• 한국정밀공학회지
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• 제13권12호
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• pp.70-77
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• 1996

Fatigue tests were carried out at high temperature on a Cr-Mo-V steel in order to assess the fatigue life of components used in power plants. The characteristics of high temperature fatigue were divided in terms of cycle-dependent fatigue and time-dependent fatigue, each crack propagation rate was examined with respect to fatigue J-integral range, .DELTA. J$_{f}$and creep J-integral range, .DELTA. J$_{c}$. The fatigue life was evaluated by analysis of J-integral value at the crack tip with a dimensional finite element method. The results obtained from the present study are summarized as follows : The propagation characteristics of high temperature fatigue cracks are determined by .DELTA. J$_{f}$for the PP(tensile plasticity-compressive plasticity deformation) and PC(tensile plasticity - compressive creep deformation) stress waveform types, and by .DELTA. J$_{c}$for the CP(tensile creep- compressive plasticity deformation) stress waveform type. The crack propagation law of high temperature fatigue is obtained by analysis of J-integral value at the crack tip using the finite element method and applied to examine crack propagation behavior. The fatigue life is evaluated using the results of analysis by the finite element method. The predicted life and the actual life are close, within a factor of 2.f 2.f 2.

• 한국표면공학회지
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• 제38권1호
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• pp.14-20
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• 2005

The control of internal stress is extremely important in electroforming because of the deliberately low adhesion between the electro form and the mandrel. Excessive tensile or compressive stress can cause distortion, separation problem, curling, peeling or separation of electroform prematurely from the mandrel, buckling and blistering. Nickel sulfamate bath has been widely used in electroforming because of its low internal stress and moderate hardness. In this study, real-time stress sensor has been used for stress control in chloride-free nickel sulfamate bath for 400 mm x 300 mm x 500 ㎛ nickel electroform. It was found that compressive stress found at low current density indicated the contamination of electrolyte, which is very useful in procuring buckling and peeling of electroform. No compressive stress is allowed for plate electroform. The real-time stress can also be used for accurate stress control of nickel electroform. The tensile stress was found to be increased slightly with increase in nickel electroform thickness, i.e., from initial 1.47 ksi to 2.02 ksi at 320 ㎛.

• Structural Engineering and Mechanics
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• 제52권1호
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• pp.89-95
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• 2014

We analyze the buckling of a thin elastic plate due to intrinsic stresses in thin films attached to the surfaces of the plate. In the case of cylindrical buckling, it is shown that for a plate with clamped edges compressive intrinsic film stresses can cause flexural buckling of the plate, while tensile intrinsic film stresses cannot. For a plate with free edges, film intrinsic stresses, compressive or tensile, cannot cause buckling.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2010년도 제39회 하계학술대회 초록집
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• pp.157-157
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• 2010

Vertical-structure light-emitting diodes (V-LEDs) by laser lift-off (LLO) have been exploited for high-efficiency GaN-based LEDs of solid-state lightings. In V-LEDs, emitted light from active regions is reflected-up from reflective ohmic contacts on p-GaN. Therefore, silver (Ag) is very suitable for reflective contacts due to its high reflectance (>95%) and surface plasmon coupling to visible light emissions. In addition, low contact resistivity has been obtained from Ag-based ohmic contacts annealed in oxygen ambient. However, annealing in oxygen ambient causes Ag to be oxidized and/or agglomerated, leading to degradation in both electrical and optical properties. Therefore, preventing Ag from oxidation and/or agglomeration is a key aspect for high-performance V-LEDs. In this work, we demonstrate the enhanced thermal stability of Ag-based Ohmic contact to p-GaN by reducing the thermal compressive stress. The thermal compressive stress due to the large difference in CTE between GaN ($5.6{\times}10^{-6}/^{\circ}C$) and Ag ($18.9{\times}10^{-6}/^{\circ}C$) accelerate the diffusion of Ag atoms, leading to Ag agglomeration. Therefore, by increasing the additional residual tensile stress in Ag film, the thermal compressive stress could be reduced, resulting in the enhancement of Ag agglomeration resistance. We employ the thin Ni layer in Ag film to form Ni/Ag mutli-layer structure, because the lattice constant of NiO ($4.176\;{\AA}$ is larger than that of Ag ($4.086\;{\AA}$). High-resolution symmetric and asymmetric X-ray diffraction was used to measure the in-plane strain of Ag films. Due to the expansion of lattice constant by oxidation of Ni into NiO layer, Ag layer in Ni/Ag multi-layer structure was tensilely strained after annealing. Based on experimental results, it could be concluded that the reduction of thermal compressive stress by additional tensile stress in Ag film plays a critical role to enhance the thermal stability of Ag-based Ohmic contact to p-GaN.

• 한국건설순환자원학회논문집
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• 제6권4호
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• pp.349-356
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• 2018

본 연구에서는 기준압축강도 120MPa의 강섬유 보강 고강도 콘크리트(FRHSC)의 재료 특성 모델링을 수행하였다. 실험변수로서 강섬유 혼입량을 1.0%, 1.5% 및 2.0%로 설정하였다. 우선, 콘크리트의 압축 거동 특성을 파악하기 위하여 압축강도 실험을 수행하였으며, 압축응력-변형률 곡선을 산정하였다. 보통 강도 콘크리트의 경우, 응력-변형률 관계는 곡선 형태로 나타나지만, 강섬유 보강 고강도 콘크리트의 경우 거의 직선으로 비례하여 증가하는 형상을 나타냈다. 또한, 콘크리트의 인장 특성을 파악하기 위해 균열개구변위(CMOD) 실험을 수행하고 역해석을 통하여 인장응력-CMOD 곡선을 산정하였으며, 이를 토대로 인장응력-변형률 관계 곡선을 모델링하였다. 강섬유 혼입량이 1.0%에서 1.5%로 증가할 때 인장강도는 뚜렷히 증가하였으나, 강섬유 혼입량이 1.5%에서 2.0%로 증가할 때 인징강도는 뚜렷한 차이를 나타내지 않았다. 이러한 실험결과는 강섬유의 분산성과 배열이 향상되지 않아 콘크리트 재료 특성에 영향을 주었기 때문이다.

• Advances in concrete construction
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• 제3권4호
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• pp.269-282
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• 2015

In this paper, a compression to tensile load converter device was developed to determine the anisotropic tensile strength of concrete. The samples were made from a mixture of water, fine sand and cement, respectively. Concrete samples with a hole at its center was prepared and subjected to tensile loading using the compression to tensile load converter device. A hydraulic load cell applied compressive loading to converter device with a constant pressure of 0.02 MPa per second. Compressive loading was converted to tensile stress on the sample because of the overall test design. The samples have three different configurations related to loading axis; 0, $45^{\circ}$, $-45^{\circ}$. A series of finite element analysis were done to analyze the effect of hole diameter on stress concentration of the hole side along its horizontal axis to provide a suitable criterion for determining the real tensile strength of concrete. Concurrent with indirect tensile test, Brazilian test and three point loading test were also performed to compare the results from the three methods. Results obtained by this device were quite encouraging and show that the tensile strengths of concrete were similar in different directions because of the homogeneity of bonding between the concrete materials. Also, the indirect tensile strength was clearly lower than the Brazilian test strength and three point loading test.

• 한국재료학회지
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• 제12권10호
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• pp.820-824
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• 2002

It is of importance to know that the bonding strength and interfacial stress of SOI wafer pairs to meet with mechanical and thermal stresses during process. We fabricated Si/2000$\AA$-SiO$_2$ ∥ 2000$\AA$-SiO$_2$/Si SOI wafer pairs with electric furnace annealing, rapid thermal annealing (RTA), and fast linear annealing (FLA), respectively, by varying the annealing temperatures at a given annealing process. Bonding strength and interfacial stress were measured by a razor blade crack opening method and a laser curvature characterization method, respectively. All the annealing process induced the tensile thermal stresses. Electrical furnace annealing achieved the maximum bonding strength at $1000^{\circ}C$-2 hr anneal, while it produced constant thermal tensile stress by $1000^{\circ}C$. RTA showed very small bonding strength due to premating failure during annealing. FLA showed enough bonding strength at $500^{\circ}C$, however large thermal tensile stress were induced. We confirmed that premated wafer pairs should have appropriate compressive interfacial stress to compensate the thermal tensile stress during a given annealing process.

• 한국정밀공학회지
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• 제7권4호
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• pp.23-28
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• 1990

This study is to investigate the magnitude, direction and distribution of residual stresses in surface ground plate according to working conditions. The specimens were made of structural carbon steel and were machined in various grinding conditions. These were divided in two groups; heat-treated materials and non-heat-treated materials. In each working condition, let the ground specimen generate displacements using deflection-etching techniques. At the same time, these displacements were precisely measured with electronic micrometer. Through the relation formula between the plane stress and strain, which was derived using these measured data, the values of residual stress are calculated, and the results are analyzed. These results are as follows : 1. According to the working conditions in this experiment, it can be seen that the distribution of residual stress generally had same trend and the maximum residual stress remained in 20~30 ((${\mu}m$) beneath the surface. 2. It is observed that compressive residual stress changes into tensile stress in 5~20 (${\mu}m$) beneath the surface. It is suggested that such phenomenon is originated from the friction effect in grinding process. 3. As the hardness increases by the heat treatment, residual stress increases. 4. As the fatigue strength increases by the compressive residual stress, it is desirable that the dowm feed and table feed reduce. 5. It can be seen that the more great the down feed and table feed increase, the more close the changing point, where the stress changed from compressive to tensile, is colse to the surface. This is due to the resultant effects of the grinding temperature and resistence are larger than the effect of the friction.

• 한국건설순환자원학회논문집
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• 제9권3호
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• pp.311-321
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• 2021

콘크리트의 낮은 인장강도와 취성적인 특성 등의 약한 재료 특성을 개선하기 위하여 수년간 하이브리드 섬유보강 콘크리트에 관한 많은 연구가 진행되어 왔다. 그러나 비정질 강섬유와 유기섬유를 이용한 하이브리드 섬유보강 콘크리트의 특성에 관한 연구는 미진한 실정이다. 본 논문의 목적은 비정질 강섬유와 폴리아미드 섬유를 이용한 고성능 하이브리드 섬유보강 콘크리트의 압축 및 인장 거동을 평가한 후 이들에 대한 재료모델을 제안하는 것이다. 이를 위하여 목표 압축강도 40MPa 및 60MPa 각각에 대해서 비정질 강섬유와 폴리아미드 섬유를 총 부피비 1.0%로 설정하여 섬유 조합별로 고성능 하이브리드 섬유보강 콘크리트를 제작한 후, 압축 거동 및 인장 거동을 평가하였다. 고성능 하이브리드 섬유보강 콘크리트의 실험결과를 바탕으로 압축 및 인장거동에 대한 재료모델을 제안하였으며, 제안한 모델은 실험결과와 비교적 잘 일치하는 것으로 나타났다.

• Computers and Concrete
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• 제22권1호
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• pp.11-25
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• 2018

A numerical approach for the evaluation of the flexural response of Steel Fibrous Concrete (SFC) cross-sections with arbitrary geometry, with or without conventional steel longitudinal reinforcing bars is proposed. Resisting bending moment versus curvature curves are calculated using verified non-linear constitutive stress-strain relationships for the SFC under compression and tension which include post-peak and post-cracking softening parts. A new compressive stress-strain model for SFC is employed that has been derived from test data of 125 stress-strain curves and 257 strength values providing the overall compressive behaviour of various SFC mixtures. The proposed sectional analysis is verified using existing experimental data of 42 SFC beams, and it predicts the flexural capacity and the curvature ductility of SFC members reasonably well. The developed approach also provides rational and more accurate compressive and tensile stress-strain curves along with bending moment versus curvature curves with regards to the predictions of relevant existing models.