• Journal of Advanced Marine Engineering and Technology
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• 제29권8호
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• pp.938-945
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• 2005

The microstructure evolution in hot forging process is composed of dynamic recrystallization during deformation as well as grain growth during dwell time. Therefore, the control of forging parameters such as strain, strain rate. temperature and holding time is important because the microstructure change in hot working affects the mechanical properties. Modeling equations are developed to represent the flow curve. grain size. recrystallized volume fraction and grain growth phenomena by various tests. The developed modeling equations were combined with thermo-viscoplastic finite element modeling to predict the microstructure change evolution during hot forging process. The large exhaust valve spindle (head diameter of 512mm) was simulated by closed die forging with hydraulic press and cooled in air after forging. The preform was heated to each 1080 and 1150$^{\circ}C$. Numerical calculation was performed by DEFORM-2D. a commercial finite element code. Heat transfer can be coupled with the deformation analysis in a non-isothermal deformation analysis. In order to obtain the fine and homogeneous microstructure and good mechanical properties in forging. the FEM would become a useful tool in the simulation of the microstructure development. In forging, appropriate temperature, strain and strain rate and rapid cooling are required to obtain the fine grain microstructure The optimal forging temperature and effective strain range of Nimonic 80A for large exhaust valve spindle are about 1080$\∼$l120$^{\circ}C$ and 150$\∼$200$\%$.

• 한국지반공학회논문집
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• 제32권1호
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• pp.55-62
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• 2016

본 연구에서는 공극 네트워크 모델을 이용하여 사질토의 함수특성곡선을 수치해석적으로 획득하였다. 주문진표준사의 시편을 고해상도 3차원 X-ray CT 촬영하여 공극 영상을 획득하였고, 이를 공극방과 공극목으로 이루어진 관망으로 재구성하였으며 이 때 관의 반경은 공극목의 최소반경으로 정의하였다. 모세관압에 영향을 미치는 공극목의 반경은 세선화알고리즘과 유클리디언 거리변환을 통해 계산하였다. 수치해석적으로 얻은 함수특성곡선을 실험결과와 비교하였으며, 수치해석 결과는 실험결과에 비해 공기함입치가 과대평가 되었으나 전체 모세관압은 유사한 분포를 나타냈다. 또한 실험결과로부터 도출된 상대투수율은 높은 포화도에서 수치해석 결과에 비해 큰 값을 보였다.

• Journal of Astronomy and Space Sciences
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• 제26권3호
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• pp.295-304
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• 2009

대마젤란은하 중심부에 위치한 식쌍성 EROS 1010에 대한 CCD 측광관측을 2006년과 2007년에 수행하여 BV 광도곡선을 완성하였다. BV 광도곡선은 EROS팀과 OGLE팀이 각각 관측한 BR과 I 광도곡선과 함께 Wilson & Devinney 차등보정법으로 분석하여 측광학적인 해를 제시하였다. 한편 대마젤란은하의 거리지수를 $m-M=18.^m3{\sim}18.^m5$ 사이로 가정하고, EROS 1010에 대한 절대량을 구하였다.

• Interaction and multiscale mechanics
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• 제2권4호
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• pp.375-396
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• 2009

In the present work, the elasto-viscoplastic behavior, interactions between grains, and the texture evolution in polycrystalline materials subjected to finite deformations are modeled using a multiscale analysis procedure within a finite element framework. Computational homogenization is used to relate the grain (meso) scale to the macroscale. Specifically, a polycrystal is modeled by a material representative volume element (RVE) consisting of an aggregate of grains, and a periodic distribution of such unit cells is considered to describe material behavior locally on the macroscale. The elastic behavior is defined by a hyperelastic potential, and the viscoplastic response is modeled by a simple power law complemented by a work hardening equation. The finite element framework is based on a Lagrangian formulation, where a kinematic split of the deformation gradient into volume preserving and volumetric parts together with a three-field form of the Hu-Washizu variational principle is adopted to create a stable finite element method. Examples involving simple deformations of an aluminum alloy are modeled to predict inhomogeneous fields on the grain scale, and the macroscopic effective stress-strain curve and texture evolution are compared to those obtained using both upper and lower bound models.

• International Journal of Precision Engineering and Manufacturing
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• 제9권4호
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• pp.57-62
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• 2008

The aim of this investigation was to extract the fatigue properties at the designated fatigue life of copper foil and observe the mean stress and stress amplitude effects on both the fatigue life and the corresponding surface morphology. Tensile tests were performed to determine the baseline monotonic material properties of the proportional limit and ultimate tensile strength. Constant amplitude fatigue tests were carried out using a feedback-controlled fatigue testing machine. The mean stress and the stress amplitude were changed to obtain the complete nominal stress-life curves. An atomic force microscope was utilized to observe the relationship between the fatigue damage and the corresponding changes in surface morphology. A Basquin's exponent of-0.071 was obtained through the fatigue tests. An endurance limit of 122 MPa was inferred from a Haigh diagram. The specimen surface became rougher as the number of fatigue cycles increased, and there was a close relationship between the fatigue damage and the surface roughness evolution.

• Earthquakes and Structures
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• 제14권6호
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• pp.589-598
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• 2018

An attempt is conducted to explore the relationship between the macroscopic global damage and the local damage of shear-type RC frames. A story damage index, which can be expressed as multi-variate functions of modal parameters, is deduced based on the tridiagonal matrix of the shear-type frame. The global damage model is also originated from structural modal parameters. Due to the connection of modal damage indexes, the relationship between the macroscopic global damage and the local story damage is reasonably established. In order to validate the derivation, a case study is carried out via an 8-story shear-type frame. The sensitivities of modal damage indexes to the location and severity of local story damages are studied. The evolution of the global damage is investigated as well. Results show that the global damage is sensitive to the degree of story damage, but it's not sensitive to its location. As the number of the damaged stories increases, more and more modes will be involved. Meanwhile, the global damage evolution curve changes from the concave shape to the S-type and then finally transforms into the convex shape. Through the proposed story damage, modal damage and global damage model, a multi-level damage assessment method is established.

• 국제강구조저널
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• 제18권4호
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• pp.1177-1190
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• 2018

This study presents an analytical framework for estimating the thermo-mechanical behavior of a composite beam exposed to fire. The framework involves: a fire simulation from which the evolution of temperature on the structure surface is obtained; data transfer by an interface model, whereby the surface temperature is assigned to the finite element model of the structure for thermo-mechanical analysis; and nonlinear thermo-mechanical analysis for predicting the structural response under high temperatures. We use a plastic-damage model for calculating the response of concrete slabs, and propose a method to determine the stiffness degradation parameter of the plastic-damage model by a nonlinear regression of concrete cylinder test data. To validate simulation results, structural fire experiments have been performed on a real-scale steel-concrete composite beam using the fire load prescribed by ASTM E119 standard fire curve. The calculated evolution of deflection at the center of the beam shows good agreement with experimental results. The local test results as well as the effective plastic strain distribution and section rotation of the composite beam at elevated temperatures are also investigated.

• 천문학논총
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• 제32권1호
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• pp.257-261
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• 2017

We built a $8{\mu}m$ selected sample of galaxies in the NEP-AKARI field by defining 4 redshift bins with the four AKARI bands at 11, 15, 18 and 24 microns (0.15 < z < 0.49, 0.75 < z < 1.34, 1.34 < z < 1.7 and 1.7 < z < 2.05). Our sample contains 4079 sources, 599 are securely detected with Herschel/PACS. Also adding ultraviolet (UV) data from GALEX, we fit the spectral energy distributions using the physically motivated code CIGALE to extract the star formation rate, stellar mass, dust attenuation and the AGN contribution to the total infrared luminosity ($L_{IR}$). We discuss the impact of the adopted attenuation curve and that of the wavelength coverage to estimate these physical parameters. We focus on galaxies with a luminosity close the characteristic $L^*_{IR}$ in the different redshift bins to study the evolution with redshift of the dust attenuation in these galaxies.

• 자원환경지질
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• 제28권1호
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• pp.19-24
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• 1995

경상분지의 동부와 남부에 분포하는 연 아연광상에서 산출되는 방연석의 납 동위원소 비로부터 광상을 형성한 납이 어떤 기원물질로부터 유래하였는가를 연구하고, 이를 태백산 광화대 내 일부광상들의 납 동위원소 비와 비교 연구하였다. 경상분지내 연 아연광상들의 보통납(common lead) 의 동위원소 비는 $^{206}Pb/^{204}Pb=18.156{\sim}18.377$, $^{207}Pb/^{204}Pb=15.482{\sim}15.638$, $^{208}Pb/^{204}Pb=37.953{\sim}38.605$로서 매우 제한된 영역의 변화를 보인다. 또한 납 동위원소 비들은 광상 납 성장곡선(Cumming과 Richards, 1975)과 평균지각 납 진화곡선(Stacey 와 Kramer, 1975) 의 선상 혹은 하부에 점시 되어 맨틀성분의 개입이 많음을 지시하는 반면, 태백산 광화대의 경우는 진화곡선 상부에 점시 되어 지각 물질의 개입이 많음을 나타내었다. Plumbotectonics Model IV (Zartman과 Haines, 1988)에서는 경상분지의 납들은 대부분 맨틀과 조산대 사이의 영역에, 태백산 광화대의 자료들은 대부분 상부지각선의 선 상이나 그 상부에 점시 된다. 위와같은 납 동위원소 조성이 보여주는 특징과 더불어 경상분지 보통납의 동위원소 비들이 나타내는 선형관계는 낮은 U/Pb와 Th/U의 비를 갖는 기원물질 혹은 결핍맨틀(depleted mantle) 물질과 기반암과 같은 지각 물질간의 혼염에 의한 혼합 아이소크론(mixing isochron)으로 추정된다.

• 한국해양공학회지
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• 제25권2호
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• pp.92-100
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• 2011

The main goal of this paper is to provide the theoretical background for the fracture phenomena in marine structural steels. In this paper, various fracture criteria are theoretically investigated: shear failure criteria with constant failure strain and stress triaxiality-dependent failure strain (piecewise failure and Johnson-Cook criteria), forming limit curve failure criterion, micromechanical porosity failure criterion, and continuum damage mechanics failure criterion. It is obvious that stress triaxiality is a very important index to determine the failure phenomenon for ductile materials. Assuming a piecewise failure strain curve as a function of stress triaxiality, the numerical results coincide well with the test results for smooth and notched specimens, where low and high stress triaxialities are observed. Therefore, it is proved that a failure criterion with reliable material constants presents a plastic deformation process, as well as fracture initiation and evolution.