Thermal fatigue is one of the critical issues in nuclear power plant components. The crack network is a typical cracking morphology caused by a thermal loading and was observed in plant components. It was pointed out that the crack network appeared under relatively small temperature fluctuations and did not grow a lot in depth. In this study, a Monte Carlo simulation was developed in order to simulate the initiation and growth of cracks under thermal fatigue loading and the evolution of crack network. Local stress field formed by pre-existing cracks was evaluated by using the body force method and was considered for initiation and growth of cracks. The influence of crack network on initiation and growth of cracks were discussed. The conclusion obtained is the crack network appears when large stress gradient in the depth direction exists. The stress-shielding effect caused by pre-existing cracks plays an important role for the formation of crack network.

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本研究では,ステンレス鋼および炭素鋼の斜めき裂を有する円筒試験片について,繰返しねじり疲労き裂進展試験を行なった.また,き裂進展駆動力を求めるために有限要素法(FEM)解析を行ない,応力拡大係数の定式化を行なった.これに基づき疲労き裂進展特性の評価を行なうとともに,電子スペックルパターン干渉(ESPI)法によるき裂先端変位場の観察から,有効なき裂進展駆動力を実験的に求め,小規模降伏条件およびき裂開閉口挙動に基づき,疲労き裂進展特性に検討を加えた.

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This paper describes the 2D complicated crack propagation analysis using the Free Mesh Method(FMM). The computational fracture mechanics involves frequent re-meshing as the crack propagates. In order to show the effectiveness of the FMM in crack propagation analysis, we study the 2D complicated crack propagation problems with the FMM. In order to show the effectiveness of the neural network in control of crack propagation analysis result.

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Various load acts on a structure having a crack at the same time. Therefore it is important to analyze complicated load about the crack propagation. When a crack is closed there is possibility that crack surface contact each other. Free Mesh Method which is a kind of the meshless method is the technique that is particularly effective for crack propagation analysis. However, the contact of the crack surface is not considered in Free Mesh Method. This study proposes the technology analyzing that considered surface contact of the crack with Free Mesh Method. And we verified new technology with numerical analysis.

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Three point bend specimen with different ratio of K_<II> to K_I are used for ductile fracture tests. As the K_<II>/K_I, changes larger, the crack growth direction in mid-plane of the specimen curves to the loading point. Detailed observation of fracture surface is conducted, and it is shown that the average void diameter changes smaller as the K_<II>/K_I changes lager. Numerical simulation is also carried out using Gurson's yield function. In mixed-mode model, the crack propagates to the direction of loading point though the crack propagates straight in mode I model. The distribution of stress triaxiality around the crack tip is smaller in mixed-mode model than in mode I model. It can be guessed that voids diameter changes smaller as the K_<II>/K_I changes lager. The distribution of stress triaxiality corresponds with the dimple fracture zone qualitatively.

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Fatigue crack growth under mixed mode loading conditions is simulated using S-FEM. By using S-FEM technique, only local mesh should be re-meshed and it becomes easy to simulate crack growth. By combining with re-meshing technique, local mesh is re-meshed automatically, and curved crack path is modeled easily. Crack growth simulations are conducted assuming mixed mode condition in several cases. It is verified that this method is useful for the simulation of complicated crack growth problem.

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A computational strategy to predict the propagation of a crack in a three-dimensional body is discussed. Proposed methodology is consisting of i) automatic mesh generation software, ii) finite element analysis module, iii) program to evaluate the mixed-mode stress intensity factors and iv) module to predict the direction and rate (amount) of crack propagation. In this approach, virtual crack closure-integral method (VCCM) for quadratic tetrahedral finite element to compute the stress intensity factors takes the central role. Finally, an illustrative example problem is presented.

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Atomistic and electronic structures of the 90° domain wall (DW) in PbTiO_3 are investigated using ab initio (first-principles) calculations based on the density-functional theory (DFT). At the domain wall, the magnitude of polarization decreases by 20% from that of bulk and the polarization direction rotates by 90 degrees. Furthermore, we have studied the domain switching which is an abrupt movement of the domain wall induced by shear stress. Our simulation demonstrates that the domain wall begins to move in the normal direction to the wall when the stress reaches at the critical value of 152MPa. During the domain switching, a covalent Pb-O bond at the center of DW breaks and another Pb-O bond is newly constrcuted.

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Ab initio tensile simulation of Si nano-films based on the density functional theory has been performed to investigate the effect of surface structure on mechanical strength. It has been revealed that (001) surface does not affect much the tensile strength, while surface step structure, which emerges on a surface with a high Miller index, can significantly reduce the strength.

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Electronic states at metal/oxide heterointerface, Ni/MgO{111}, are investigated, using first-principles calculations based on density functional theory. The extended slab supercell in accordance with lattice constants of metal and oxide is adopted for the Ni/MgO interface with large misfit. The interfacial chemical bonding at the on-top site is local and covalent, compared with the hollow and bridge sites. In concern to the site-dependent bonding character, the atomic configuration at the interface is relaxed vertically, and then the interfacial strain appears, not negligible for evaluation of the interfacial adhesion energy.

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The convergence property for ab-initio finite-element calculation with curving-grid mesh in terms of total energy has been investigated. Through the calculation of an oxygen molecule, we found that the favorable convergence property, i.e. the variational convergence with the uniform rate, of the finite element method with uniform mesh is conserved.

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We have simulated the piezoresistance coefficients in single-crystal silicon nanowires (SiNWs) based on the first-principle calculations of model structures. Variations of band gaps and curvatures due to tensile stresses will exert an influence on carrier densities and effective masses, and frequently contribute to a drastic turn of the conductivity. In particular, in the <001> SiNW model, uniaxial tensile stress to [001] longitudinal direction causes a sharp drop in band energy of the highest valence band with a small hole effective mass, leading to a drastic decrease in the hole conductivity. It is found that p-type <001> SiNW will be one of the most suitable candidates for nano-scale piezoresistor due to a giant piezoresistivity.

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The morphological transformation and change of composition of gold-silicon alloy at the interface between a Au thin film and a Si substrate are studied using classical molecular-dynamics analysis. The simulation employs modified embedded-atom method potential extended to the Au-Si system by Kuo and Clancy. After the system is equilibrated at room temperature, it is annealed the above eutectic temperature. We study the most stable composition and structure of Au-Si alloy at the interface.

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This paper presents the analysis for the dependency of surface stress and elastic module to the change in the crystal face on grain boundary using MD. It happens rarely that a crystalline material is filled with only one crystal, and usually consists of the set of several crystals. The interface between these several crystals is called a grain boundary. In this study, the grain boundary models for materials with two different crystal face are used in MD. We obtain a surface stress on the grain boundary. Influence of grain boundary is larger than that of normal surface. A distribution of surface stress in atomic layers is influenced by step of grain boundaries.

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It is examined by numerical calculation whether a finite plate with a 2D crack provides intrinsic natures of oscillation of an infinite plate, emphasizing the application to geometrical characterization of a geothermal reservoir crack. Boundary Integral Equation Method is employed for the numerical calculation. In the case of a crack with fluid, the intrinsic nature of an infinite plate oscillation appears clearly in the data of oscillation of the finite plate. The ratio of the plate half width to the crack half length (size ratio) has effects on eigen angular frequency of crack wave. In the case that the size ratio is larger than two, eigen angular frequency are same values of oscillation of an infinite plate. Radiation pattern of crack wave depends on boundary condition of a finite plate e.g., how to stimulate the oscillation.

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The Free Mesh Method (hereinafter referred to as "FMM") has a good compatibility with the parallel computing. But, the conventional FMM has a problem in accuracy. Recently, The Enriched FMM (hereinafter referred to as "EFMM") was proposed by Yagawa et al. This paper describes the parallel EFMM, intending the large scale analysis by saving of times. Saving of times is attained by domain decomposition. The parallel EFMM code is developed and employed on a cluster system.

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In recent years, the object for numerical analysis becomes complicated and the meshing is more and more difficult. The Free Mesh Method, which is one of the mesh-less methods, is considered to make large-scale analysis easier without the so-called meshing method. Enriched Free Mesh Method is a method based on Free Mesh Method. Analysis result of Enriched Free Mesh Method becomes better than analysis result of Free Mesh Method. In recent years, frequent re-mesh is required of n umerical analysis. However mesh generation still needs mush computation time. In order to reduce the computation time for meshing, we develop a new method using semi-circle with searching circle of nodes when employing the Free Mesh Method.

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Effects of additives with isocyanate group such as LDI and LTI on the mechanical properties of PLLA were investigated. It was found that bending properties of PLLA effectively increase due to addition of these additives. Fracture property of PLLA is also dramatically improved by adding 2phr of LDI and LTI. Isocyanate group of LDI and LTI and hydroxyl group of PLLA are chemically reacted and as a result, farm entangled structures are created. This kind of micro-structural modification is thought to be the primary mechanism of the improvement of the macro-mechanical properties.

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Polylactic acid (PLA) is a biodegradable plastic made from lactic acid, and can be produce by renewable raw materials. In addition, PLA has better mechanical properties and commercial availability than those of other biodegradable plastics. The mechanical properties of PLA are, however, not sufficient for structural materials. In order to overcome the deficiency, carbon nanofiber reinforced PLA was fabricated, and the mechanical properties were measured at room and elevated temperatures. Vapor grown carbon fiber (VGCF) was used for reinforcement, which has 150 nm in diameter and 10 tm in length. No surface treatment of VGCF was conducted. The weight fraction of VGCF ranged from 1wt% to 10wt%. Three point bending specimens were fabricated by using injection molding, and then bending stiffness, bending strength and fracture toughness were measured for amorphous and crystallized specimens. The bending stiffness increased, but the bending strength and fracture toughness slightly decreased at room and elevated temperatures.

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Continuous natural fiber reinforced thermoplastic materials are expected to replace inorganic fiber reinforced thermosetting materials. However, natural fiber itself has low strength and in the process of fabricating the composite, it is difficult to impregnate the thermoplastic resin into reinforcement fiber. In our previous studies, it have been clarified that a spun natural yam had high strength. Moreover intermediate material, which allows highly impregnation during molding, can be obtained for fabricating continuous fiber reinforced thermoplastic composite by aligning resin fiber alongside reinforcing fiber with braiding technique. This intermediate material has been called "Micro-braided-yam". In this investigation, completely naturally-derived micro-braided-yarn was fabricated by using thermoplastic biodegradable resin fiber (PLA) as the resin fiber and jute spun yam as the reinforcement. Using the micro-braided-yam, continuous natural fiber reinforced biodegradable resin composite plates can be molded by hot press molding with various molding conditions. Observation of impregnation state and evaluation of mechanical properties were performed, and consequently, the relationship between the molding conditions and the mechanical properties of the composite was clarified.

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Biodegradable PLLA was toughened by blending with four different ductile bio-polymers such as PBS, PCL, PBSL and PBSC by melt-mixing. Fracture properties were measured and compared to assess their effectiveness as a blending partner for PLLA. It was found that the bending absorbed properties are the best with PBS; on the other hand, the mode I fracture property and impact absorbed energy are most improved with PBSC. For PLLA/PBSC, FESEM observation showed that the fracture surface exhibited ductile fracture with elongated fibrils, suggesting the highest energy dissipation in the crack-tip region.

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Press process was performed in order to toughen HA/PLLA bio-composite material. Mode I fracture properties and bending mechanical properties of presssed and unpressed HA/PLLA were then evaluated to assess the effect of press process on these properties. FE-SEM was also used to characterize the micro-structural change and toughening mechanism due to pressing. Testing results showed that the bending strength and modulus slightly increased due to pressing; on the other hand, the bending absorbed energy and the mode I fracture property dramatically improved. FE-SEM observation of the fracture surfaces showed that ductile deformation and fracture of PLLA fibrils were enhanced and in addition, interfacial cohesion strength was improved. These microstructural modifications appear to result in the dramatic toughness improvement.

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Bioactive ceramics/poly-L-lactide (PLLA) composites have been expected as a material for the bone fracture fixations which have more biocompatibility than monolithic PLLA. In this study, β-tricalcium phosphate (β-TCP)/PLLA composites containing three different β-TCP contents and monolithic PLLA specimens were prepared by injection molding to clarif,' the effect of hydrolysis and strain rates on the mechanical properties. These specimens were immersed in phosphate buffered solution, and then tensile tests were performed at the strain rates ranging from 10^<-4> to 10^<-1> [s^<-1>]. In the results of these tests, both β-TCP/PLLA composites and PLLA specimens showed that their modulus have no dependence on the strain rate and their strengths increased with increasing strain rates. After 8 weeks immersion, strength of specimens were decreased slightly.

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The composite made from hydroxyapatite (HAp) and poly (L-lactic acid) (PLLA) is a high-revel candidate for scaffold materials in bone tissue engineering. We made the HAp/PLLA composites which have comparable in bending strength to neat PLLA owing to the improvement in interface strength, resulting from the interfacial control via surface treatment of HAp particles. In this study, we evaluated the effect of the hydrolysis of PLLA on the flexural properties of interface-controlled HAp/PLLA by conducting the three point flexural tests of these materials after immersed in phosphate buffer solution (PBS, 37℃, pH7.4) up to 12 weeks. For comparison, we also carried out tests for neat PLLA and untreated HAp/PLLA. Bending modulus and strength of untreated HAp/PLLA deteriorated faster than those of interface-controlled HAp/PLLA. Finally, the deterioration mechanisms were discussed from the viewpoint of the change in interfacial microstructure. The observation of fracture surfaces indicated that the hydrolysis of PLLA at the interface was locally accelerated in untreated HAp/PLLA.

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The poly lactic acid (PLLA) is tried to use for the biomaterial, for example, the bone screw. However, the tensile strength of PLLA is lower than that of the metal such as the titanium alloy. The objective of this study is to describe on the improvement of mechanical properties of PLLA by using the extension process. By using stretched specimens, the effects of the extension rate, molecular weight and strain velocity on the tensile strength are examined experimentally. To evaluate the work process of the born screw, the bending strength of the specimens with the notch processed by cut or press is compared.

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We developed novel fabric structural composite scaffolds using a bioabsorbable polymer for a regeneration of alveolar bone. The composites consist of the high strength poly (L-lactide) fiber fabric and the bioabsorbable polymer coating. To clarif,' the biodegradable properties of scaffolds, two kinds of resins (polyε-caprolactone and poly (DL-lactide-co-glycolide) copolymer) as a coating binder are employed. For the evaluation of biodegradation and cell growth, mouse osteoblastic cell (MC3T3-E1) seeded in a scaffold was cultured in vitro for eight weeks. As a result, the rigidity and yield strength of PLGA coating scaffold that had a membrane structure between fibers was large increase compared with PCL coating scaffold. However, DNA content per unit area as a cell proliferation and ALP activity per unit DNA as the regeneration activities were about the same to each resin.

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Hydrolysis tests were conducted on the poly(lactic acid) (PLA) fibers to evaluate the effect of hydrolysis on the mechanical properties. In the hydrolysis tests, phosphate buffered solution with pH 7.4 was used. The immersion periods are 4 and 8 weeks. After hydrolysis, single fiber tensile tests were conducted. The results of tensile test indicated that the hydrolysis occurs at 8 weeks.

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The quantitative estimation is required since the fatal defect called a ring crack will occur when a ceramic ball is used for ceramic bearing. We considered the application of stress intensity factor and effective volume by performing FEM analysis and the contact examination of balls. As the results, it did not depend on ball size for the scatter of ring crack initiation load, and it was clarified that a ring crack progressed aslant to the ball surface. The stress intensity factor was discussed based on these results. It was clearly showed that the crack length involved in the formation of a ring crack was approximately 1.5μm, without dependence of ball size. This analytical result is well agreement with an experimental result. Therefore, it was proved theoretically that it is not necessary to consider the influence of effective volume in ring crack initiation of a ball.

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Epoxy resin is widely used for engineering components as adhesive and matrix materials due to its prior properties. However, the pure epoxy resin is brittle. To improve the fracture toughness of the epoxy resin, rubber particles are mixed with epoxy resin. In the present study, we measured the fracture toughness of unmodified epoxy resin and rubber-modified epoxy resin using single-edged notch bending specimens. The distributions of strain around a crack tip were measured using a micro-video-scope and the digital image correlation method (DICM). The measured distribution of strain is compared with that estimated using the finite element method (FEM) in conjunction with Gurson's model, which can well describe the yielding of porous materials such as rubber-modified epoxy resin. The measured distribution of strain around a crack tip was similar to that obtained by the three dimensional FEM with Gurson's model.

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Fatigue life prediction of solder joints is one of the most important areas of research in the development of reliable electronic packages. In the present days, a new method of estimating the crack propagation, which was based on finite element analysis without geometric crack model, was examined. On the basis of damage model for solder, the new method called 'damage path simulation' was verified for CT specimens of Sn-37Pb solder. The extensions of damage path in the cases of different mesh sizes and element types were compared. The extensions of damage path showed comparatively good agreement with the behavior of the crack propagation observed in the fatigue tests using CT specimens, in the case that the mesh size was comparable to the grain size of solder.

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In this study, soil effect on mechanical and morphological properties of Hiba (Thujopsis dolabrata) root was investigated concerning growth of seedling. Seedlings brought up in different soils for two years were used as samples. Tensile test and image analysis were performed to measure mechanical and morphological properties of the roots. As a result, effect of the upbringing soils was clarified from these properties of the roots. The tensile strength and the fracture load for the center region of the cross section were found to be dependent on the soil. The fractal dimension of the root, the total root length, and the number of the root tip were also found to be related to the soil environment.

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Cavitation normally courses severe damage in hydraulic machinery such as pumps and valves, as severe impacts were produced at cavitation bubble collapses. However, cavitation impact was utilized for surface improvement in the same way as shot peening. Peening method using cavitation impacts was called "cavitation peening CP" or "cavitation shotless peening CSP", as shots were not required. The great advantage of cavitation peening was that surface roughness after peening was very smooth compared with that of shot peening, because of no solid collision. Recently, new system for cavitation has been developed to enhance cavitation impact to increase peening effects such as fatigue strength, introduction of compressive residual stress and work hardening. This research and development was partly supported by Ministry of Economy, Trade and Industry.

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