This paper summarizes recent advances in the experimental, mathematical and computational studies on dynamic fracture phenomena, made by the author and coworkers. In the section for the, experimental studies, explained topics are (1) high-speed photography of laser-caustics for mixedmode impact fracture, (2) the shapes of propagating crack front and the surface singularities of corner-point, (3) governing criterion of dynamic crack bifurcation, and (4) transonic interfacial fracture. For the mathematical studies, the following items are briefly explained: (1) asymptotic near-tip fields of unsteadily propagating crack, (2) a unified solution for dynamically propagating interfacial cracks, (3) concept of separated dynamic J integral for dynamic interfacial fracture mechanics, and (4) dynamic interfacial fracture mechanics for piezoelectric ceramics. For the computational studies, the following recently developed simulation technologies are explained: (1) dynamically curving fracture path prediction using a moving finite element method based on mapping technique, (2) impact fracture path prediction using a moving finite element method based on Delaunay automatic triangulation, (3) moving finite element simulations of subsonic, intersonic, and supersonic interfacial crack propagation, (4) simulations of dynamic crack bifurcation phenomenon, and (5) hybrid experimental-numerical methods for dynamic fracture phenomena.

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A 3D finite element analysis program is newly developed for fluid-structute interaction problems with structural buckling and large domain changes. An ALE finite element method with automatic mesh updating is formulated for large domain changes. In order to cope with the numerical instability caused by the buckling of a thin shell structure, a strong coupling strategy is taken together with some stabilization techniques for fluids and structures. The simulation of the pulsation of a 3D simplified artificial heart model with structural buckling and large domain changes are demonstrated to show the capability of the proposed methods. The reasons for the numerical instability caused by the structural buckling are clarified.

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For understanding the mechanical behavior of nano-scale metallic contact, the first principle simulations are conducted for atomic chain and bulk single crystal of aluminum under tension. The elastic constants calculated for the bulk coincides well with the experimental data reported. The distance between the adjacent atoms in the chain under no load is 0.247 nm, which is much smaller than the distance between the nearest atoms in the bulk (0.283 nm) . The chain breaks at the maximum load of 1.1 nN and at the strain of about 0.2. The chain shows higher strength and lower elongation in comparison with the bulk.

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The aged piping of a nuclear power plant has possibility to have some defects such as SCC cracks and local thinning. In order to evaluate tolerance limit of the degraded piping, a series of large scale four-point bending, low cycle fatigue tests for the degraded pipes were carried out as a part of the "Aged Piping Committee"project sponsored by National Research Institute for Earth Science and Disaster Prevention. The purpose of the study in this paper is to estalish analytical approaches by which the failure behaviors of the locally thin-walled piping can be simulated well against seismic loading. The summary of results obtained through the present study is as follows. Fully three dimensional elasto-plastic analyses have been carried out for the locally thin-walled piping by using ABAQUS. It has been found that the ratcheting or ovaling/buckling behaviors observed in the experiment can be simulated well through the finite element analysis.

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The authors have proposed the estimating method of thermal fatigue reliability for solder joints of electronic package, using the total equivalent inelastic strain range Δε_in and Manson-Coffin's law. However, as it is difficult to calculate it in the case of huge models, it is needed to establish a simple calculation method of Δε_in. In this study, the authors proposed a simple approach of FEM analysis for calculation of Δε_in. Approximate formula for relative displacements of solder joint using the results of the elastic analysis was examined in order to simplify huge inelastic analysis. In addition, using Statistical Design Support System (SDSS), they made the equation of inelastic strain range in the solder joints. And proposed simple estimation method of thermal fatigue life.

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Polygonal type wear is often seen on a circumference of a rotating viscoelastic disk under contact load. This phenomenon is caused by a combination of physical and mechanical features, such as a dynamic behavior of the material, a heat generation due to loss energy, a heat conduction and various modes of vibrations in the viscoelastic disk including the loading system. The authors had already discussed about the dominant factor of the peculiar phenomenon using an experimental-theoretical hybrid type simulation. In this study, numerical analyses were attempted by using four simplified numerical models, namely a multistage modeling, for a finite element analysis in order to investigate a dominant factor and an opportunity of the periodic stress and temperature distribution of the rotating viscoelastic disk. The criterion of the occurrence of the periodic stress and temperature distribution in a rotating viscoelastic disk was investigated accurately, comparing the results of the experimental/theoretical hybrid simulations and FE analysis.

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Stress singularities at the edge of a piezoelectric bi-material bonded structure with hourglass-shape and straight sides under an axisymmetric deformation are analyzed. The bonded materials are assumed to be transversely isotropic to the symmetric axis of deformation. One of the two materials is piezoelectric and another one is either a dissimilar piezoelectric or a usual elastic material. The analysis assumes perfect bonding and is based on a general solution consisting of three quasiharmonic functions. The eigenfunction expansion method is used to determine the singularities. The discussions are given for the difference of the stress singularities between the cases where the electromechanical coupling effect is considered or not. Moreover, the difference between the axisymmetric deformation and the plane strain deformation are made clear. Finally, the theoretical results are compared with the numerical ones and the good agreement between them are demonstated.

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An equation for a numerical calculation is derived for a heating cycle during the laser beam scan whose power varies as a function of time. Simplified equations are also derived for some special cases. These equations are used for calculating heating cycles during both continuous beam and pulse beam scans imposed on a gray cast iron. The results provide such parameters as a power/ velocity combination to heat the surface to a given temperature, the quench-hardenable thickness, and the time duration being heated above a given temperature. Pulse beam scan is shown to give a fast heating cycles, although the same effect can be obtained with a continuous beam when the scanning speed becomes very fast. Such a fast heating cycle can result in inhomogeneous carbon distribution in the austenite. Effects of quenching such inhomogeneous austenite are discussed.

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The method of arbitrary lines (MAL) constitutes a general dimensional reduction methodology for elliptic boundary value problems (BVP) in arbitrary two-and three-dimensional domains by solving systems of one-dimensional boundary value ordinary differential equations (ODEs) . It has been already applied to two-dimensional problem, and the good results have been reported. In this work, we consider the extension of the MAL to three-dimensional elasto-plastic stress analysis. We first give the MAL formulation of three-dimensional elasto-plastic problems. Although the MAL formulation is derived from the principle of three-dimensional increment virtual work as well as the finite element method (FEM), the MAL is different from FEM in that displacement increment and virtual displacement increment are expressed continuous functions along one direction and shape-functions along other two directions. Substituting displacement increment and virtual displacement increment into the principle of three-dimensional increment virtual work, we have a system of ODEs. The three-dimensional elasto-plastic analysis of BGA model, which was a method of the solder joints of electronic component, was carried out. As results, it was confirmed that to solve 3D elasto-plastic problem at the good accuracy was possible by the MAL.

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The austenitic Shape Memory Alloy (SMA) can be loaded up to a large strain without any plastic deformation and behaves superelastically (pseudelastically) above its characteristic transition temperature. When loaded, the superelastic SMA undergoes a stress-induced martensitic transformation that will result in a large recoverable strain up to 8%. On unloading, the superelastic SMA experiences a large hysteresis loop that makes the alloy an useful candidate for strain energy absorption. By taking advantage of this effect, SMA is used in various fields. The SMA can be used to improve the impact damage tolerance of composite materials and to induce the relaxation of stress distribution. This investigation deals with the transient thermoelastic problem for a superelastic SMA strip under the heat exchange. In order to simplify the SMA model, a linearization of one dimensional constitutive equations is made. The piecewise linear stress-strain relation is separated into two parts : before and after the stress-induced martensitic phase transformation. Another assumption is the identical SMA stress-strain relation in tension and in compression. Results for the temperature and stresses distributions are obtained numerically. We discuss about the variation of the martensitic phase and the thermal stress relaxation.

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Microscopic expressions of generalized Cauchy's relations for higher-order stresses and heat flux are disscused on the basis of the equation of motion of atom. A macroscopic solid is modeled as an assembly of atoms, and the concept of the mesodomain is introduced in order to describe macroscopic quantities with microscopic motions of atoms. Furthermore, a mesoscopic tetrahedral element which is composed of a number of atoms is introduced in the mesodomain. Higher-order moments such as bimoment do not satisfy the equilibrium of the tetrahedron. The microscopic expression of generalized Cauchy's relation for the higher-order stresses can be derived from the first-order tensor product of equation of motion of the atom and position vector. On the other hand, the microscopic description of generalized Cauchy's relation for the heat flux can also be obtained from the summation of higher -order tensor product of motion of atom and position vectors.

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As a part of effective use of domestic Sugi wood, the technique to manufacture compressed wood holds the spotlight. In use of compressed wood, both bending strength and dimensional stabilities are important factors. However, the latter is emphasized, studies about the former is poor. In this study, the effect of manufacturing conditions on the bending strength of compressed wood was investigated. As a result, the fact that Plasticizing Temperature has no effect was obtained. On the other hand, Fixation Temperature and its Holding time have remarkable effect. The higher Fixation Temperature and the longer its Holding time is set, the lower bending strength of compressed wood falls. Moreover apperance of fracture observed in bending test differs in these conditions. This seems to be due to the structural transformation within the cellwall in fixation process. On the basis of these fact, the knowledge was obtained that manufacturing process of compressed wood can acquire greater save-energy and high-efficiency by introducing low temperature plasticizing and high temperature-short fixation than current process.

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The interlaminar peeling test of a beetle fore-wing for A. dichotoma was conducted and the reinforcement mechanism, especially for interlaminar reinforcement, was investigated. As a result, many load peaks in load-displacement curves were observed and the number for them was equal to the number of trabeculae found in chitin fiber laminas. This showed that the fracture of one trabecula contributed to one load peak, which then contributed to interlaminar strength. The increment of interlaminar strength by the trabeculae in chitin fiber laminas was about 30 times in a 10cal region and about 3 times in a whole region as large as that in the chitin fiber laminas without trabeculae. It was found that chitin fibers connected between the chitin fiber laminas and the trabeculae were of a curved shape and continuous, and the trabeculae bonding chitin fiber laminas were distributed in two dimensions in the plane of a chitin fiber lamina. Furthermore, a strong reinforcement mechanism in nature was made clear and a model for the reinforcement mechanism was proposed.

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In order to clarify a crack type degradation mechanism of a directionally solidified Ni-base superalloy CM 247 LC, the effects of environment and microstructural changes on the crack initiation and growth behavior under creep condition have been investigated using an optical and scanning electron microscopes. Metallographic examinations revealed that the surface crack initiation and growth were closely associated with the internal oxidation, nitriding and decomposition of grain boundary carbides. The surface crack initiated at AIN/γ-matrix interface and propagated along a γ′ and carbide-free grain boundary. It was also found that the grain boundary M₂3C₆ carbide was preferential site for creep cavity initiation, and the transformation of M₆C to M₂3C₆ during creep was the critical factor of the internal crack type degradation. Furthermore, the transgranular cracking occurred primarily along the γ-channel, which became lamellar constituent perpendicular to the loading direction because of the raft structure formation.

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Shape memory TiNi fiber reinforced/epoxy matrix composite (SMA-FEC is fabricated to demonstrate the suppression effect of crack-tip stress concentration and the fracture toughness (K value) under mixed mode stresses in the composite. The test specimens have the two types of angled notches to the transverse direction of the tensile-type specimen. i. e. θ=0°, 15°, 30°, 45°, 60° with several crack lengths. The stress intensity factor at the notch tip is experimentally determined by photoelastic fringes. The decreases of K-value are attributed to the compressive stress field in the matrix which is induced when the pre-strain value of TiNi fiber ( T >A_f). The dependencies of K value on the pre-strain value of TiNi fibers as well as on the compressive domain size between a crack-tip and are discussed.

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Shape memory TiNi wire reinforced/acrylic matrix smart composite has been studied by the authors to enhance the strength and fracture toughness of the machinery components by using a shape recovery force associated with inverse phase transformation of shape memory alloy (SMA) . It is also well known that SMA shows the remarkable changes of stiffness and damping between lower temperature martensite and upper temperature austenite phases. Therefore, in the present paper, the authors try to study the active control of deflection and vibration of the base floor plate of the smart bridge model by using direct heating method for the embedded TiNi wires in the composite. The effect of heating time and prestrain of SMA wire on the deflection and damping of the TiNi/acrylic composite structure are experimentally investigated.

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In order to evaluate the reliability of ceramics under transient thermal stresses, a thermal shock test equipment was designed and fabricated. Cylindrical specimens made of Si₃N₄ and SiC were tested by the test equipment. Each specimen was preheated uniformly at 1573 K in an electric furnace and then it was pulled down rapidly into a cooling chamber beneath the furnace. At the same time, it was cooled locally by high-velocity helium gas passed through a narrow slit. The transient temperature distributions were measured by optical pyrometers and thermo-couples. The moment of the fracture was confirmed by the sound of the fracture itself and fracture originated at the outer surface just under the cooling nozzle. The results showed that Si₃N₄ Specimens were fractured more severe cooling conditions than that of SiC.

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The aim of the present study is to make clear the factors controlling the fatigue strength of the locally hardened machine parts. Rotational bending fatigue tests were performed with special focus on the effect of a peak tensile residual stress generated at the quenching boundary on the fatigue properties of the super rapid induction hardened steel. Results are summarized as follows. (1) The final failure starts at the position of the specimen surface where the peak tensile residual stress has been generated near the quenching boundary. (2) The fatigue strength at 10⁷ cycles of the locally hardened specimen is inferior to that of the non-hardened one because of such a peak tensile residual stress. (3) The effect of the peak tensile residual stress on the fatigue strength becomes smaller at higher stress amplitude because the relaxation of the residual stress occurs under stress cycling at such high stress levels. (4) Reduction of the fatigue strength due to localized hardening can be removed by introducing the cold working of the specimen surface after quenching.

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Rotating bending fatigue tests were carried out to investigate the influence of radical nitriding on the surface integrity and the fatigue strength for a maraging steel. Fatigue strength increased by nitriding. Although the initiation site of fracture was the surface in the aged steel, it was the specimen surface at high stress levels and the interior at low stress levels in the nitrided steel. The main reason for the surface fracture was the surface embrittlement and the one for the interior fracture was the surfce hardening due to nitriding. As the results, S-N curves of the nitrided maraging steel showed a double staged curve.

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Fatigue test has been performed to investigate the effect of strain aging on fatigue properties of pre-strained carbon steel (S45C) using Ono-type rotary bending fatigue testing machine. Main results summarized in this study are as follows : (1) It is considered that both of the pre-strain and annealing condition influence the fatigue limit of pre-strained specimen. (2) Both the fatigue limit and hardness are increased by annealing. (3) Under the same annealing condition, the fatigue limit of pre-strained specimen increases with increase of pre-strain. (4) The optimal heat treatment temperature for obtaining the maximum value of fatigue limit decreases with increase of pre-strain.

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Compressor stator blades of gas turbines are subjected to severe conditions such as dynamic pressure oscillation due to rotating stall, corrosive operating environments, and a high number of operations. To improve the reliability of such blades, an analytical method for estimating fatigue damage of these blades has been developed. This method is based on blade-vibratory-stress analyses, stress-peak counting, and use of actual environmental data. The blade-vibratory-stress analyses took superposition of multi-peaks of the stress spectrum into account. And the numerically simulated stress from these analyses showed better agreement with measured stress than that from a traditional stress analysis, which is based on frequency-response analysis considering a single peak of the lowest single eigen vibration mode. Furthermore, the fatigue damage of the blade under rotating stall was estimated by the stress-peak counting of synthesized historical stress waves which were given by the blade-stress analyses and from material data.

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In the present study, a new apparatus was devised to measure stress-strain characteristics of thin adhesive layers under biaxial loadings. The combined stress provided to the adhesive layer consisted of pure shear and axial compression stresses. In the measurement, pure shear and four kinds of combined stresses were provided to the adhesive layer. And stress-strain curves in bulk tensile and compressive specimens were also measured. From the results of the experiments, the followings were found. (1) The apparatus designed in the present study can measure stress-strain curves of thin adhesive layer in a small scatter. (2) Bulk and adhesive layer specimens have essentially the same stress-strain characteristics. And under 15% strain of equivalent plastic strain, their equivalent stress-equivalent plastic strain curves are the same. (3) The stress-strain curves under different loading paths can be expressed by single equivalent stress-equivalent plastic strain curve.

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Many requirements such as safety, lightness and comfort are necessary to helmets for sports and riding. Many studies regarding basic functions of helmets including safety problems were reported. In the previous studies on safety problems, however, it seems to be difficult to say that those studies were accurate reviews because a helmet model integrated with a headform model or a simplified helmet model different from a real one had been used in their simulations. In this paper, FE models were made from the real shapes of the full face type helmet of JIS C Snell and the headform of the Hybrid-III dummy model, which were used in the drop impact tests. Computer simulations for the falling impact of helmets with headform onto playing surfaces were conducted by using FE models. In simulations, the material properties of shell and liner of helmet were determined as elastic and the headform was assumed as a rigid body. As a result, the acceleration responses of headform from the simulated results were compared with those of drop impact tests to evaluate the head protection abilities of the helmet by comparing HIC values. Additionally, the parametric studies were conducted to investigate the factors that affect mainly the headform acceleration. From this study, the main physical parameters, which are important in the optimal design of helmets under impact loadings, can be decided.

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Under the influence of mechanical environment, bone structure is formed and maintained by adaptive remodeling which involves osteoclastic resorption and osteoblastic formation. In the mechanotransduction system in osteoblasts, it is believed that intracellular calcium plays fundamental roles and actin filament is a crucial component for the signal transduction processes. To clarify the role of actin filament in deformation-induced Ca²⁺ signaling, osteoblast-like cells (MC3T3-E1) with different volume fraction of actin filament controlled by cytochalasin D were subjected to tensile strain in vitro. And, the change in intracellular Ca²⁺ density labeled by fluo-3 was observed by using a laser-scanning confocal microscope. An a result, disruption of the actin filament greatly suppressed the deformation-iuduced Ca²⁺ response that was regulated according as the degree of actin filament organization. This result indicates that the actin filament is indispensable in quantitative regulation of the Ca²⁺ signaling in response to the mechanical stimulus in osteoblasts.

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