• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.310-313
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• 2009

오스테나이트계 고질소 스테인레스강 4 가지 후보강종인 HNS C1, C2, C3, C4에 대해 질소함량, 가압조건 따른 단조재와 온도 및 동일한 가압조건에서 Ni 합금원소 첨가유무에 따른 용체화 처리재(HNS C1, C3)의 물성평가와 실제 냉연 공정의 pass schedule 검증을 위한 모사실험을 통해 다음과 같이 요약 하였다. 1) 가압증가에 따른 질소함량의 증가로 강도 및 경도가 동시 상승하며, 결정립의 미세화 및 기계적 쌍정 발생이 많았다. 동일한 가압조건에서 Ni 첨가된 HNS C3가 Ni 미첨가한 HNS C1 보다 강도-연성 조합평가에서 열/냉간 가공성 등의 특성이 우수하였다. 2) 고질소강 스테인레스강의 적정한 용체화 처리온도 범위는 상용화된 AIS1304와 유사한 $1050^{\circ}C{\sim}1100^{\circ}C$이며, 용체화처리 후냉각방법의 변경(수냉과 공냉)에 따른 경도의 편차는 없었다. 냉연 pass schedule 검증을 위한 모사실험에서 초기 압연의 최적조건은 roll 속도 5mpm, pass당 압연율 $15{\sim}17%$였다.

• 한국지반공학회지:지반
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• 제10권2호
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• pp.25-40
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• 1994

연약지반상에 편재하중이 작용할 경우에 지반의 소성화에 의한 측방유동과 그에 따른 안정관리에 대한 거동을 규명하기 위하여 기존의 이론적인 배경을 조사하고,모형실험에 의한 실측결과와 서로 비교 분석하여 보았다. 모형실험은 모형재하장치를 제작하고 토조안에 연약지반의 재성형시료를 채워 일정한 시간간 격으로 하중단계를 증가시켜 가면서 변형의 거동상태를 관측하였다. 실험결과 한계하중 및 극한하중은 각각 Jaky와 Meyerhof의 제안값에 접근하는 경향을 가졌 으며,측방유동압은 최대값을 z/H=0.26+1.71cu에서 산정한 깊이에 작용시키고 지표면에 최대 값의 113을 작용시킨 사변형분포에 근접하였으며, 최대값으로는 홍의 제안식이나 Tschebotarioff의 제안식의 유동압계수 ($\alpha=0.4$)를 ($\alpha=K_0$)로 대체한 간편식에서 산정할 수 있다. 또한 [(q/$y_m$)-q] 및 [$S_y-(y_m/S_y)$] 의 관계에서 구한 파괴하중은 극한하중보다 작게 나타 났으며, 특히 [$S_y(y_m/S_y)$] 관리도의 파괴기준선은 다음식에서 산정할 수 있다. $S_y.=3.15exp[-0.58(y_m/S_y)$

• 대한토목학회논문집
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• 제26권2A호
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• pp.363-370
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• 2006

프리스트레스 긴장재에 현재 작용하고 있는 응력을 계측하는 방법으로서 고저항 전도체의 전기기계적 상관작용을 이용하는 방법을 제안했다. 사용 고저항 전도체를 선택하기 위해 탄소섬유와 금속계 열선의 특성을 일반적인 응력제어와는 달리 변형률제어를 통해 실험적으로 연구했다. 탄소섬유의 경우 변형 초기에는 일반적으로 알려진 포물선 형태의 상관관계를 보였으나 재하-제하시 상관관계의 기울기가 일정하지 않아서 본 목적에는 부합하는 않는 것으로 확인되었다. 금속계 열선은 거의 전 구간에서 탄성 재하, 제하 및 재재하시 일정한 선형 상관계수를 보여 본 목적에 매우 적합한 것으로 확인되었다. 금속계열선의 전기기계적 상관관계를 예측하기 위해 완전소성론에 기초한 간단한 식을 제안하였다. 또한 금속계 열선을 이용한 긴장력 측정이 가능한 긴장재를 최초로 제안했다. 본 연구의 부수적인 결과로서 함침되지 않은 탄소섬유의 경우, 특정 변형률 이후 추가 변형에 대해 거의 선형적인 전기기계적 상관관계를 갖는 새로운 경향을 발견했다.

• 소성∙가공
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• 제23권2호
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• pp.82-87
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• 2014

The plastic strain ratio is one of the factors that affect the deep drawability of Al alloy sheet. The deep drawability of Al alloy sheet is limited because of its low plastic strain ratio. Therefore an increase in the plastic strain ratio to improve the deep drawability of Al alloy sheet is needed. The current study investigated the increase of the plastic strain ratio and the change in texture of AA5083 Al alloy sheet after a 2 step asymmetric rolling with heat treatments. The average plastic strain ratio of initial AA5083 Al alloy sheets was 0.83. After the first asymmetric rolling step of 88% deformation and subsequent heat treatment at $320^{\circ}C$ for 10 minutes the value was still 0.83. After the second asymmetric rolling of 14% reduction and subsequent heat treatment at $330^{\circ}C$ for 10 minutes the plastic strain ratio rose to 1.01. The average plastic strain ratio after the 2 step asymmetric rolling and heat treatment is 1.2 times higher than that of initial AA5083 Al alloy sheet. This result is related to the development of ND/<111> texture component after the second asymmetric rolling and heat treatment.

• 대한기계학회논문집A
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• 제21권7호
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• pp.1042-1049
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• 1997

LMFBR(Liquid Metal Fast Breeder Reactor) vessel is operated under the high temperatures of 500-550.deg. C. Thus, transient thermal loads were severe enough to cause inelastic deformation due to creep-fatigue and plasticity. For reduction of such inelastic deformations, Y-piece structure in the form of a thermal sleeve is used in LMFBR vessel under repeated start-up, service and shut-down conditions. Therefore, a systematic method for inelastic analysis is needed for design of the Y-piece structure subjected to such loading conditions. In the present investigation, finite element analysis of heat transfer and inelastic thermal stress were carried out for the Y-piece structure in LMFBR vessel under service conditions. For such analysis, ABAQUS program was employed based on the elasto-plastic and Chaboche viscoplastic constitutive equations. Based on numerical data obtained from the analysis, creep-fatigue damage estimation according to ASME Code Case N-47 was made and compared to each other. Finally, it was found out that the numerical predictio of damage level due to creep based on Chaboche unified viscoplastic constitutive equation was relatively better compared to elasto-plastic constitutive formulation.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 1991년도 가을 학술발표회 논문집
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• pp.133-138
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• 1991

The aim of the present paper is to develop a computer program predicting ultimate fracture strength of initially cracked structure under monotonically increasing external loads. For this purpose, two kinds of 3-D isoparametric solid elements, one 6-node wedge element and another 8-node brick element are formulated along the small deformation theory. Plasticity in the element is checked using von Mises' yield criterion. Elasto-plastic stiffness matrix of the element is calculated taking account of strain hardening effect. If the principal strain at crack tip which is one nodal point exceeds the critical strain dependin on the material property, crack tip is supposed to be opened and the crack tip node which was previously constrained in the direction perpendicular to the crack line is released. After that, the crack lay be propagated to the adjacent node. Once a crack tip node is fractured, the energy of the newly fractured node should be released which is to be absorbed by the remaining part. The accumulated reaction force which was carried by the newly fractured node so far is then applied in the opposite direction. During the action of crack tip relief force, since unloading may be occured in the plastic element, unloading check should be made. If a plastic element unloads, elastic stress-strain equation is used in the calculation of the stiffness matrix of the element, while for a loading element, elasto-plastic stress-strain equation is continuously used. Verification of the computer program is made comparing with the experimental results for center cracked panel subjected to uniform tensile load. Also some factors affecting ultimate fracture strength of initially cracked plate are investigated. It is concluded that the computer program developed here gives an accurate solution and becomes useful tool for predicting ultimate fracture load of initially cracked structural system under monotonically increasing external loads.

• 소성∙가공
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• 제7권2호
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• pp.158-167
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• 1998

Hot restoration mechanism flow stress and stain of the Al2024 composites reinforced with 1,8,15,36, and $44{\mu}m\;SiC_p$(10 vol. %) were studied by hot torsion tests. The hot restoration mechanism of all the composites was found to be dynamic recrystallization(DRX) at $320^{\circ}C$ while that of the composites reinforced with 1 and $8{\mu}m\;SiC_p$ was found to be dynamic recovery(DRX) at $480^{\circ}C$. It was found that the Al2024 composite with $15{\mu}m\;SiC_p$ showed the highest flow stress(${\sim}$223 MPa) at $320^{\circ}C$ under a strain rate of 1.0/sec. Also the highest flow strain of the composites was obtained at $430^{\circ}C$. The com-posites reinforced with 1 and $8{\mu}m\;SiC_p$ showed lower flow stress and higher flow strain at $480^{\circ}C$ than those of the composites reinforced with 15, 36, and $44\;{\mu}m\;SiC_p$ These result were discussed in relation to the transition of the hot restoration mechanism. $DRX{\leftrightarrow}DRV$. The dependence of flow stress on strain rate and temperature was attempted to fit with the hyperbolic sine equation ($\dot{\varepsilon}=A[sinh({\alpha}{\cdot}{\sigma}_p]^n$ exp(-Q/RT)and Zener-Hollomon parameter($Z=\;\dot{\varepsilon}\;exp(Q/RT))$.

• 소성∙가공
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• 제16권7호
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• pp.521-529
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• 2007

This paper is concerned with the analysis on the surface expansion of AA 2024 and AA 1100 aluminum alloys in backward extrusion process. Due to heavy surface expansion appeared usually in the backward can extrusion process, the tribological conditions along the interface between the material and the punch land are very severe. In the present study, the surface expansion is analyzed especially under various process conditions. The main goal of this study is to investigate the influence of degree of reduction in height, geometries of punch nose, friction and hardening characteristics of different aluminum alloys on the material flow and thus on the surface expansion on the working material. Two different materials are selected for investigation as model materials and they are AA 2024 and AA 1100 aluminum alloys. The geometrical parameters employed in analysis include punch corner radius and punch nose angle. The geometry of punch follows basically the recommendation of ICFG and some variations of punch geometry are adopted to obtain quantitative information on the effect of geometrical parameters on material flow. Extensive simulation has been conducted by applying the rigid-plastic finite element method to the backward can extrusion process under different geometrical, material, and interface conditions. The simulation results are summarized in terms of surface expansion at different reduction in height, deformation patterns including pressure distributions along the interface between workpiece and punch, comparison of surface expansion between two model materials, geometrical and interfacial parametric effects on surface expansion, and load-stroke relationships.

• Composites Research
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• 제12권4호
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• pp.42-54
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• 1999

섬유-금속 적층 복합재료(FMLC)는 섬유와 금속 박판으로 구성된 새로운 형태의 구조재로 가볍고 우수한 피로 특성을 가지며 금속과 같이 가소성과 충격저항성이 우수하고, 가공성이 뛰어나다. 본 연구에서는 여러 하중 조건하에 있는 섬유-금속 적층 복합재료를 유전자 알고리듬을 이용하여 최적 설계하였다. 전단변형이론에 근거한 유한요소법을 사용하여 적층판을 해석하였으며, 설계변수로 금속판의 강도와 섬유 층의 수에 따른 적층각도를 두었다. 섬유층과 금속판의 적합도 함수로는 각각 Tasi-Hill failure criterion과 Miser yield criterion을 사용하였다. 유전자 알고리듬의 연산자로는 토너먼트 선택과 균일 교배를 사용하였다. 효율적인 진화를 위해 엘리티스트 모델을 사용하며, 높은 정확도를 가진 해를 얻기 위해 크리프 무작위 탐색(creeping random search) 방법을 통해 더 우수한 자손을 얻었다. 여러 가지 하중 조건에 대하여 최적설계 결과를 나타내었으며, 파괴 지수 측면에서 탄소섬유강화복합재료(CFRP)와 비교하였다. 해석 결과 섬유-금속 적층 복합재료는 탄소섬유강화복합재료에 비하여 집중하중이나 분포하중 형태에 대하여 우수한 특성을 보였으며, 파괴 지수의 편차가 적어 예기치 않은 하중에 잘 견딜 것으로 사료된다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.1740-1743
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• 2007

In this study, two types of fatigue tests were conducted. First, cyclic bending tests were performed using the micro-bending tester. A four-point bending test method was adopted, because it induces uniform stress fields within a loading span. Second, thermal fatigue tests were conducted using a pseudo power cycling machine which was newly developed for a realistic testing condition. The pseudo-power cycling method makes up for the weak points in a power cycling and a chamber cycling method. Two compositions of solder are tested in all test condition, one is lead-free solder (95.5Sn4.0Ag0.5Cu) and the other is eutectic lead-contained solder (63Sn37Pb). In the cyclic bending test, the solder that exhibits a good reliability can be reversed depending on the load conditions. The lead-contained solders have a longer fatigue life in the region where the applied load is high. On the contrary, the lead-free solder sustained more cyclic loads in the small load region. A similar trend was detected at the thermal cycling test. A three-dimensional finite element analysis model was constructed. A finite element analysis using ABAQUS was performed to extract the applied stress and strain in the solder joints. A constitutive model which includes both creep and plasticity was employed. Thermal fatigue was occurred due to the creep. And plastic deformation is main damage for bending failure. From the inelastic energy dissipation per cycle versus fatigue life curve, it can be found that the bending fatigue life is longer than the thermal fatigue life.