• Transactions of Materials Processing
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• v.10 no.5
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• pp.383-388
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• 2001

In describing the plastic deformation behaviour of ultrafine-grained materials, a phase mixture model in which a polycrystalline material is regarded as a mixture of a crystalline phase and a grain boundary phase has been successful. The deformation mechanism for the grain boundary phase, which is necessary for applying the phase mixture model to polycrystalline materials, is modelled as a diffusional flow of matter along the grain boundary. A constitutive equation for the boundary diffusion creep of the boundary phase was proposed, in which the strain rate is proportional to (stress/grain siz $e^{2}$). The upper limit of the stress of the boundary phase was set to equal to the strength to the amorphous phase. The proposed model can explain the strain rate and grain size dependence of the strength of the grain boundary phase. Successful applications of the model compared with published experimental data are described.

• Steel and Composite Structures
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• v.34 no.4
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• pp.581-597
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• 2020

Light-Weight Concrete containing Expanded Poly-Styrene Beads (EPS-LWC) is an approved structural and non-structural material characterized by a considerably lower density and higher structural efficiency, compared to concrete containing ordinary aggregates. The experimental campaign carried out in this project provides new information on the mechanical properties of structural EPS-LWC, with reference to the strength and tension (by splitting and in bending), the modulus of elasticity, the stress-strain curve in unconfined compression, the absorbed energy under compression and reinforcement-concrete bond. The properties measured at seven ages since casting, from 3 days to 91 days, in order to investigate their in-time evolution. Mathematical relationships are formulated as well, between the previous properties and time, since casting. The dependence of the compressive strength on the other mechanical properties of EPS-LWC is also described through an empirical relationship, which is shown to fit satisfactorily the experimental results.

• Journal of The Korean Society of Agricultural Engineers
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• v.48 no.5
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• pp.63-71
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• 2006

In the present study, the dynamic deformation characteristics of clay, including the effect of loading rate in large strain ranges, were examined by performing undrained cyclic triaxial test. The test results showed that the loading rate to failure decreased with increasing loading amplitude and decreasing loading frequency. While the stress-strain relationships was not affected by loading frequency, excess pore pressure was affected significantly with the change in loading frequency. The change for 0.1 Hz was larger for than that of 0.01 Hz, resulting in inclined effective stress paths. Furthermore, the lower the frequency was, the higher the excess pore pressure was in the first loading.

• Proceedings of the KIEE Conference
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• 1999.07e
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• pp.2051-2053
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• 1999

In this work, PD patterns and electrical trees are observed by use of the XLPE cable insulation. and the tree initiation voltage related to the contamination are also measured in order to calculate the electric stress necessary for the design of cable insulation. Throughout these works, correlation between the PD pattern and the shape of electrical tree has been observed that there may exist three distinct pattern of PD dependence on the stage of tree propagation, Such correlation could bring a basic information in connection with the partial discharge of cable system. It was also found that the electrical stress necessary for the initiation of electrical tree was measured to the 290kV/mm for the 154kV XLPE cables manufactured in KOREA.

• Transactions of Materials Processing
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• v.13 no.8
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• pp.696-703
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• 2004

The purpose of this study is to examine the bulk/sheet forming characteristics of bulk amorphous alloys in the super cooled liquid state. Recently it is reported that amorphous alloys exhibit stress overshoot/undershoot and non-Newtonian behaviors even in the super cooled liquid state. The stress-strain curves with the temperature-dependences as well as strain-rate dependence of Newtonian/non-Newtonian viscosities of amorphous alloys are obtained based on the previous experimental works. Then, those curves are directly used in the thermo-mechanical finite element analyses. Upsetting and deep drawing of amorphous alloys are simulated to examine the effects of process parameters such as friction coefficient, forming speed and temperature. It could be concluded that the superior formability of an amorphous alloy can be obtained by taking the proper forming conditions.

• Journal of Korea Foundry Society
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• v.17 no.5
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• pp.502-509
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• 1997

The mechanical behavior and microstructural evolution in the ignition-proof Mg-Zn-Ca-Zr alloy produced by the semisolid squeeze casting are clarified and the mechanical properties are also compared with those of squeeze cast Mg-Zn-Ca-Zr alloy. The tensile strength and elongation increase slightly as the solid fraction depending on temperature decreases, while the 0.2% proof stress decreases. The size of primary crystal increases with increasing holding time. The tensile strength and 0.2% proof stress of the semi-solid squeeze cast Mg-Zn-Ca-Zr alloy decrease as the size of primary crystal increases, indicating the dependence of strength on the size of primary crystal. The elongation of the semi-solid squeeze cast Mg-Zn-Ca-Zr alloy is two times as large as the squeeze cast Mg-Zn-Ca-Zr alloy and the tensile strength is unchanged despite the growth of primary crystal, resulting from the refining of the melted ${\alpha}Mg$ phase and the brittle eutectic compound as well as the reduction of solidification shrinkage and porosities.

• The Journal of Engineering Research
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• v.6 no.2
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• pp.33-38
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• 2004

To investigate the reliability of the MTJs on the roughness of insulating tunnel barrier, we prepared two MTJs with the different uniformity of barrier thickness. Namely, the one has uniform insulating barrier thickness; the other has non-uniform insulating barrier thickness as compared to different thing. As to depositing amorphous layer CoZrNb under the pinning layer IrMn, we achieved MTJ with uniform barrier thickness. Toinvestigate the reliability of the MTJs dependent on the bottom electrode, time-dependent dielectric breakdown (TDDB) measurements were carried out under constant voltage stress. The Weibull fit of out data shows clearly that $t_{BD}$ scales with the thickness uniformity of MTJs tunnel barrier. Assuming a linear dependence of log($t_{BD}$) on stress voltages, we obtained the lifetime of $10^4$years at a operating voltage of 0.4 V at MTJs comprising CoNbZr layers. This study shows that the reliabilityof new MTJs structure was improved due to the ultra smooth barrier, because the surface roughness of the bottom electrode influenced the uniformity of tunnel barrier.

• Transactions of Materials Processing
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• v.13 no.5
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• pp.435-440
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• 2004

The temperature in hot forming of metallic materials, such as hot extrusion and hot forging, ranges from $300^{\circ}C$ to $900^{\circ}C$. Correspondingly, the die also exhibits high temperatures close to that of a work piece and its life is limited generally by high temperature fatigue. Thus, the analysis of high temperature fatigue would need the mechanical properties over the wide ranges of temperature. However, very few studies on the high temperature fatigue of brittle materials have been reported. Especially, the study on the fatigue behavior over such transition temperature regime is very rare. In this paper, the stress-strain curves and stress-life curves of a die steel such as STD61 are experimentally obtained. The wide ranges of temperature from $300^{\circ}C$ to $900^{\circ}C$ are considered in experiments and the transition temperature zone is carefully examined.

• Structural Engineering and Mechanics
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• v.50 no.1
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• pp.89-103
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• 2014

According to experimental studies made so far, design formula of shear characteristics suggested by ISO 22762 and JEAG 4614, representative design code for Lead Rubber Bearing(LRB) shows dependence caused by changes in compressive stress. Especially, in the case of atypical special structure, such as a nuclear power structure, placement of seismic isolation bearing is more limited compared to that of existing structures and design compressive stress is various in sizes. As a result, there is a difference between design factor and real behavior with regards to shear characteristics of base isolation device, depending on compressive stress. In this study, a full-scale low hardness device of LRB, representative base isolation device was manufactured, analyzed, and then evaluated through an experiment on shear characteristics related to various compressive stresses. With design compressive stress of the full-scale LRB (13MPa) being a basis, changes in shear characteristics were analyzed for compressive stress of 5 MPa, 10 MPa, 13 MPa, 15 MPa, and 20 MPa based on characteristics test specified by ISO 22762:2010 and based on the test result, a regression analysis was made to offer an empirical formula. With application of proposed design formula which reflected the existing design formula and empirical formula, trend of horizontal characteristics was analyzed.

• Elastomers and Composites
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• v.39 no.2
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• pp.142-152
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• 2004

Tensile tests of two types of injection-molded polystyrene(PS) samples have been carried out over a wide range of temperature and strain rates in order to characterize their crazing behaviors. Mechanical properties were affected by the formation of crazes as well as test variables. Below the brittle-ductile transition temperature, the tensile stress and the ultimate elongation increased with the molecular weight, strain rate, and with decreasing temperature while the number and average length of crazes also increase. The crazing stress increased with molecular weight, strain rate, and with decreasing temperature. However, the dependence was small compared to the tensile stress. The gap between crazing stress and tensile stress which represents time fur craze formation and growth increased with molecular weight, strain rate, and with decreasing temperature. Crazing was activated near the ${\beta}$-relaxation temperature; crazing stress abruptly decreased at this temperature. During the tensile test, the craze density changed exponentially with the applied stress. At the initial stage, crazes formed slowly. Once a certain number of craze formed, however, the craze density increased rapidly. Craze nucleation and growth occur simultaneously.