There have hardly been experimental studies of the mechanical properties of Sn-7Zn-0.006Al lead-free solder. The temperature dependence of Young's modulus E and internal friction Q^-1 were investigated in Sn-7Zn-0.006Al, Sn-7Zn, and Sn-8Zn-3Bi solders using flexural vibration. Sn-7Zn-0.006Al solder showed the smallest E drop with increasing temperature, representing heat-softenig. Three Q^-1 peaks appeared at around 90-100°C (1^st), 135-140° (2^nd), and 175-180°C (3^rd) in Sn-7Zn-0.006Al and Sn-7Zn, and two Q^-1 peaks at around 90°C (1^st) and 135°C (2^nd) in Sn-8Zn-3Bi. To investigate the origin of these Q^-1 peaks, vibration experiments at lower frequencies were also carried out. From Arrhenius plots for the Q^-1 peaks in two vibration experiments, the 3^rd Q^-1 peak in Sn-7Zn-0.006Al, the 2^nd Q^-1 peak in Sn-7Zn and those at low frequencies in Sn-8Zn-3Bi were relaxation phenomena obeying the Arrhenius equation, whose activation energies were determined to be 83kJ/mol, 144kJ/mol, and 98kJ/mol, respectively. It was estimated that the origin of these Q^-1 peaks was related to the grain boundary from these values, and that Sn-7Zn-0.006Al alloy was refined in microstructure by the addition of a very small amount of Al and superior to other Sn-Zn system solders with regard to mechanical properties.

View full abstract

Currently, the flip chip package is one of the standard semiconductor packages in the world market. Also, there are variety of materials and configurations used in flip chip packages with further high performance or miniaturization. From the encapsulate materials point of view, many types of assembly methods are presently studied on a current standard process as well as novel proposed processes. A highly reliable package depends on properties of constituent. Additionally, there is a possibility that the assembly process to encapsulation affects the warpage behavior and reliability performance. The present paper reports the relationship between warpage and other parameters depending on not only the material properties but also the encapsulating process.

View full abstract

To investigate the effects of post-coat substrate quenching on the tribological properties of TiN film, a specimen was prepared in which the steel substrate (carbon tool steel, JIS SK3) was quenched after TiN coating, and the ball-on-disk type wear test was carried out using a ZrO₂ ball. The delamination initiation life of TiN film was improved by the high adhesive strength of TiN film obtained by post-coat substrate quenching. The specific wear rate was also improved by post-coat substrate quenching, although TiN hardness was lower than that of the conventional type specimen, which was coated with TiN after substrate quenching. The improvement in the specific wear rate could be explained as follows. In the post-coat substrate quenching process, TiN was partly oxidized and titanium oxide, which acted as a lubricant, was formed on the TiN surface. The formation of titanium oxide resulted in a reduction in the friction coefficient and consequently a lower specific wear rate was obtained.

View full abstract

This paper describes an evaluation method of primary stress in three dimensional (3-D) structures. In “Design by Analysis” for nuclear components, the stresses are classified into the primary and the secondary stresses. The primary stress in axisymmetric structures can be evaluated by linearization of stress distribution in the specified section, however this procedure is hardly applicable to 3-D structures from difficulties to define the evaluation section. From this reason, the alternative evaluation method is needed. In this study, the evaluation method of primary stress in 3-D structures with elastic-plastic analysis is proposed utilizing the feature of primary stress that is independent from stress redistribution. The proposed method is verified through application to example problems.

View full abstract

An attempt has been made to study the microstructural deformation and micro damage evolution process in the thermoplastic polypropylene (PP) blended with the ethylene-propylene rubber (EPR) and talc. The in situ observation is conducted during uniaxially stretching within TEM step by step to investigate the deformation events depending on the elongation of samples. In addition, the microstructural deformation and damage evolution process were simulated by coarse-grained molecular dynamics (MD). The experimental result shows that the micro damage initiates at the interface between PP matrix and talc particle. Then, the micro void is generated there, leading to the fibrils of the PP matrix and EPR particles. The similar trend of the micro damage evolution process is obtained by MD simulation. Finally, the effects of the interfacial strength of PP-EPR and PP-talc on the microstructural damage evolution and macroscopic mechanical properties are investigated. It is indicated that increasing the interfacial strength between PP-EPR is more effective for improving the stiffness and the maximum stress, compared to the PP-talc interfacial strength. In addition, to enhance the material ductility, it is important to increase the interfacial strength of PP-EPR.

View full abstract

In this paper, we propose a finite displacement modeling technique based on the total Lagrangian formulation for the structural analysis of two-dimensional (2D) framed structures connected by flexible joints. Our proposed model consists of beam segments with three-node elements and flexible joints with multi-rotational nodes, which have higher rotational degrees of freedom for deformation compared with those of a classical continuum beam model. Therefore, the category of this model can be regarded as a piecewise C¹ continuum beam model with rotational discontinuity on the joints. The objective of this study is to understand both the buckling modes and the post-buckling behaviors, which are key issues to understanding the integrity of the framed structures. Large elastic deformation simulations of the proposed model reveal that a 2D repetitive structure with finite square grids has biaxial symmetry buckling as a higher-order mode under uniaxial compression and has redundant post-buckling deformability, which rigid-jointed structures might not have.

View full abstract

The methodologies to calculate failure probability and to estimate the reliability of fatigue loaded structures are developed. The applicability of the methodologies is evaluated with the help of the fatigue crack growth models suggested by Paris and Walker. The probability theories such as the FORM (first order reliability method), the SORM (second order reliability method) and the MCS (Monte Carlo simulation) are utilized. It is found that the failure probability decreases with the increase of the design fatigue life, and the decrease of the initial edge crack size, the stress range, and the stress ratio. Furthermore, according to the sensitivity analysis of random variables, it is found that the slope of Paris equation affects the failure probability dominantly among other random variables in the Paris and the Walker models.

View full abstract

Porous ceramics possess the excellent penetration and adiabatic characteristics, etc., and are used as heatproof filter materials for environmental equipments, etc. Moreover, porous ceramics controlled with porosity and pore size in the wide range have been actively developed. However, how the strength characteristics of porous ceramics are influenced by porosity and pore size of the material are not understood still enough. In this research, the evaluation tests on fracture strength, fracture energy and fracture toughness of porous alumina ceramics which porosity are almost equal, while pore sizes are different mutually were performed, and the relation between the pore size and the fracture strength was studied. The tests results show that the dispersion of fracture strength data is few though fracture strength of porous ceramics is lower than that of high-density ceramics. The relation based on linear fracture mechanics between the defect size and the fracture strength is valid when the one that a pore accompanies with the peculiar defect of the material was regarded as a defect size. In addition, fracture energy increases with the increase of pore size, and this seems based on a crooked propagation path of a crack. Finally, the process zone fracture model with considering the effect of the pore and grain size of the material is proposed. According to this model, for all pore size and crack length, it was shown that the fracture strengths of cracked specimens are evaluated.

View full abstract

Computer aided topology optimization design is a relatively new but rapidly expanding area of structural mechanics. Topology optimization design is used in an increasing rate by for example building and bridge engineering as well as the car, machine and aerospace industries. The reason for this is that it often achieves great savings and design improvements by solving the basic engineering problem of distributing a limited amount of material in a design space, in which a certain objective function has to be optimized. In static problems a common objective is to minimize the compliance such as strain energy and in dynamic problems the first natural eigenfrequency is often maximized in order to evaluate the stiffest structures. The goal of this study is to get a wide use for structural designs in the topology optimization. Therefore with respect to SIMP dynamic topology optimization problems are treated in comparisons with static problems in this research. For the topology optimization problems implemented algorithms can be the optimality criteria method and the method of moving asymptotes (MMA). Numerical applications topologically maximizing the first natural eigenfrequency or minimizing strain energy of plates verify the generality and wide use of topology optimization with respect to structural designs.

View full abstract

At first, we analyze the stress intensity factors at a crack tip of multiple kinked crack from a horizontal initial crack in a half-space due to rolling/sliding contact with frictional heat. On the basis of the results of stress intensity factors and applying the maximum energy release rate criterion to each kinks in order, the two-dimensional fatigue crack growth path are simulated, and making use of the fatigue crack growth law obtained from fatigue experimental results for a high carbon chromium bearing steel (AISI-52100), the propagation fatigue lives until the surface tribological failures are predicted due to the repetition of rolling-sliding contact. And we show that the increase of frictional coefficient, sliding/rolling ratio reduce the propagation fatigue lives, and the shortening of the depth of a horizontal initial subsurface crack reduce the propagation life and is apt to be easily induced the fatigue surface tribological failures.

View full abstract

This paper focuses on a solution of the ill-posed inverse problem of the impact-force reconstruction. The solution approximates the impact-force with a regularized quadratic spline having an uniform distribution of knots along the time domain of analysis. The approximation results in a small size, ill-posed, and rank-deficient system equation, which is then solved by the truncated singular value decomposition and L-curve methods. The proposal that is verified by three impact-force problems provides acceptable estimated impact-forces. However, the estimated impact-force in the unloading part rather oscillates in particular when the data are severely contaminated with the noise. Therefore, the present method needs to be further developed to allow a non-uniform knots distribution such that the regularization in the unloading part can be increased.

View full abstract

The authors have performed micro computed tomography (μCT) with synchrotron radiation of SPring-8 to evaluate the fatigue crack propagation behavior inside specimens. A double-spindle type compact rotating bending fatigue testing machine was developed for fatigue testing by the side of a beam line of SPring-8. Preparatory experiments demonstrated that the resulting data by the machine developed were consistent with the data by conventional Ono-type rotating bending fatigue testing machine. In this study, results from μCT technique were compared to the well-established replication technique in order to confirm the applicability of μCT to the study of fatigue crack propagation behavior. Fatigue crack propagation curves and da/dN-ΔK relation obtained by μCT technique corresponded to the data by replication technique on the surface of the specimens, however, it was also shown that the replication technique without information on inside would not always present relevant information on the crack behavior.

View full abstract

The present study is about an automatic diagnostic method for the structural health monitoring. In this study, a new diagnostic method applicable to existing structures from the present moment is proposed. In the proposed method, structural condition is diagnosed without information about damaged condition. The proposed method statistically diagnoses structural condition by means of investigating the change of a response surface. The response surface is calculated as a regression model of relationship between multiple sensors. The shape of the response surface is changed reflecting the change of the structural condition. In this method, the change of the response surface is statistically investigated with the F-test. In the F-test, the threshold of normal or damaged condition is decided with only theoretical F-probability distribution. This theoretical F-distribution is easily calculated using the response surface parameters. Therefore, diagnosis is conducted by means of only intact data used for the reference data. This means the proposed method doesn't require information about the damaged condition. In this study, the health monitoring system of the jet fan was developed to investigate the effectiveness of the proposed method. In this study, field test was conducted using an actual jet fan in a tunnel. In the field test, robustness of the proposed method was investigated. As a result, the structural condition of the jet fan was successfully diagnosed and effectiveness of proposed method was confirmed.

View full abstract

Results of thermal fatigue tests of the magnesium base alloy AZ31 for the case of out-of-phase-loading in a temperature range between -50°C and +290°C are presented. Specimens were loaded under constant total strain and uniaxial homogeneous stress. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. It is well known that rolled AZ31 shows different stress-strain behavior during tensile and compressive loading at lower temperatures due to the fact that mechanical twinning occurs preferentially during the compression loading phase and not during tension. However untwinning processes may occur during the tensile phase of a loading cycle. As a consequence, during the first thermal loading cycles, typical consequences of the formation and the dissolution of twins can be observed. The interaction of deformation, recovery and recrystallization processes, characteristic for individual temperature ranges are discussed in detail to analyze the damage progress during thermal fatigue.

View full abstract

This paper presents dynamic failure mechanism of the spot weld nugget under the shear loading mode. The data collected from an impact experiment are presented. In addition, a finite element model with the bilinear hardening material model is established. The model responses agree well with experiment data for the cases with and without nugget crack. The study suggests that the yield strength of the nugget increases by four times of the base material due to the manufacturing process of the spot weld and due to the high strain rate loading.

View full abstract

In this paper, the long-term effect of noncharacteristic harmonics resulting from a slip energy recovery induction motor drive (SERIMD) on the fatigue life expenditure of turbine-generator shafts is analyzed. A feed-water pump (FP) in power plants is one of the most essential pieces of auxiliary equipment and consumes considerably large quantities of energy. An SERIMD has many advantages and is an adequate candidate for the purpose of variable speed control. However, it gives rise to sustainable variable frequency subharmonics which induce electromechanical subsynchronous oscillations in turbine shafts through proposed deductions. Accordingly, the author has determined that the long-term effect of these subharmonics is a cause of fatigue damage on turbine shafts even under normal operating conditions through fatigue life estimation.

View full abstract

MCrAlY, M means Co and/or Ni, sprayed coating is used to protect a super alloy substrate from corrosion or oxidation in a gas turbine blade. However, the mechanical properties are not well-known, because there are few proper measurement methods for a thin coating at high temperature. Authors have developed the new easy method to measure the mechanical properties using the lateral compression of a circular tube. The method is useful to apply to a thin coating because it does not need chucking and manufacturing a test piece is very easy. The method is also easily applicable to high temperature measurement. In this study, high temperature mechanical properties, Young's modulus, bending strength and fracture strain, of CoNiCrAlY coatings by HVOF were systematically measured. The results obtained were as follows: Young's modulus and bending strength suddenly decreased beyond 400∼450°C. The Young's modulus and bending strength thermally treated at higher than 1050°C was significantly higher than that of virgin CoNiCrAlY coating. It was found that higher thermal treatment in atmosphere was the most effective in increasing the Young's modulus and bending strength. It was also found that the improvement of Young's modulus was primarily caused by not the effect of TGO but the sintering and diffusion of unfused particles. On the contrary, the fracture strain increased beyond 400°C differently from the bending strength. The fracture strains of CoNiCrAlY thermally treated in vacuum were higher than those of CoNiCrAlY treated in atmosphere. It was found that higher thermal treatment in vacuum was the most effective in increasing the fracture strain.

View full abstract

The Preisach model is applied to the hysteresis of the energy required for reorientation of martensitic variants of shape memory alloys. To express tension-compression asymmetric behavior, three variants of the martensitic phase are considered—thermal-induced, tensile-stress-induced, and compressive- stress-induced martensitic phases. Residual stresses due to unconformity at interfaces between grains are also considered in the required energy. Stress-strain hysteresis loops for a bar under tension-compression cyclic loading are simulated and they are compared with the available measured data. Results show that this model can effectively capture the asymmetric stress-strain deformation behavior for tension and compression and the difference between the stress-strain relationships in the first and second loops. This indicates that the proposed model can be used for explaining the deformation behavior of shape memory alloys and for the design of structural elements containing shape memory alloys.

View full abstract