A method for identifying the material constants of an elastic/visco-plastic meterial under tensile impact loading is proposed. In this method, the Kalman filter and the finite element method are used together with a few test data. To obtain data for high strain rates, notched specimens are employed, because arbitrarily shaped specimens are available in this method. To increase the accuracy of identification, a test under a relatively low strain rate is needed in addition to a test under a high strain rate. The optimum strain rate is determined in such a way that the largest eigen value of the covariance matrix of the estimated parameters becomes minimum. A numerical simulation in performed to demonstrate the validity and usefulness of this method.

View full abstract

The adaptively shifted integration (ASI) technique and continuum damage mechanics are applied to the nonlinear finite element analysis of framed structures modeled by linear Timoshenko beam elements. In order to remove the mesh-dependence caused by the strain-dependency of damage, a new evolution equation of damage dependent on plastic rotation is introduced. The elasto-plastic damage behavior of framed structures including yielding, damage initiation and growth can be sucessfully predicted by the combination of the ASI technique and the new damage-evolution equation. Several numerical studies are carried out in order to show the validity, especially the mesh-independency of the proposed computational technique.

View full abstract

FMM (Fast Mutlipole Method) has been developed as a technique to reduce the computational time and memory requirements in solving big sized multibody problems. This paper proposes a BIE formulation of FMM using Wigner-3 j symbols and collocation for the scattering of scalar waves by cracks in 3 D. The resulting algebraic equation is solved with GMRES (Generalized Minimum RESidual method). It is shown that FMM is more efficient than the conventional method when the number of nuknowns is greater than several thousands.

View full abstract

Large scale molecular dynamics simulation are performed to examine the fracture mechanism of an amorphous metal. Crack propagation properties in a model amorphous material made from FS-potential for Fe atom are analyzed for a thick-plate model with periodic structure and a thin-plate model. In the former model, being promoted by development of band-like shear deformation emanating from the crack tip and density decrease in the crack front region, smooth crack blunting with round tip proceeds. On the other hand in the thin plate model, after blunting slightly, the crack sharpening proceeds, which is accompanied by a narrow damaged region in the crack front where a void nucleates thereafter. And coalescence of the crack and void drives further crack advance. Fracture in amorphous metal is revealed to be ductile locally but brittle-like macroscopically, which are quite different from those in the materials with crystalline structure.

View full abstract

The fracture toughness test was carried out by using small CNS (Compact Normal and Shear) specimens subjected to mixed-mode loading. Small CNS specimens made of 62Sn-38Pb and epoxy resin enable us to carry out the experiment under various kinds of mixed-mode loading. The complex stress-intensity factor associated with an interface crack was evaluated by the virtual crack extension method. K_I and K_II values at unstable crack growth were measured under various mixed-mode loading. The energy release rate of interface crack was not constant under various mixed-mode loading. The relationship between K_I and K_II values shows the elliptic law. The stress intensity factor of CNS specimen can be estimated by the polynomial approximation with high accuracy.

View full abstract

Circumferential wave propagation within pipe wall has been analyzed with an explicit finite element method under plane strain for thin walled steel pipes with an axial slot on the outer or inner surface as an artificial flaw. In the analysis, a plastic wedge of large attenuation is also taken into consideration to simulate the real pitch-catch measurement configuration with angle beam transducers. The numerical results show the pulse energy is not propagated along the path predicted by the ray theory. Namely, the pulse is propagated macroscopically along the circumference, however, the energy is carried locally along the radial direction. At the inner and outer surfaces of the pipe, the reflected and mode-converted waves are excited, thus the wave train becomes longer after multiple reflections. The numerical results of the received signal amplitude for various slot depths are in good agreement with the measured.

View full abstract

In this work, stress concentration factors (SCFs) K_t of a flat bar with double circular-arc notches or double V-shaped notches are considered on the basis of exact solutions now available for special cases and accurate numerical results. First, for the limitting cases of deep and shallow notches, the body force method is used to calculate the SCFs ; then the formulas are obtained as K_td and K_ts. On the one hand, upon comparison of K_t and K_td, it is found that K_t is nearly equal to K_td if the notch is deep or blunt. On the other hand, if the notch is sharp or shallow, K_t is mainly controlled by K_ts and the notch depth. The notch shape is classified into several groups according as the notch radius and depth ; then, the least squares method is applied for calculation of K_t/K_td and K_t/K_ts. Finally, a set of convenient formulas useful for any shape of notch in a flat test specimen are proposed. The formulas yield SCFs With less than 1% error for any shape of notch.

View full abstract

In this work, stress concentration factors (SCFs) of a round bar with fillet K_t are considered on he basis of exact solutions now available for special cases and accurate numerical results. First, for he limiting cases of large and small values of the step including fillets, the body force method is used to calculate the SCFs ; then the formulas are obtained as K_td and K_ts. Here, K_td is a SCF of a deep shoulder fillet and K_ts is a SCF of a shoulder fillet in semi-infinite plate. On the one hand, upon comparison of K_t and K_td, it is found that K_t is nearly equal to K_td if the step is large or the radius of the fillet is small. On the other hand, if the step is small or the radius of the fillet is large, K_t is mainly controlled by K_ts and the relative step length. The fillet shape is classified into several groups according as the fillet radius and the step length ; then, the least squares method is applied for calculation of K_t/K_td and K_t/K_ts. Finally, a set of convenient formulas useful for any shape of fillet in a round test specimen are proposed. The formulas yield SCFs for any shape of notch with less than 1% error in most cases.

View full abstract

In order to evaluate the tensile strength or fatigue strength of a plate material, usually a plate specimen is used. The plate specimen has two grips whose breadth is different from that of the plane part. When the plate specimen has notch or crack, the value of stress intensity factor or stress concentration factor is affected by the existence of the grips. The effect of the grips depends on the length of plane part. In this paper, the effects of the length of plane part on the stress intensity factor were evaluated by the versatile program of body force method and the necessary length of plane part was discussed based on the results.

View full abstract

The effect of prior fatigue damage on the creep characteristics of 316FR stainless steel was studied to investigate the applicability of two creep-fatigue life evaluation procedures, Time Fraction Rule and Ductility Exhaustion Method, widely used in the recent high temperature structural design codes. Creep tests were conducted at 550°C by using the specimens exposed to prior fatigue cycles at the same temparature and interrupted at 1/4 Nf, 1/2 Nf and 3/4 Nf cycles. The creep strength of the pre-damaged material showed almost no reduction compared with the virgin material until the fatigue damage of 1/2 Nf cycles, and simple summation of creep damage and fatigue damage by using Time Fraction Rule was not formed. On the other hand, the creep ductility of the predamaged material showed monotonic reduction with the prior fatigue damage from the initial stage of the damage, and the relationship between creep damage and fatigue damage evaluated by the Ductility Exhaustion Method was almost linear. These results show that the application of the stress-based Time Fraction Rule is unsuitable and the application of the strain-based Ductility Exhaustion Method is promising to the exaluation of creep-fatigue interaction damage of the 316FR stainless steel.

View full abstract

The effect of prior creep-fatigue damage on the creep and the fatigue characteristics was studied to investigate the creep-fatigue life evaluation procedure of 316FR stainless steel. Creep and fatigue tests were conducted at 550°C by using the specimen exposed to prior creep-fatigue cycles at the same temperature and interrupted at 1/4 Nf, 1/2 Nf and 3/4 Nf cycle. The creep and fatigue strength of the pre-damaged material showed monotonic reduction with the prior creep-fatigue damage compared with the virgin material. The creep ductility also showed monotonic reduction with the prior creep-fatigue damage. These results were evaluated by the stress-based Time Fraction Rule and the strain-based Ductility Exhaustion Method. The result showed that the application of the Ductility Exhaustion Method to the creep-fatigue damage evaluation is more promising than the Time Fraction Rule.

View full abstract

Fatigue crack initiation and propagation behaviors in DCB specimens were investigated using a rubber modified epoxy as adhesive. Glass substrates were used to observe the failure transition (cohesive→interfaicial) in fatigue crack growth. In order to know the effect of stress singularity at the interface edges on the failure transition, tapered and chamfered DCB specimens. (S55C substrates) having nominaly zero/greatly reduced stress singularities were also used. The experimental results revealed that a cohesive fatigue crack in the adhesive layer once stopped with decreasing a strain energy release rate range. Simultaneously a new interfacial crack initiated and propagated near the arrested crack tip. Tapering and chamfering the edges of DCB adherends delayed the interfacial crack initiation. It was found that the interfacial crack initiation due to stress singularity at the interfacial edges was the major driving force for failure transiton.

View full abstract

Creep-fatigue crack propagation tests using type 304 stainless steel have been carried out under intermediate tensile load hold during an unload process in order to study the effect of the load hold position. A finite element analysis was carried out to investigate the fracture mechanics parameter. The results obtained were as follows. (1) As for the finite element analysis, the application of the fracture mechanics parameter, creep J-integral J′, was found to be effective under the intermediate tensile load hold condition as well as under the maximum load hold condition. (2) The crack propagation rate was correlated well with the fracture mechanics parameter, creep J-integral range (ΔJ_c), under the intermediate tensile load hold condition as well as under the maximum load hold condition.

View full abstract

This paper describes the fatigue strength and subsurface crack initiation behaviour in gasnitrided low alloy steel, SCM 435. Cantilever type rotating bending fatigue tests have been conducted at room temperature in laboratory air using smooth specimens with different nitrided depths, and the effect of nitrided depth on fatigue strength was evaluated and the fracture process with fish-eye was studied on the basis of detailed fractographic examination. The fatigue strength increased with increasing nitrided depth and could be evaluated by the equation of σ_w/σ_n=D/(D-2d_n), where σ_w is the fatigue limit of nitrided materials, σ_n is the maximum applied stress in the core at the fatigue limit, D is the specimen diameter, and d_n is the nitrided depth. Subsurface crack mitiation was observed in all nitrided specimens regardless of nitrided depth and stress amplitude and a fish-eye was always seen with an inclusion at the crack initiation site. Based on some detailed analyses of fish-eye, it was indicated that the fatigue process of nitrided specimens was dominated by the crack growth from inclusion to the formation of fish-eye.

View full abstract

Fatigue properties of the nitrided rolled steel are investigated on the basis of the observations of the substructures produced by nitriding and fatigue. It is found that the fatigue limit of the nitrided rolled specimens is much higher than that of the nitrided annealed ones and the substructures before the nitriding are effective for the improvement of the fatigue strength. The substructure of the rolled steel consists of cells with dislocation clusters within them. By means of nitriding treatment, cell structures and twins appear in the inner part of the compound layer, while uniformly distributed dislocation clusters in the region just below the compound layer and the inner part of the diffusion layer. After fatigue test, most of these substructures are preserved except for the region just below the compound layer where the cell structures due to active dislocation motions were formed.

View full abstract

Two types of the amorphous high polymer materials were used in this paper, one was PC with a ductile property, and another was PMMA with brittle property. The changes of the dynamic Visco elasticity and the fracture phase in the fatigue process were determined to the fatigue fracture mechanism in the difference stress amplitude at room temperature. The results are following as (1) By the tensile testing, PC showed an obvious yield phenomenon so it is a type ductile material, PMMA with the strain below 8% showed the brittle fracture. (2) The fatigue life of PMMA would be low companion with the increase in the stress amplitude, however, the fatigue life of PC was not a simple low, and it would be reverse increase on the certain range of the stress amplitude. The reason may be form a part of ductile fracture on the maximum fracture stress direction of the about 45°. (3) The fatigue fracture properties of PMMA and PC was deeply relative to the change trend of E′, tan δ, ΔL and the fracture surface property.

View full abstract

Electric current induces atom transport in an LSI conductor, which is termed "electromigration (EM)". The EM along grain bondary/interface (GB/IF) often brings about a cavity. Since the EM, flux of which is proportional to the electric current density, does not satisfy the law of mass conservation, stress is generated along the GB/IF. In other words, the migration due to the stress, ""stress-induced migration (SM) ", compensates the mass conservation. Thus, the atoms flow not only by the EM but also by the SM under the condition without external loading. In this study, the functional of atom flux due to the EM/SM is derived and a numerical simulation method of migration along an arbitrary GB/IF network is proposed. The mechanism of cavity growth in a polycrystalline LSI conductor under an electric current is investigated on the basis of the numerical simulation. The SM transports the atoms around a cavity into the GB through the cavity tip and they are moved away by the EM. The cavity growth rate is strongly influenced by the IF/GB network.

View full abstract

Cracking is studied for a ceramic film coating (Al₂O₃) on a metal substrate (WC-Co) subjected to indentation by a spherical diamond indenter. The focus of the study is the pattern of crack growth and apparent variation of strength of the film with respect to size of indenters. To this end, we perform indentation experiments with indenters of five different radii. Based on the experimental results and FEM stress analysis, we discuss the apparent strength of the ceramic film using a Weibull distribution. It is found that only radial cracks are induced for a smaller indenter, but only circumferencial cracks are induced for a larger indenter. It is also revealed that the apparent strength of the film is strongly dependent on the size of indenters. For indenter radii above 500 μm, the variation of apparent strength with respect to the size of indenters is not so strong. Thus, the use of an indenter with a radius larger than 500 μm is recommended in order to obtain valid results. "

View full abstract

Lsystem (LS) is a technique expressing a formation algorithm creating the various shape patterns by the simple rules. Using LS, the complex branch phenomena of plants could be described by simple rules. This paper has tried the methodology to apply LS to the structural design. This methodology grows up the structure as if plants grow up, and produces the optimum structure that is appropiate for the design purpose. It is difficult to acquire LS rules for the optimum structure because the rules have many degrees of freedom. Therefore, this paper proposes an idea that evolves LS rules using Genetic Algorithms (GA) . The methodology is called as an evolutionary LS (ELS) . Using the methodology, the design problems to decide optimum distribution of material and plate thickness are solved.

View full abstract

This article is intended as an investigation on material composition in functionally graded materials (FGMs) that enable to achieve two purposes simultaneously, securement of strength against thermal stress and improvement of heat resistance against thermal load, from the standpoint of multipurpose optimization. Application of genetic algorithm technique is investigated to find the set of the Pareto-optimal solutions in the multipurpose optimization problem of material composition in FGM. In this study, a multipurpose optimization problem of material composition is illustrated for a functionally graded circular plate. Assuming that zirconia/titanium alloy type of functionally graded circular plate is subjected to uniform heat supply from upper surface, we investigate how much two purposes, securement of strength and improvement of heat resistance, are achieved simultaneously by numerical calculation.

View full abstract

This paper describes an analytical formulation and a numerical solution of the elastic dynamic problems of fluid-saturated porous moderately thick shells of revolution. The equations of motion and relations between the strains and displacements are derived from the Reissner-Naghdi shell theory. As the constitutive relations, the consolidation theory of Biot for models of fluid-solid mixtures is employed. The fundamental equations derived are numerically solved by the finite difference method. As a numerical example, the simply supported cylindrical shell under a semisinusoidal internal load with respect to time is analyzed, and the variations of pore pressure, displacements and internal forces with time are discussed.

View full abstract

This paper is concerned with the numerical analysis of transient thermal stress and deformation for moderately thick shells of revolution made of functionally graded material by using finite element method. The material properties of the shell are continuously inhomogeneous along the shell thickness and dependent on the temperature. The temperature distribution through the shell thickness is expressed with a curve of high order, and the temperature field in the shell is determined using the equations of heat conduction and heat transfer. As numerical examples, two kinds of functionally graded material shells composed of SUS304 and PSZ subjected to thermal loads due to fluid are treated ; one is a cylindrical shell and another is an axisymmetric shell having a constant meridional curvature. In comparison with the results from analytical solutions, good agreement is obtained. And it is found that deformations and stress distributions are significantly influenced by temperature dependent properties of the materials.

View full abstract

The friction welding for dissimilar-metal is useful to minimize the diffusion thickness at the welded interface. The minimization of diffusion thickness is required for maintaining the joint strength. However, it is generally difficult to maintain the joint strength. Because brittle intermetallic compounds are necessarily formed at the friction welded interface of dissimilar-metal. The design techniques on joint strength characteristics have not fully been clarified. In this study, stress singularities at the intersection of free surfaces and the interface of various dissimilar-metal were investigated using singular analysis and finite element analysis. As a result, it was made clear that the no free-edge stress singularities existed in the range of apex angles, that were determined by the combination of dissimilar-metal. Especially, it was confirmed that the compressive stress could be observed in the no singular range below an apex angle of 60°by filite element analysis. Finally, the design technique of dissimilar-metal friction welded joint was investigated by paying attention to the geometry of welded interface. "

View full abstract

In the plastic deformation, the material anisotropies have a great influence on the deformation capacity especially in the change of strain path. The anisotropies of sheet metals are caused from the texture developed by a cold rolling process. Because the strength of the texture is different from that of the surrounding material, we assume that the principal axes of anisotropies do not move similarly to the material fiber. According to this assumption, a dynamical model of the principal axes is introduced by using a dumping factor to the motion of the material fiber. Furthermore, some neumerical integrations of this dynamic equation are shown in connection with the change of strain path. Finally, an identification method with the revolution angle of principal axes is proposed by investigating the distribution of r-values with respect to the tension directions.

View full abstract

Electromagnetic acoustic transducers (EMATs) were developed for flaw detection in steel pipes. The EMAT, which consists of an electromagnet and a coil surrounding the pipe, can generate and detect the longitudinal wave propagating along the axial direction of the pipe. In this study, we investigated the properties of the EMAT. It was ensured that the optimum strength of the static magnetic field exists for transmitter and receiver, respectively, because the magnetostriction concerns the operating mechanism of the EMAT for ferromagnetic steel pipes. Thus the dual EMAT configuration is recommended to obtain the high amplitude signal, while narrowband burst wave should be used as excitation signal to avoid the distortion of the signal by dispersion. On the other hand, broadband pulse excitation is accessible for the single EMAT configuration. We compared the flaw detectability of both configurations.

View full abstract

In this paper, an optimal method of numerical analysis was proposed for the musculotendinous forces and joint reaction forces in the index finger. The finger model at each position in motion was constructed taking three dimensional locations and directions of all musculotendons into account. The bones were modeled as massless rigid bodies, and each musculotendon was as a straight element from the origin to the insertion. A Iocal orthogonal coordinate system at each finger joint was used to define force and moment. Because the control mechanism of the finger is very complex, musculotendinous forces and joint reaction forces can not be calculated by only the static equilibrium equations of force and moment. The SQP method of the non-linear optimization was applied to solve this statically indeterminate problem. The results obtained from this analysis gave good agreements with well-known physiological phenomena. The optimal finger position, at which the sum of musculotendinous forces was the minimum, was determined in tip-loaded finger motion like touchtyping motion. The findings are very important to understand the functional anatomy and pathologic deformities of the hand as well as to develope a prosthetic design for finger joints.

View full abstract

A bone structure is a highly differentiated coupling tissue and alternatively grows or declines regarding to biological and/or physiological environments. Therefore, it is an important subject in the bioengineering and practical needs in development of new medicines to reinforce bone structures to investigate the evaluation method of bone density and its elastic modulus and also to clear the mechanism of the correlation between the structure change of a bone tissue and the external environments. In the present paper, density and elastic modulus distributions of a bone tissue are analyzed via distributions of surface wave velocity and acoustic impedance of micro parts of a bone tissue measured using a scanning acoustic microscope (SAM) and examined to be correlative with stress distributions and piezoelectric effects obtained by FEM analysis.

View full abstract