• Journal of the Korean Society for Heat Treatment
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• v.16 no.2
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• pp.90-96
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• 2003

The effect of betatizing temperature on microstructure and transformation characteristics in a Cu-AI-Ni based pseudoelastic alloy fabricated by heated mold continuous casting by using metallography, XRD and calorimetry. The microstructure of cast rod betatized at $600^{\circ}C$ revealed a ${\beta}_1$ parent phase and a ${\gamma}_2$ phase precipitated along the casting direction. When the cast rod was betatized at the elevated temperature above $600^{\circ}C$, the ${\gamma}_2$ phase is completely dissolved into the matrix so that the volume fraction of the ${\gamma}_2$ phase was decreased. The parent phase was stabilized by betatizing at $600^{\circ}C$. However, the ${\beta}_1$ parent phase was transformed to both ${{\beta}_1}^{\prime}$ and ${{\gamma}_1}^{\prime}$ martensites with increasing betatizing temperatures above $600^{\circ}C$, while $M_s$ and $A_s$ temperatures were decreased. The stress-strain curves for compression test were not same with betatizing temperature; the stress-strain curves of the specimen betatized at $600^{\circ}C$ and $700^{\circ}C$ were linear but those of the specimen betatized at $800^{\circ}C$ and $900^{\circ}C$ were not linear.

• Journal of the Korean Geosynthetics Society
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• v.12 no.1
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• pp.93-100
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• 2013

Frost heave occurs when ground temperature decreases under $0^{\circ}C$ and soil volume expands, which causes roadway and buried pipe line failure due to differential heaving. There are several models to predict volumetric strain caused by frost heave, but they requires expertises who have professional background and experience related to numerical analysis. This study presents an evaluation method to predict volumetric strain caused by frost heave with fundamental physical properties of soils. The evaluation method is assessed with experimental results.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.10a
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• pp.190-194
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• 2007

The forging process design and microstructure evolution for gas turbine disk of a Waspaloy is investigated in this study. Parameters related to deformation are die and preform geometry, and forging temperature of die and workpiece. Die and preform design are considered to reduce the forging load, and to avoid the forging defects. Blocker and finisher dies for multistage forging are designed and the initial billet geometry is determined. The control of hot forging parameters such as strain, strain rate and temperature also is important because the microstructure change in hot working affects the mechanical properties. The dynamic recrystallization evolution has been studied in the temperature range 900-$1200^{\circ}C$ and strain rate range 0.01-1.0s-1 using hot compression tests. Modeling equations are required represent the flow curve, recrystallized grain size, recrystallized volume fraction by various tests. In this study, we used to thermo-viscoplastic finite element modeling equation of DEFORM-2D to predict the microstructure change evolution during thermo-mechanical processing. The microstructure is updated during the entire thermal and deformation processes in forging.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.06a
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• pp.15-32
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• 1999

A relationship between stress and stain is very important to design a die to avoid defects of products during semi-solid forming process. Since the liquid will be of eutectic composition in alloys liquid segregation will result in significant or undesirable situation. The materials used in this experiment are A 357. A390, Al2024 alloys that is fabricated by the electro-magnetic stirring process from Pechiney in France. The compression test was performed by induction heating equipment and MTS. In order to prevent the liquid segregation these measured temperature would be useful to control of strain rate during compression test. The liquid segregation is controlled as change of the strain rate and solid fraction during the compression process, The characteristics of flow between solid and liquid phase considering liquid segregation is examined through the above experiments. In the case of medium and high volume fractions of solid the distribution of strain rate is calculated by using compression test data of semi-solid materials (SSM). The thixoforming experiments with the designed die are carried out successfully. The die filling patterns of SSM for variation of die temperature and pressing force have been investigated. The hardness of the thixoformed scroll products is evaluated in terms of the microstructure for each position.

• Structural Engineering and Mechanics
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• v.76 no.4
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• pp.551-559
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• 2020

On the basis of nonlocal strain gradient theory, considering the material properties of porous FGM changing with thickness and the influence of moment of inertia, the wave equation of FG nano circular plate is derived by using the first-order shear deformation plate theory, by introducing dimensionless parameters, we transform the equations into dimensionless wave equations, and the dispersion relations of bending wave, shear wave and longitudinal wave are obtained by Laplace and Hankel integral transformation method. The influence of nonlocal parameter, porosity volume fraction, strain gradient parameters and power law index on the propagation characteristics of bending wave, shear wave and longitudinal wave in FG nano circular plate.

• Steel and Composite Structures
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• v.33 no.3
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• pp.463-472
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• 2019

Initial thermo-elastic and steady state creep deformation of a rotating functionally graded simple blade is studied using first-order shear deformation theory. A variable thickness model for cantilever beam has been considered. The blade geometry and loading are defined as functions of length so that one can define his own blade profile and loading using any arbitrary function. The blade is subjected to a transverse distributed load, an inertia body force due to rotation and a distributed temperature field due to a thermal gradient between the tip and the root. All mechanical and thermal properties except Poisson's ratio are assumed to be longitudinally variable based on the volume fraction of reinforcement. The creep behaviour is modelled by Norton's law. Considering creep strains in stress strain relation, Prandtl-Reuss relations, Norton' law and effective stress relation differential equation in term of effective creep strain is established. This differential equation is solved numerically. By effective creep strain, steady state stresses and deflections are obtained. It is concluded that reinforcement particle size and form of distribution of reinforcement has significant effect on the steady state creep behavior of the blade.

• Journal of the Society of Naval Architects of Korea
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• v.46 no.6
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• pp.611-621
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• 2009

This paper first reviews the physical meanings and the expressions of two representative strain rate models: CSM (Cowper-Symonds Model) and JCM (Johnson-Cook Model). Since it is known that the CSM and the JCM are suitable for low-intermediate and intermediate-high rate ranges, many studies regarding marine accidents such as ship collision/grounding and explosion in FPSO have employed the CSM. A formula to predict the material constant of the CSM is introduced from literature survey. Numerical simulations with two different material constitutive equations, classical metal plasticity model based on von Mises yield function and micromechanical porous plasticity model based on Gurson yield function, have been carried out for the stiffened plates under impact loading. Simulation results coincide with experimental results better when using the porous plasticity model.

• Korean Journal of Metals and Materials
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• v.49 no.1
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• pp.1-8
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• 2011

To measure the sensitivity of the hydrogen embrittlement from plated high strength bolts (SWCH18A, F11T), the bolt was stressed by a corresponding to the amount of tensile strain of 5% and 20%, and then the bolt developed a crack on the surface. The bolt that didn't have concentrated hydrogen, even though it was stressed by a corresponding the amount of tensile strain of 5% and 20%, no crack developed. However, the bolt that had concentrated hydrogen, developed cracks from the thread crest to the root. It is impossible to measure the hydrogen amount from plated high strength bolts using Hydrogen-Determinator, because of the limitation of the minimum sample size (about 1g as a mass or $5{\times}5{\times}5 mm^3$ as a volume). Therefore, the sensitivity of hydrogen embrittlement can be measured by observing the presence of cracks on the surface of plated high strength bolts which are stressed by a corresponding to the amount of tensile strain of 5% and 20%.

• Structural Engineering and Mechanics
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• v.77 no.3
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• pp.369-381
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• 2021

The seismic responses of elevated tanks considering liquid-structure interaction are presented under horizontal earthquake. The scaled model tank is fabricated to study the dynamic responses of anchored tank and newly designed uplift tank with replaced dampers. The natural frequencies for structural mode are obtained by modal analysis. The dynamic responses of tanks are completed by finite element method, which are compared with the results from experiment. The displacement parallel and perpendicular to the excitation direction are both gained as well as structural acceleration. The strain of tank walls and the axial strain of columns are also obtained afterwards. The seismic responses of liquid storage tank can be calculated by the finite element model effectively and the results match well with the one measured by experiment. The aim is to provide a new type of tank system with vertical constraint relaxed which leads to lower stress level. With the liquid volume increasing, the structural fundamental frequency has a great reduction and the one of uplift tank are even smaller. Compared with anchored tank, the displacement of uplift tank is magnified, the strain for tank walls and columns parallel to excitation direction reduces obviously, while the one perpendicular to earthquake direction increases a lot, but the values are still small. The stress level of new tank seems to be more even due to uplift effect. The new type of tank can realize recoverable function by replacing dampers after earthquake.

• Advances in concrete construction
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• v.12 no.6
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• pp.451-459
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• 2021

The mechanical behaviors and chloride resistance performance of fiber reinforced cementitious composites (FRCC) with hybrid polyethylene (PE) and steel fiber (in total 2% by volume) were investigated. Based on micro-mechanics and fracture mechanics, the reason why the tensile strain capacity of FRCC changed obviously was obtained. Besides, the effects of the total surface area of fiber in FRCC on compressive strength and chloride content were clarified. It is found that the improvement of the tensile strain capacity of FRCC with hybrid fiber is attributed to the growth of strain-hardening performance index (the ratio of complementary energy to crack tip toughness). As the total surface area of fiber related with the interfacial transition zone (ITZ) between fiber and matrix increases, compressive strength decreases obviously. Since the total surface area of fiber is small, the chloride resistance performance of FRCC with hybrid PE and steel fiber is better than that of FRCC containing only PE fiber.