated from the fracture surface. These transition temperatures were lower in comparison to those of both the as-welded joint and the hot-rolled base metal. (3)In the case of the normalized joint in the transition region, the crack propagation resistance was more markedly increased than that of the as-welded joint. (4)The impact bending strength of the normalized joint after friction welding was almost equal to that of the hot-rolled base metal. The value, however, was somewhat lower than that of the as-welded joint.

View full abstract

Three- and four-point bending of a layered beam with a delamination are analyzed on the basis of classical beam theory. Axial tensile and compressive forces are induced by bending in the parts of the beam above and below the delamination. When one of the constituent beams is thin enough, the axial compression causes local delamination buckling. The problem of determining the critical buckling load is reduced to the solution of a characteristic equation. The critical load and the relation between the magnitude of the applied load and the deflection at the point of load application predicted from the analyses are shown to agree well with experiments. The nonzero energy release rates are also predicted for four-point bending, which implies that the delamination can spread via local buckling.

View full abstract

Microscopic damage progress under static tensile loading in quasi-isotropic CFRP laminates is observed using a scanning acoustic microscope (SAM) and an optical microscope. Both plain specimens and specimens with circular holes are tested. By observing the edges of the plain specimens, the transverse crack density is measured as a function of laminate stress. By SAM observation of the specimens with holes, delamination onset and growth are detected. To discuss the delamination onset around the hole, the energy release rate associated with the delamination onset is calculated following O'Brien and Raju (AIAA-84-0961, (1984) 526-536). The energy release rate predictions are qualitatively consistent with experimental observations.

View full abstract

Interface control of GFRP was investigated using glass fiber treated with ammonia gas plasma. In this paper, discussions, particularly concerning the effects of magnetic field under a pair of electrodes on the bending strength of GFRP prepared with plasma-treated fiber, are presented. The plasma treatments under various plasma condition; of treatment pressure, treatment power, electrode distance and treatment time were carried out in a bell-jar reactor for the glass fibers. Three-point flexural tests of GFRP were performed using an Instron testing machine in order to examine the effects of various plasma treatments on the composite materials. As a result, direct implantation of amino and imino groups onto the surface of glass fibers using magnets under plasma treatment are shown to be effective to improve the interfacial adhesion between glass fiber and epoxy resin. The bending strength of GFRP with plasma treatment became higher than that of GFRP made with untreated fiber. The bending strength of GFRP varied considerably with the plasma treatment parameters. The adhesion state of epoxy resin onto the surface of glass fiber after ammonia gas plasma treatment was observed in scanning electron micrographs of fracture surfaces. It can be verified from these results that interfacial adhesion between fiber and resin was increased by this process.

View full abstract

Composite materials are widely used because of their excellent functional properties. However, in practice, they have the disadvantage that their microstructural parameters vary uncertainly. In this paper, the authors will propose a new stochastic finite element procedure combined with the homogenization method for the efficient analysis of the stochastic nature of composite materials with uncertain microstructures. In the proposed procedure, the stochastic homogenization method (SHM), the first order perturbation technique and the first order second moment method are applied to the homogenization method. The SHM, which can be used to compute the expectation and variance of microscopic stresses as well as those of macroscopic stresses, is implemented in a deterministic homogenization analysis code based on the finite element method. The results of numerical analysis indicate that the proposed method is usefull from the points of view of both efficiency and accuracy.

View full abstract

In this paper, we propose a new homogenization method for composite materials in order to predict effective dynamic viscoelastic material properties in the frequency domain. The loss tangent, which is the ratio of storage modulus to loss modulus, is an important design variable for products made of viscoelastic materials. The proposed procedure, which makes it possible to compute the effective loss tangent of composite materials with periodic structure, is implemented in a homogenization analysis system based on the general purpose finite element analysis code, ABAQUS. The results of the numerical analysis are very close to experimental results obtained using rubber composite containing a stiff rubber and a soft rubber.

View full abstract

This paper deals with a row of equally spaced equal elliptical and circular-arc notches in a semi-infinite plate subjected to tension. Based on the concepts of the body force method, the problems are formulated as a system of singular integral equations with Cauchy-type singularity, where the densities of body forces distributed in the x-and y-directions of semi-infinite plate are unknown functions. In order to satisfy the boundary conditions along the notches, eight kinds of fundamental density functions proposed in our previous paper are used. Then the body force densities are approximated by a linear combination of the fundamental density functions and polynomials. In the analysis, the shape and position of notches are varied systematically; then, the magnitude and position of the maximum stress are examined. For any fixed shape and size of notches, the maximum stress is shown to be linear with the reciprocal of the number of notches.

View full abstract

This paper deals with the numerical solution of singular integral equations of the body force method in the interaction problem for a row of ellipsoidal cavities under uniaxial tension. The problem is solved by the superposition of two auxiliary loads: (i)biaxial tension and (ii)plane state of pure shear. These problems are formulated as a system of singular integral equations with Cauchy-type singularities, where the densities of body forces distributed in the r, θ and z directions are unknown functions. In order to satisfy the boundary conditions along the ellipsoidal boundaries, eight kinds of fundamental density functions proposed in our previous paper are applied. In the analysis, the number, shape, and distance of cavities are varied systematically; then the magunitude and position of the maximum stress are examined. For any fixed shape and size of cavities, the maximum stress is shown to be linear with the reciprocal of the squared number of cavities.

View full abstract

The elastic moduli of 20 kinds of engineering steels were measured by the ultrasonic pulse method. For quench-tempered carbon and low alloy steels, a linear model can be used to explain the moduli with an accuracy of 0.3%. The equation derived is E, G or v=A₁C+A₂Si+A₃Ni+A₄Cr+A₅Cu+A₆(1/T)+A₀, where E, G and v are Young's modulus, the shear modulus and Poisson's ratio, respectively. C, Si etc. are chemical elements and T is the tempering temperature. The effects of heat-to-heat variation, material specification and tempering temperature on the elastic moduli are expressed fairly well by the equations.

View full abstract

In plastic deformation of damaged materials, the influence of the distribution of voids on macroscopic mechanical properties is an important problem. Since the distribution of voids in actual damaged materials is heterogeneous, we have to evaluate the heterogeneous distribution of voids quantitatively. In this paper, we propose a method using stereology and voronoi tessellation to evaluate the heterogeneous distribution of holes and propose an anisotropic yield function for perforated sheets with randomly distributed holes used as models of damaged materials. It is shown that the method is valid for evaluating the heterogeneous distribution of holes.

View full abstract

Development of a method for measuring the residual strain in graphite components in the gas-cooled nuclear reactors which is nondestructive is a major research aim. In order to measure the residual strain in graphite components, the applicability of a micro indentation method was investigated using a micro hardness testing machine. In the test indentation load to depth characteristics and micro hardness were determined at several levels of tensile residual strain in the graphite material. We describe the experimental results and discusses the applicability of the micro indentation method to residual strain measurement for graphite components.

View full abstract

This paper presents an analytical method of thermal stresses in ZrO₂-8%Y₂O₃/MCrAlY graded thermal barrier coatings produced by plasma spraying. The formation of sprayed particles in graded coatings is investigated in order to construct the analytical model. The model is characterized by random dispersion of particles which have each physical properties in graded coatings. The validity of this methodology is investigated by comparison with more detailed stress analysis on a simple pattern with MCrAlY particle in ZrO₂ matrix. This analytical method is applied to graded thermal barrier coatings on gas turbine blades to determine the thermal stresses. Meanwhile the vertical crack in the 100% ZrO₂ top coat of the coating surface layer and the delamination in the ZrO₂ rich area of ZrO₂/MCrAlY graded layer in graded coating specimens are generated in heat cycle tests. The relationship of thermal stresses to these fractures in graded coatings is discussed.

View full abstract

Creep deformation of SUS 304 stainless steel is experimentally investigated at low temperature by applying combined axial and torsional loads to thin-walled tubular specimens. The specimens are cooled by soaking in liquid nitrogen. The strain of specimens is measured by an image processing method. Creep deformation is remarkable in the stress range from 700MPa to 900MPa at low temperature. Creep deformation depends on the loading direction. Compressive creep deformation is much less than the tensile one at the same equivalent stress level. Torsional creep deformation is also much less than the tensile one at the same stress level. From the results of step-up tests, it is found that the stainless steel is hardened by creep deformation and its hardening value is nearly equal to that produced by the same magnitude as plastic strain.

View full abstract

A quantitative relation (F-N curve) was obtained between constant impact load due to cavitation bubble collapse and the number of cycles to the incubation period termination. The relation was derived from the linear cumulative damage law, in which all impact loads including ones of very low intensity (0.3 N) are assumed to cause damage. The slope of the F-N curve corresponds well with that of the S-N curve for a smooth specimen obtained from a standard fatigue test, provided that the contact area is obtained by the Hertz theory. The incubation period can be evaluated regardless of cavitation test conditions from the cumulative cycle ratio Σ(n_i/N_i), using both the number n_i. of impact loads and N_i from an extrapolated F-N curve (modified Miner's law) for the corresponding impact load F_i. Similarly, the F-N' curve was obtained for the steady-state period, where N' was defined as the number of cycles required to achieve a mean depth of penetration (MDP) of 1 micron. The slope of the curve coincides well with that of the S-N curve for a slightly notched specimen. The Σ(n_i/N'_i) has a linear relation with the MDP in the steady-state period. Since N'_i is proportional to 1/F²_i from the F-N' curve, the cumulative cycle ratio was reduced to ΣF²_i·n_i, which is consistent with the impact energy from the authors' previous report. It is concluded that the ratios Σ(n_i/N_i) and Σ(n_i/N'_i) are suitable parameters for evaluation of the cavitation damage in the incubation and steady-state periods, respectively.

View full abstract

In order to estimate the mechanical behavior of coal in a hopper, an experimental and analytical investigation was performed. The experiments were carried out using two-dimensional hopper models. Steel plates or cones were inserted in some hopper models in order to examine the preventative effect of coal arching. A simplified equilibrium equation was derived to obtain the stress distribution in a hopper. It was shown that arching can be quantitatively analyzed using the obtained stress distribution and Jenike's theory.

View full abstract

To enable the wider use of CFRP, damage especially in the laminated direction such as transverse cracking, delamination or fiber-matrix debonding should be easily and economically detected and a reasonable test method for detecting damage nondestructively should also be devised. We present the application of a hierachical neural network to damage identification in a CFRP laminated beam and discuss the accuracy and the efficiency of this method. It is found that the neural network is a useful and practical nondestructive method for the first stage approximation of damage identification in the CFRP laminated beam. Although the network is developed through an iterative calculation, this network is suited for field measurement because the damage can be identified by simple operations and multiplications.

View full abstract

A new method for estimating the nonuniform distribution of moisture content in solids is developed using inverse analysis. Since this inverse problem is ill-conditioned, direct application of the conventional method for solving inverse problems results in an oscillatory solution. To avoid this difficulty, the nature of the material, that the distribution of moisture content is governed by the diffusion equation, is taken into account, and the number of unknown parameters is reduced. The multiple-domain boundary element method is employed for analyzing the electric field within the solid. Some example problems are solved using this method in order to demonstrate the accuracy of the estimation.

View full abstract

This paper describes an application of a hierarchical neural network to defect identification with the ultrasonic method. The present method consists of three subprocesses. First, sample data of identification parameters vs. dynamic responses of displacements at several monitoring points on the surface are calculated using the dynamic finite element method. Second, a back-propagation neural network is trained using the sample data. Finally, the well-trained network is utilized for defect identification. This method is applied to the identification of location, length and inclination of a defect hidden in solid. Its performance and robustness are quantitatively discussed in detail.

View full abstract

Atomic diffusion near an aluminum grain boundary is analyzed in order to investigate a basic mechanism of stress-induced migration (SM) in a thin wiring line in LSI. Molecular dynamics which is based on the effective-medium theory (EMT) is applied to a model containing a surface and Σ=5[001] symmetrical tilt grain boundary with bamboo-like structure. EMT is constructed using no empirical knowledge and is known to be effective for analysis of inhomogeneous atomic structure. Above the transition temperature from low-to high-temperature modes of SM, jump motion of atoms near the grain boundary occurs and the diffusion coefficients (DC) which are estimated from the mean square displacement of atoms increase remarkably. DC has the largest value in the region where the grain boundary intersects the surface. The magnitude of DC in the grain boundary region and its temperature dependence agree well with the results obtained by Plimpton et al., who have used several pair-wise interatomic potentials. It is proven that DC increases with an exponential function of the tensile strain.

View full abstract

We describe the effects of temperature and interatomic force interactions between tool and workpiece on the atomic-scale cutting mechanism, by means of molecular dynamics simulation. The interatomic force interaction between tool and workpiece is assumed to be derived from the Morse potential function. Molecular dynamics simulations of the cutting process using a rigid pin tool were carried out, changing the temperature and the values of Morse potential parameters γ₀, D and α. The increase in the potential parameters D and α brought about the positive effect of surface roughness, but the increase in the parameter γ₀ and temperature brought about the negative effect of surface roughness. Chip formation and side flow resulted due to the collision force between tool and workpiece, which lead to temperature increase of the workpiece. The surface of workpieces observed experimentally in microscale cutting was similar to that in atomic-scale cutting by molecular dynamics simulation.

View full abstract

In tightening critical structural members such as pressure vessels of nuclear reactors and chemical plants and important parts of diesel engines, a hydraulic tensioner is widely used because of its high accuracy in controlling clamping force. The ratio of the desired clamping force to initial tension, which is termed the effective tensile coefficient, is the most important factor to be predicted in the actual operation of given joint configurations. It is reported, however, that a certain amount of scatter in clamping force cannot be avoided. In this paper, an elementary approach to analyze the tightening process is proposed using spring elements, where the effects of incorrect geometry at contact surface on the coefficient are taken into account. The influences of pitch error and flatness deviation at the nut-loaded surface are discussed. Finally, a simple equation for estimating the coefficient is presented, where the major factors influencing scatter in clamping force are considered.

View full abstract

This paper deals with the approximated evaluation method for failure probability of structural systems using conditional safety indices. In general, it is difficult to compute an exact solution of the structural reliability problem with correlations of design variables and limit state functions. It is expected that the conditional safety index proposed will be a useful measure for the safety of complicated structural systems. The merit of this new measure is that the characteristics of statistics and geometry are identical on standardized space. Therefore the conditional safety index yields a practical effective interval of system reliability. When the high-order product sets of a failure event is obtained, the new assessment of the bounds for the estimation of failure probability is also significant. However, the evaluation process of conditional safety indices requires a sequential conditional correlation matrix so that it is the complicated process to calculate high-order simultaneous probability. This paper also proposes the method for transforming the vector of original safety indices into the vector of conditional safety indices using cholesky decomposition of the correlation matrix. In addition, it is suggested that the multi-integral can be approximated by the product of simple integrals using conditional safety indices.

View full abstract

An improved simulation method is proposed for the assessment of structural failure probabilities. Basic random variables are assumed to be stochastically independent normal variates. The sampling region expressed in polar coordinates is divided into sampling segments by several concentric spheres with the center at the origin. The probability element at a certain radial distance from the origin is the product of the probability density, a volume element of the corresponding sampling segment and an indicator factor. A "hit and miss" Monte Carlo simulation is applied to estimate the indicator factor, which is the proportion of samples falling in the failure region for each sampling segment. The structural failure probability is estimated as the summation of a probability element of each sampling segment. The proposed method gives good estimates of the structural failure probability with small sample size. Numerical examples are provided to show the validity of the method.

View full abstract

In this paper we present a numerical analysis method for shape determination of continua based on a strength criterion to control the stress distribution to a desired one. As an objective functional, we introduce a squared stress error norm on the prescribed domains. Using the Lagrange multipliers, an optimization problem subjected to a volume constraint is formulated. The shape gradient function and the optimality condition are derived using the material derivative method and the adjoint method. With the traction method, the domain variation that minimizes the objective functional is numerically and iteratively determined. The shape determination system is developed using a general-purpose FEM code, which is used to solve the state equation and the adjoint equation. The calculated results show the effectiveness of the proposed method in controlling the stress distribution.

View full abstract