In the present paper, researches which investigate the influence of surface energy and surface stresses on mechanical behaviors in nano-sized elastic domains are reviewed. A dynamic equilibrium relationship involving surface stresses and bulk stress on a general curved interface is firstly derived. Surface wave propagation and stress distribution around a prismatics dislocation loop existing near an interface are discussed. Furthermore, a contact analysis based on three-dimensional theory of elasticity and a nano-adhesion theory considering surface stresses is presented. Green function considering surface stresses as a tool for analyzing surface defects, steps, undulation, 2D and 3D islands on surfaces is derived in the framework of the theory of elasticity.

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Voids in a solder joint can reduce their fatigue life. Voids are caused by the presence of flux in solder paste during reflow soldering, and they are difficult to remove completely. From numerical analysis, we aimed to obtain criteria for evaluating the effect of voids on fatigue life in a solder joint.
We investigated crack propagation in the micro-solder joints of a semiconductor and developed a new crack propagation model. In our model, the fatigue life of a solder joint is evaluated based on damage that is accumulated during crack propagation, and crack paths are automatically calculated. The crack-propagation behavior of a center-cracked-plate (CCP) specimen calculated using the new model agreed well with that obtained from measurement.
Using our model, we analyzed the effect of positions and sizes of voids on crack paths and the fatigue life of a ball grid array (BGA) structure. The crack paths and the fatigue life were both found to strongly depend on the positions and sizes of voids. We have achieved a reliable method of evaluating the effects of voids in a solder joint.

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To clarify the validity of evaluation of the threshold value of fracture toughness K_th on the surface layer of ceramics by sphere indentation test, indenters of various diameters 2R were used for sphere indentation tests with using Si₃N₄ specimens made by HIP-sintering and numerical calculation of the stress intensity factor K_I was performed for surface cracks under ball-plate contact loading. The crack length c_i was estimated from experimental results using K_I, where c_i is the length of the crack leading to a ring crack and the conditions for ring crack initiation were assumed to be K_I>K_th. The average values of c_i increased with increasing 2R in the case of small 2R, but the averages of c_i gradually approached a constant value in cases with large 2R. The constant value of c_i was estimated as 7.9-8.6 μm using K_th=5.3 MPa·m^½ and was almost equivalent to the grain size of the test material. The same results were obtained in the previous study with Si₃N₄ specimens made by gas-pressure-sintering. Therefore, sphere indentation tests can be used to evaluate K_th of ceramics using K_I for surface cracks.

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Bio-compatible piezoelectric materials are becoming increasingly important for actuators and sensors in medical devices, that is Bio-MEMS such as health monitoring systems and drag delivery systems. In this study, we challenged to derive new piezoelectric materials with bio-compatibility by first principles calculation. Firstly, constituent elements of bio-compatible piezoelectric materials have been specified by HSAB method from the viewpoint of interaction energy with in-vivo molecules. Secondly, in order to create a perovskite-type crystal structure with good piezoelectric response, the combination of bio-compatible elements was selected to satisfy geometric stable condition defined by tolerance factor. As a result, we discovered 7 kinds of new piezoelectric materials. We focused on one of them, MgSiO₃ that is known to be a mineral with perovskite-type crystal structure, and analyzed the stable cubic structure at paraelectric non-polar phase and the stable tetragonal structure at ferroelectric phase by first principles DFT. Additionally, structural phase transition of MgSiO₃ has been investigated on the assumption of linear structural change from cubic structure to tetragonal one. DFT calculation indicated that MgSiO₃ can change spontaneously to tetragonal structure with high tetragonality and polarization, and that MgSiO₃ can present a good piezoelectricity.

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A technique for generating automatic hexahedral mesh in a complex solid model was developed. This technique can automate the interactive operations (such as model editing) that are carried out as a pretreatment for hexahedral-mesh generation. Consequently, in our tests, the time taken by these interactive operations was significantly shortened, and high-speed generation of a hexahedral mesh was thus possible. The technique was applied to generation of a hexahedral mesh of an engine-block model. It was confirmed that the mesh generation time is shortened to one tenth of the conventional method's time.

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In order to elucidate the complex effects of surface roughness, the transition of microstructures from the surface to the interior, defects and residual stress, tension-compression fatigue tests have been conducted by using shot blasted ferrite-pearlite ductile cast irons with as-cast surfaces. Regardless of the applied stress levels, the fracture origins of the specimens were mostly situated at a defect near an as-cast surface, slag, or pinhole at N_f<10⁷. At a relatively lower stress amplitude, some fatigue fractures such as shrinkage occurred from an inner defect far from the surface, mostly at N_f<10⁷. The √area parameter model was applied to the quantitative evaluation of the fatigue limit. The equivalent hardness considering the transitional layers and the effective defect size with the interaction between the surface roughness and a defect were defined. Moreover, the relief of the residual stress during the fatigue tests was detected. The mean value of the relieved residual stress acting on a defect was employed for the quantitative evaluation. By using these parameters, the complex effects of ductile cast iron have been successfully evaluated for practical use.

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Diamond films were synthesized on a Mo substrate using combustion flame. During cooling process, the most diamond films delaminated from the Mo substrate because of their thermal expansion mismatch. To prevent the delamination, a three-step synthesis method was proposed. The first step was synthesis of the Mo₂C and the diamond phases on the Mo substrate, and the second and the third steps were synthesis of the diamond phase. The interfacial stress between the film and substrate was calculated by a finite element method. According to the results, the stress in the film made by the method was smaller than that by an one-step synthesis method. The three-step method is useful for synthesizing the diamond film.

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In existing literatures, the closed-form solution for stress distribution around an elliptic hole has been expressed in the elliptic coordinate in textbooks of theory of elasticity. However, these conventional solutions are inconvenient for application to strength analysis. In the present paper, a new concise closed-form solution for the stress distribution around an elliptic hole under remote uniform stresses σ_x, σ_y, and τ_xy is given in Cartesian coordinate. The analysis of crack growth path from the elliptic hole under pure shear stress is carried out with combination of FEM analysis. The contradiction between the crack growth direction from an elliptic hole and that from a crack under pure shear is made clear by the numerical analysis.

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It is well known that nuclear pressure vessel steel shows embrittlement under thermal aging and strong neutron irradiation. We focus on nanoscale copper-rich precipitates and try to clarify the effect of the nanoscale copper-rich precipitates on embrittlement of reactor pressure vessel steels. Our final goal is to evaluate such embrittlement from microscopic viewpoint based on atomistic simulation. In this study, we simulate interaction between motion of an edge dislocation and copper clusters using Molecular Dynamics method with a PBC, where uniform shear strain is applied to the boundaries parallel to the slip plane (1 1 2) in the system. As the results, we clarify the effects of size, distance and pinning of the copper clusters on dislocation motion.

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The influences of microstructure and specimen size on creep properties of Sn-37Pb and Sn-3.5Ag solders were investigated by nano-indentation tests and tensile creep tests for fine wire solders. The nano-indentation tests enabled us to measure the indentation creep properties of the specific microstructures individually. Tensile creep experiments were performed on our original creep testing machine for fine wire solders with the diameters of 0.2, 0.5 and 1.0 mm. For the Sn-37Pb solder, the creep deformation properties were uniform for both α and β phases and grain boundary sliding was the dominating creep mechanism. For the Sn-3.5Ag solder, creep deformations of α phase were observed under much smaller stress levels than those for (α+ε) phase, and thus the creep deformations occurred only in α phase, which resulted in the nucleation of microvoids around the ε phase. The tensile creep tests for wire solders revealed that the significance of specimen size on the creep properties is strongly influenced by the microstructure of the solders.

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In order to clarify the effects of a hydrogen gas environment on the fatigue crack growth characteristics of stable austenitic stainless steels, bending fatigue tests were carried out in a hydrogen gas, in a nitrogen gas at 1.0 MPa and in air on a SUS316L using the Japanese Industrial Standards (type 316L). Also, in order to discuss the difference in the hydrogen sensitivity between austenitic stainless steels, the fatigue tests were also carried out on a SUS304 using the Japanese Industrial Standards (type 304) metastable austenitic stainless steel as a material for comparison. The main results obtained are as follows. Hydrogen gas accelerates the fatigue crack growth rate of type 316L. The degree of the fatigue crack growth acceleration is low compared to that in type 304. The fracture surfaces of both the materials practically consist of two parts; the faceted area seemed to be brittle and the remaining area occupying a greater part of the fracture surface and seemed to be ductile. The faceted area does not significantly contribute to the fatigue crack growth rate in both austenitic stainless steels. The slip-off mechanism seems to be valid not only in air and in nitrogen, but also in hydrogen. Also, the main cause of the fatigue crack growth acceleration of both materials occurs by variation of the slip behaviour. The difference in the degree of the acceleration, which in type 316L is lower than in type 304, seems to be caused by the difference in the stability of the γ phase.

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