• 한국가스학회지
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• 제25권5호
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• pp.30-36
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• 2021

정유 및 석유화학산업의 개질로 튜브에 사용되는 mod HP강에 대한 잔여수명의 예측법을 튜브의 표면 금속조직에 석출된 탄화물의 면적분율에 대한 정량적인 평가를 통해 연구하였다. 표면복제법을 활용한 미세조직 분석을 통하여 탄화물의 면적분율과 잔여수명 사이의 관계를 실험적으로 구하였다. 신재(Virgin)와 950 ℃의 개질로에서 1.8, 6.0, 7.2, 8.5, 9.7 및 16.2년 실제 사용한 튜브들을 광학현미경(OM)을 사용하여 관찰하였으며, 석출물의 면적분율을 Image J 프로그램을 이용한 정량적 이미지 분석에 의해 측정하였다. 석출물의 면적분율이 증가함에 따라 잔여수명이 감소하여 서로 반비례하는 결과를 보여주었다. 이 관계를 사용온도와 수명을 결합한 파라미터인 LMP로 나타내면, 신재에서 LMP 값이 가장 높게 나타났고, 사용시간이 증가할수록 LMP가 감소하여 잔여수명이 줄어 듦을 나타내었다. LMP와 탄화물의 면적분율과의 상관관계를 나타내는 관계식을 도출하였다. 도출한 관계식은 현장에서 잔여수명 평가를 위해 표면복제법 적용으로 면적분율을 측정하여 잔여수명을 평가하는데 사용될 수 있다.

• Structural Engineering and Mechanics
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• 제20권6호
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• pp.673-693
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• 2005

A general step-by-step simulation for the time-dependent analysis of segmentally-erected prestressed concrete box-girder bridges is presented. A three dimensional finite-element model for the balanced-cantilever construction of segmental bridges, including effects of the load history, material nonlinearity, creep, shrinkage, and aging of concrete and the relaxation of prestressing steel was developed using ABAQUS software. The models included three-dimensional shell elements to model the box-girder walls and Rebar elements representing the prestressing tendons. The step-by-step procedure allows simulating the construction stages, effects of time-dependent deformations of materials and changes in the structural system of the bridges. The structural responses during construction and throughout the service life were traced. A comparison of the developed computer simulation with available experimental results was conducted and good agreement was found. Deflection of the bridge deck, changes in stresses and strains and the redistribution of internal forces were calculated for different examples of bridges, built by the balanced-cantilever method, over thirty-year duration. Significant time-dependent effects on the bridge deflections and redistribution of internal forces and stresses were observed. The ultimate load carrying capacities of the bridges and the behavior before collapse were also determined. It was observed that the ultimate load carrying capacity of such bridges decreases with time as a result of time-dependent effects.

• 대한기계학회논문집A
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• 제22권3호
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• pp.595-602
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• 1998

Specimens of 304 stainless steel were tested to failure at elevated temperatures under multiaxial stress states, uniaxial tension using smooth bar specimens, biaxial shearing using double shear bar specimens, and triaxial tension using notched bar specimens. Rupture times are compared for uniaxial, biaxial, and triaxial stress states with respect to the maximum principal stress, the von Mises effective stress, and the principal facet stress. The results indicate that the principal facet stress gives the best correlation for the material investigated, and this parameter can predict creep life data under multiaxial stress states with rupture data obtained with specimens under uniaxial stresses. The results also suggest that grain boundary cavitation, coupled with localized deformation processes such as grain boudary sliding, controls the lifetimes of the specimens.

• 대한기계학회논문집A
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• 제33권10호
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• pp.1177-1186
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• 2009

High temperature steam pipes of thermal power plant are subject to a severe thermal range and usually operates well into the creep range. Cyclic operation of the plant subjects the piping system to mechanical and thermal fatigue damages. Also, poor or malfunctional supports can impose massive loads or stress onto the piping system. In order to prevent the serious damage and failure of the critical piping system, various inspection methods such as visual inspection, computational analysis and on-line piping displacement monitoring were developed. 3-dimensional piping displacement monitoring system was developed with using the aluminum alloy rod and rotary encoder sensors, this system was installed and operated on the high temperature steam piping of "Y" thermal power plant successfully. It is expected that this study will contribute to the safety of piping system, which could minimize stress and extend the actual life of critical piping.

• Structural Engineering and Mechanics
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• 제46권1호
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• pp.53-73
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• 2013

Prediction of prestressed concrete girder integral abutment bridge (IAB) load effect requires understanding of the inherent uncertainties as it relates to thermal loading, time-dependent effects, bridge material properties and soil properties. In addition, complex inelastic and hysteretic behavior must be considered over an extended, 75-year bridge life. The present study establishes IAB displacement and internal force statistics based on available material property and soil property statistical models and Monte Carlo simulations. Numerical models within the simulation were developed to evaluate the 75-year bridge displacements and internal forces based on 2D numerical models that were calibrated against four field monitored IABs. The considered input uncertainties include both resistance and load variables. Material variables are: (1) concrete elastic modulus; (2) backfill stiffness; and (3) lateral pile soil stiffness. Thermal, time dependent, and soil loading variables are: (1) superstructure temperature fluctuation; (2) superstructure concrete thermal expansion coefficient; (3) superstructure temperature gradient; (4) concrete creep and shrinkage; (5) bridge construction timeline; and (6) backfill pressure on backwall and abutment. IAB displacement and internal force statistics were established for: (1) bridge axial force; (2) bridge bending moment; (3) pile lateral force; (4) pile moment; (5) pile head/abutment displacement; (6) compressive stress at the top fiber at the mid-span of the exterior span; and (7) tensile stress at the bottom fiber at the mid-span of the exterior span. These established IAB displacement and internal force statistics provide a basis for future reliability-based design criteria development.

• 플랜트 저널
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• 제10권1호
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• pp.40-46
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• 2014

500 MW급 석탄화력발전소인 당진화력 4호기 텐덤형 증기 터빈 로터의 고온부위 재질열화를 평가하기 위하여 고중압 터빈 로터의 고온증기가 통과하는 부위인 로터의 고압 터빈 입구 측과 중압 터빈 입구에 대해서 표면복제 관찰을 통해서 금속조직의 크리프 기공 성장 및 미세조직 변화를 분석하였다. 연구결과, 74,628시간 사용된 당진화력 4호기 고중압 터빈 로터의 미세조직 열화정도는 핀란드 Technical Research Centre 미세조직열화등급 기준으로 B등급이다. 그리고 Neubauer 크리프 기공 평가를 통해서 잔여수명은 201,523시간 이상으로 판단된다.

• Advances in materials Research
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• 제8권1호
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• pp.11-24
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• 2019

This paper presents a review study for energy-efficient gas turbines (GTs) with cycles which contributes significantly towards sustainable usage. Nonetheless, these progressive engines, operative at turbine inlet temperatures as high as $1600^{\circ}C$, require the employment of highly creep resistant materials for use in hotter section components of gas turbines like combustion chamber and blades. However, the gas turbine obtain its driving power by utilizing the energy of treated gases and air which is at piercing temperature and pushing by expanding through the several rings of steady and vibratory blades. Since the turbine blades works at very high temperature and pressure, high stress concentration are observed on the blades. With the increasing demand of service, to provide adequate efficiency and power within the optimized level, turbine blades are to be made of those materials which can withstand high thermal and working load condition for longer cycle time. This paper depicts the recent developments in the field of implementing the best suited materials for the GTs, selection of proper Thermal Barrier Coating (TBC), fracture analysis and experiments on failed or used turbine blades and several other designing and operating factors which are effecting the blade life and efficiency. It is revealed that Nickel based Superalloys were promising, Cast Iron with Zirconium and Pt-Al coatings are used as best TBC material, material defects are the foremost and prominent reason for blade failure.

• Nuclear Engineering and Technology
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• 제51권6호
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• pp.1575-1588
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• 2019

Fracture near the U-10Mo/cladding material interface impacts fuel service life. In this work, a mesoscale stress model is developed with the fuel foil considered as a porous medium having gas bubbles and bearing bubble pressure and surface tension. The models for the evolution of bubble volume fraction, size and internal pressure are also obtained. For a U-10Mo/Al monolithic fuel plate under location-dependent irradiation, the finite element simulation of the thermo-mechanical coupling behavior is implemented to obtain the bubble distribution and evolution behavior together with their effects on the mesoscale stresses. The numerical simulation results indicate that higher macroscale tensile stresses appear close to the locations with the maximum increments of fuel foil thickness, which is intensively related to irradiation creep deformations. The maximum mesoscale tensile stress is more than 2 times of the macroscale one on the irradiation time of 98 days, which results from the contributions of considerable volume fraction and internal pressure of bubbles. This study lays a foundation for the fracture mechanism analysis and development of a fracture criterion for U-10Mo monolithic fuels.

• 한국주조공학회지
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• 제42권6호
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• pp.392-397
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• 2022

In order to compliant the stringent exhaust emission regulations, higher fuel efficiency and cleaner exhaust gas in combustion engines have been required. To improve combustion efficiency, an exhaust gas temperature is increasing, therefore higher temperature resistance is required for components in exhaust system, especially turbine wheel in turbocharger. IN100 looks quite attractive candidate as it has high temperature properties with low density, however it has low castability due to poor ductility at high temperature. In this study, the balance of Al and Ti composition was optimized from the base alloy IN100 to improve the high temperature ductility by expanding the γ single phase region below the solidification temperature, while obtaining the high temperature strength by maintaining the volume fraction of γ' phase equivalent to IN100 around 1000℃. Furthermore, the high temperature creep rupture life increased by adding a small amount of Ta. The alloy developed in this study has high castability, low density and high specific strength at high temperature.

• 한국재료학회지
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• 제4권3호
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• pp.349-356
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• 1994

재료의고온소성변형과 수명예측 및 수명향상을 위하여 재료의 변형기구를 규명하는 것이 매우 중요하다. 이를 위하여 전위환모델이 자주 사용되며, 현재 실험적인 결과를 토대로 한 두개의 중요한 전위환모델로서, Orlova등고 Mills등이 제시한 모델들이 있다. 이들은 모두AI-5.5at.%MG을 사용하였으나 상호 상반된 전위환모델을 설명하고 있다. 그러므로 본 연구에서는 상반된 전위환 모델을 확인하기 위하여 AI-5.5at.%MG을 사용하여 573K의 약 30MPa에서 $\varepsilon$=0.03까지 크\ulcorner시험을 하고, 이러한 크\ulcorner시험후 이어서 각각 약 15,10 및 oMPa의 응력감소 시험도 수행하였으며, 동시에 응력감소 시험 전과 후의 전위구조를 관찰하여 전위환모델을 고찰하였다.