• 열처리공학회지
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• 제29권5호
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• pp.220-226
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• 2016

The mechanical properties of Ti-6Al-4V can be improved by microstructural control through the heat treatment in ${\alpha}+{\beta}$ region. The heat treatment was carried out with a variety of heat treatment temperatures and holding times to find the optimized heat treatment conditions and it was analyzed by linking the microstructural characteristics and mechanical properties. The part of ${\beta}$ phase with $10{\pm}2wt%$ vanadium was transformed into ${\alpha}^{{\prime}{\prime}}$ martensite phase after quenched, so the hardness and tensile properties were decreased below $900^{\circ}C$. The higher the heat treatment temperature is, the smaller is the vanadium-rich region, which leads to transformation into hcp ${\alpha}^{\prime}$ martensite above $900^{\circ}C$. The hardness and tensile properties were improved due to the hard ${\alpha}^{\prime}$ martensite. As the holding times were longer, the hardness and tensile properties decreased below $900^{\circ}C$ because of the softening effect by the grain growth. When varying the holding times above $900^{\circ}C$, the change of mechanical properties was slight because the softening effect of grain growth and the strengthening effect of ${\alpha}^{\prime}$ phase were counteractive. Therefore, the best conditions of heat treatment, which is in the range of $920{\sim}960^{\circ}C$, 40 min, WQ, can effectively improve the mechanical properties of Ti-6Al-4V.

• Structural Engineering and Mechanics
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• 제84권3호
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• pp.375-391
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• 2022

This paper discussed and analyzed the interfacial stress distribution characteristic of adjacent cracks in Carbon Fiber Reinforced Polymer (CFRP) plate strengthened concrete slabs. One un-strengthened concrete test beam and four CFRP plate-strengthened concrete test beams were designed to carry out four-point flexural tests. The test data shows that the interfacial shear stress between the interface of CFRP plate and concrete can effectively reduce the crack shrinkage of the tensile concrete and reduces the width of crack. The maximum main crack flexural height in pure bending section of the strengthened specimen is smaller than that of the un-strengthened specimen, the CFRP plate improves the rigidity of specimens without brittle failure. The average ultimate bearing capacity of the CFRP-strengthened specimens was increased by 64.3% compared to that without CFRP-strengthen. This indicites that CFRP enhancement measures can effectively improve the ultimate bearing capacity and delay the occurrence of debonding damage. Based on the derivation of mechanical analysis model, the calculation formula of interfacial shear stress between adjacent cracks is proposed. The distributions characteristics of interfacial shear stress between certain crack widths were given. In the intermediate cracking region of pure bending sections, the length of the interfacial softening near the mid-span cracking position gradually increases as the load increases. The CFRP-concrete interface debonding capacity with the larger adjacent crack spacing is lower than that with the smaller adjacent crack spacing. The theoretical calculation results of interfacial bonding shear stress between adjacent cracks have good agreement with the experimental results. The interfacial debonding failure between adjacent cracks in the intermediate cracking region was mainly caused by the root of the main crack. The larger the spacing between adjacent cracks exists, the easier the interfacial debonding failure occurs.

• 열처리공학회지
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• 제3권2호
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• pp.37-44
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• 1990

The effect of the microstructural change on the near threshold fatigue crack growth rate in SM40C steel has been studied using the ${\Delta}K$ decreasing method. Below the total strain amplitude of 0.56%, cyclic softening occured, whereas above this value cyclic hardening occurred in the pearlitic lamellar structure. However, in the spherodized structure the cyclic hardening solely occurred. The crack growth rate in the near-threshold region was decreased with increasing prior austenite grain size and this was due to surface roughness. The crack growth rate of the spherodized structure was lower than that of the pearlite lamellar structure and the ${\Delta}K_{th}$ of the former was higher than that of the latter. It was understood that the crack propagates preferentially through the ferrite phase. The intergranular facets in the near-threshold region appeared in the spherodized structure.

• International Journal of Korean Welding Society
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• 제3권2호
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• pp.46-52
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• 2003

The refinement of microstructure and phase transformation near the interface of pure copper/carbon steel dissimilar metals joints with various friction welding parameters have been studied in this paper. The microstructure of copper and carbon steel joints were changed to be a finer grain compared to those of the base metals due to the frictional heat and plastic deformation. The microstructure of copper side experienced wide range of deformed region from the weld interface and divided into very fine equaxied grains and elongated grains. Especially, the microstructures near the interface on carbon steel were transformed from ferrite and pearlite dual structure to fine ferrite, grain boundary pearlite and martensite due to the welding thermal cycle and rapid cooling rate after welding. These microstructures were varied with each friction welding parameters. The recrystallization on copper side is reason for softening in copper side and martensite transformation could explain the remarkable hardening region in carbon steel side.

• 대한토목학회논문집
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• 제9권3호
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• pp.31-38
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• 1989

본 논문은 점진적 증가하중에 의한 철근 콘크리트 구조물의 전반적인 거동을 고찰하기 위한 것으로써 콘크리트의 인장균열, 철근 및 콘크리트의 응력-변형을 관계의 비선형성을 고려하였다. 콘크리트는 인장영역에서는 선형 탄성체로 가정하였으며 압축영역에서 탄소성체로 가정하였다. 압축영역의 콘크리트 거동을 파악함에 있어 Kupfer가 제안한 파괴표면 식을 항복한계로 사용 하였으며 associated flow rule에 의해 거동한다고 가정하였다. 철근은 일축응력을 받는 선형의 변형경화 재료로 모델링하였다. 콘크리트의 균열 발생시 인접한 균열 사이의 tension stiffening effect를 고려하였으며 콘크리트 구조물의 해석시 나타나는 유한요소의 크기에 따른 수치해석 오차를 콘크리트 인장부분의 변형연화 영역의 기울기를 보정함으로써 감소시키는 에너지 개념에 의한 ${\epsilon}_0$의 결정 모델 제안하였다.

• E2M - 전기 전자와 첨단 소재
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• 제10권3호
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• pp.210-216
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• 1997

The th ermal and optical properties of multicomponent oxide glass optical fiber by adding heavy metal oxide Ga$_{2}$O$_{3}$(0-20wt%) were investigated. The fiber samples were made by the method of rod in tube. The optical loss of fiber was measured in 0.3-1.8.mu.m wavelength region. As Ga$_{2}$O$_{3}$ increased up to 20wt%, the transition and softening temperature of bulk glass were increased from 495.deg. C to 579.deg. C and from 548.deg. C to 641.deg. C, respectively. Whereas the thermal expansion coefficient was decreased from 102 to 79.1x10$^{-7}$ /.deg. C. The refractive index was increased from 1.621 to 1.665, and IR cut-off wavelength was enlarged from 4.64.mu.m to 6.1.mu.m. The optical loss of fiber was remarkably decreased in 1.146.mu.m-1.8.mu.m wavelength region.

• 한국정밀공학회지
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• 제21권10호
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• pp.162-169
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• 2004

High temperature uniaxial compression tests in the alpha single phase region were carried out on the Ti -43mo1%Al intermetallic compound, in order to obtain oriented lamellar microstructure. The compression deformation temperatures and strain rates are from 1573k to 1623k and 1.0x10$^{-4}$ s to 5.0x10$^{-3}$ s, respectively. Fully lamellar microstructure was observed after the uniaxial compression deformation in a single phase region followed by cooling to room temperature. Lamellar colony diameter depended on strain rates and test temperatures. The diameter varied between 8601m and 300fm. Stress-strain curve showed a work softening and the size of lamellar colony diameter varied depending on peak stresses. This shows the occurrence of dynamic recrystallization. Texture measurements after the uniaxial compression deformation, showed the development of fiber during dynamic recrystallization. It is seen that the area for the maximum pole density existed in 35 degrees away from the compression plane. The texture sharpens with a decrease in strain rate

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 1996년도 추계학술대회 논문집
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• pp.128-134
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• 1996

In this study, the thermal and optical properties of multicomponent glass optical fiber by adding heavy metal oxide Ga$_2$O$_3$were investigated. The fiber samples were made by rod in tube method. The optical loss of fiber was measured in 0.3~1.8${\mu}{\textrm}{m}$ wavelength region. As Ga$_2$O$_3$increased up to 20wt%, the transition and softening temperature of bulk glass were increased from 495$^{\circ}C$ to 579$^{\circ}C$ and from 548$^{\circ}C$ to 641$^{\circ}C$respectively. Whereas the thermal expansion coefficient was decreased from 102 to 79.1$\times$10$^{-7}$ $^{\circ}C$. The refractive index was increased from 1.621 to 1.665, and IR cut-off wavelength was enlarged from 4.64${\mu}{\textrm}{m}$ to 6.1${\mu}{\textrm}{m}$. The optical loss of fiber was decreased and more remarkably decreased in 1.146${\mu}{\textrm}{m}$~1.8${\mu}{\textrm}{m}$ wavelength region.

• E2M - 전기 전자와 첨단 소재
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• 제9권10호
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• pp.1047-1052
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• 1996

In this study, the thermal and optical proper-ties of multicomponent oxide glass fiber for IR sensor by adding heavy metal oxide Ga$_{2}$O$_{3}$ were investigated. The fiber samples were made by rod-in tube method. The optical loss of fiber was measured in 0.3-1.8/M wavelength region. As Ga$_{2}$O$_{3}$ increased up to 12wt%, the transition and softening temperature of bulk glass were increased from 495.deg. C to 564.deg. C and from 548.deg. C to 612.deg. C respectively. Whereas the thermal expansion coefficient was decreased from 102 to 88.2*10$^{-7}$ /.deg. C. The refractive index was increased from 1.621 to 1.662, and IR cut-off wavelength was enlarged from 4.64.mu.m to 5.22.mu.m. The optical loss of fiber was decreased and more remarkably decreased in 1.146.mu.m-1.8.mu.m wavelength region.

• 콘크리트학회지
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• 제8권1호
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• pp.145-153
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• 1996

콘크리트의 파괴진행영역은 콘크리트의 균열선단의 브리징영역과 미세균열영역으로 구성되는 비선형영역으로서 콘크리트의 파기거동을 지배한다. 파괴진행영역을 고려한 파괴역학은 콘크리트에 유용하게 적용될 수 있으며 파괴진행영역 모델의 개발은 콘크리트의 파괴현상을 규명하는데 매우 중요하다. 본 논문에서는 콘크리트의 균열진행을 해석하기 위하여 선형 인장 연화곡선을 사용한 Dugdale-Barenblatt형 모델로 콘크리트의 브리징영역을 모델링하였고 이를 이산균열방법을 사용하여 단지 요소경계면에 파괴진행영역을 발생시켜 유한요소 해석하는 방법과 요소내의 불연속 균열면을 도입한 균열요소를 사용함으로써 이산균열방법의 결점을 보완한 해석방법을 제시하였다. 또한 해석 예를 통해 균열진행해석에 사용된 유한요소모델을 검증하였다.