• Journal of Welding and Joining
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• 제31권6호
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• pp.50-57
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• 2013

Characterization of microstructures and cryogenic mechanical properties of electro beam (EB) welds between cast and forged Inconel 718 superalloys has been investigated. Optimal EBW condition was found in the beam current range of 36~39 mA with the constant travel speed of 12 mm/s and arc voltage of 120 kV for 10 mm-thick specimens. Electron beam current lower than 25 mA caused to occur the liquation microfissuring in cast-side heat affected zone (HAZ) of EB welds. The HAZ liquation microfissure was found on the liquated grain boundaries with resolidified ${\gamma}/Laves$ and ${\gamma}/NbC$ eutectic constituents. EBW produced welds showing a fine dendritic structure with relatively discrete Laves phase due to fast cooling rate. After post weld aging treatment, blocky Laves phase and formation of ${\gamma}^{\prime}+{\gamma}^{{\prime}{\prime}}$ strengtheners were observed. Presence of primary strengthener and coarse Laves particles in PWHT weld may cause to reduce micro-plastic zone ahead of a crack, leading to a significant decrease in Charpy impact toughness at $-196^{\circ}C$. Fracture initiation and propagation induced by Charpy impact testing were discussed in terms of the dislocation structures ahead of crack arisen from the fractured Laves phase.

• Composites Research
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• 제12권4호
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• pp.62-70
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• 1999

건축 구조물의 재령이 증가함에 따른 손상은 구조물의 안전성 및 신뢰성 확보에 매우 심각한 문제이다. 최근에는 이렇게 손상을 입은 콘크리트 구조물의 보수 보강을 위해 탄소섬유시트가 광범위하게 사용되고 있지만 탄소섬유시트를 보강한 콘크리트의 파괴기구가 명확하게 규명되지 않았다. 따라서 본 연구에서는 탄소섬유시트를 보강한 콘크리트의 손상거동 및 미시적 파손기구를 이해하기 위해 4가지 경우의 시험편, 즉 순수한 콘크리트, 철근 보강 콘크리트, 탄소섬유시트 보강 콘크리트, 철근과 탄소섬유시트 보강 콘크리트 시험편에 대하여 3점 굽힘 시험을 실시하였다. 이러한 시험편들의 파손기구를 명확히 하기 위해 음향방출기법이 사용되었으며 균열 발생, 성장을 모니터링하기 위해 2차원 AE 발생원 위치추정 기법이 적용되었다.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2004년도 춘계 학술발표대회 개요집
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• pp.324-326
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• 2004

l2wt%Cr Steel has been applied on turbine bucket and nozzle partition material of power plant. Turbine bucket and nozzle get damaged by solid particle within steam, therefore they are protected by surface treatments such as ion nitriding, boriding and chrome carbide HVOF spray coating. In this study, solid particle erosion(SPE) characteristics after these surface treatments are examined at operating temperature 540$^{\circ}C$ and 590$^{\circ}C$ of fossil power plant and the mechanism of damage was studied. Erosion of 12wt%Cr steel is originated by micro cutting and that of boriding and chrome carbide HVOF spray is originated by these mechanism - repeating collision, crack initiation and propagation. As the results of SPE test at 540$^{\circ}C$ and 30$^{\circ}$ impact angle that is the most commonly occurred in power plant, Boriding had the best SPE -resistance property, Cr$_2$C$_3$-25(Ni20Cr) HVOF spayed and ion nitrided samples were also better than bare metals(l2wt%Cr Steels). At 590$^{\circ}C$ and 30$^{\circ}$ impact angle, Boriding had also the most superior characteristic and HVOF spay sample was better than bare metal.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2007년도 정기 학술대회 논문집
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• pp.3-17
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• 2007

This paper provides an introduction to life-span simulation and numerical approach to support the performance design processes of reinforced concrete structures. An integrated computational system is proposed for life-span simulation of reinforced concrete. Conservation of moisture, carbon dioxide, oxygen, chloride, calcium and momentum is solved with hydration, carbonation, corrosion, ion dissolution. damage evolution and their thermodynamic/mechanical equilibrium. Coupled analysis of mass transport and damage mechanics associated with steel corrosion is presented for structural performance assessment of reinforced concrete. Multi-scale modeling of micro-pore formation and transport phenomena of moisture and ions are mutually linked for predicting the corrosion of reinforcement and volumetric changes. The interaction of crack propagation with corroded gel migration can also be simulated. Two finite element codes. multi-chemo physical simulation code (DuCOM) and nonlinear dynamic code of structural reinforced concrete (COM3) were combined together to form the integrated simulation system. This computational system was verified by the laboratory scale and large scale experiments of damaged reinforced concrete members under static loads, and has been applied to safety and serviceability assessment of existing structures. Based on the damage details predicted by the nonlinear finite element analytical system, the life-span-cost of RC structures including the original construction costs and the repairing costs for possible damage during the service life can be evaluated for design purpose.

• Advances in materials Research
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• 제12권3호
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• pp.227-242
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• 2023

Polyester composites play a vital role in civil engineering applications, especially in bridge and car park structures. Therefore, the addition of waste silica-based fillers will both improve the mechanical and durability performance of composites and produce an environmentally friendly material. In this study, the mechanical performance of polyester composites was investigated experimentally and numerically by adding micro and nano-sized silica-based fillers, marble powder, silica fume and nano-silica. 24 cubes for the compression test and 18 prisms for the flexural test were produced in six different groups containing 30% marble powder, 5% silica fume and 1% nano-silica by weight. SEM/EDS testing was used to investigate the distribution of filler particles in the matrix. Experimentally collected results were used to validate tests in the Abaqus software. Additionally, the Extended Finite Element Method (XFEM) was used to estimate the fracture process for the flexural test. The results show that the added silica fume, marble powder and nano silica improves the compressive strength of polyester composites by 32-38% and the flexural tensile strength by 10-60% compared to pure polyester composite. The numerically obtained results matched well with the experimental data, demonstrating the accuracy and feasibility of the calibrated finite element model.

• 한국세라믹학회지
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• 제47권6호
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• pp.498-502
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• 2010

Aluminium titanate($Al_2TiO_5$) has extremely anisotropic thermal expansion properties in single crystals, and polycrystalline material spontaneously microcracks in the cooling step after sintering process. These fine intergranular cracks limit the strength of the material, but provide an effective mechanism for absorbing strain energy during thermal shock and preventing catastrophic crack propagation. Furthermore, since machinable BN-ceramics used as an insulating substrate in current micro-electronic industry are very expensive, the development of new low-cost machinable substrate ceramics are consistently required. Therefore, cheap $Al_2TiO_5$-machinable ceramics was studied for the replacement of BN ceramics. $Al_2O_3-Al_2TiO_5$ ceramic composite was fabricated via in-situ reaction sintering. $Al_2O_3$ and $TiO_2$ powders were mixed with various mol-ratio and sintered at 1400 to $1600^{\circ}C$ for 1 h. Density, hardness and strength of sintered ceramics were systematically measured. Phase analysis and microstructures were observed by XRD and SEM, respectively. Machinability of each specimens was tested by micro-hole machining. The results of research showed that the $Al_2TiO_5$-composites could be used for low-cost machinable ceramics.

• 지질공학
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• 제6권2호
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• pp.59-64
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• 1996

암석 시료가 파괴될 때에 발생되는 AE신호는 미세한 균열 발생시의 갑작스런 변형에너지 해바에 기인한다. 압전 압력형 탐촉자와 다채널 기록장치를 이용하여 AE 신호파를 기록하여 분석하므로 외적인 하중조건과 그에 따른 미세균열의 특징에 대하여 연구하였다. 연구결과 미세균열의 체적은 수 $\mu\textrm{m}^3$ 내지 $150,000\mu\textrm{m}^3$로 산출되어서 그 크기가 넓은 범위로 분포하였고 인장형 미세균열이 대체적으로 전단형 보다 큰 체적을 보였다. 또한, 균열원에서의 에너지 강도는 모드 I 하중조건하에서 발생하는 AE 신호가 혼합모드 조건하에서 발생한 신호보다 약 3배정도 크게 나타났으나,시료가 파괴되는 동안 기록된 AE 신호의 숫자는 반대로 모드 I의 경우가 혼합모드의 25%에 불과하였다. 이러한 사실은 같은 크기의 파괴면을 형성하는데 필요한 에너지 요구량이 대체적으로 동일함을 암시하는 것으로 보인다.

• 대한금속재료학회지
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• 제48권1호
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• pp.28-38
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• 2010

High temperature high cycle and low cycle fatigue deformation behavior of automotive heat resistant aluminum alloys (A356 and A319 based) were investigated in this study. The microstructures of both alloys were composed of primary Al-Si dendrite and eutectic Si phase. However, the size and distribution for eutectic Si phase varied: a coarse and inhomogeneous distributed was observed in alloy B (A319 based). A brittle intermethallic phase of ${\alpha}-Fe\;Al_{12}(Fe,Mn)_3Si_2$ was detected only in B alloy. Alloy B exhibited high fatigue life only under a high stress amplitued condition in the high cycle fatigue results, whereas alloy A showed high fatigue life when stress was lowered. With regard to the low-cycle fatigue result ($250^{\circ}C$) showing higher fatigue life as ductility increased, alloy A demonstrated higher fatigue life under all of the strain amplitude conditions. Fractographic observations showed that large porosities and pores near the outside surface could be the main factor in the formation of fatigue cracks. In alloy B. micro-cracks were formed in both the brittle intermetallic and coarse Si phasese. These micro-cracks then coalesced together and provided a path for fatigue crack propagation. From the observation of the differences in microstructure and fractography of these two automotive alloys, the authors attempt to explain the high-temperature fatigue deformation behavior of heat resistant aluminum alloys.

• 대한조선학회논문집
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• 제32권1호
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• pp.118-131
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• 1995

수중폭발을 받게 되는 해군 함정이나 충격하중을 받게 되는 초고속선의 구조에 대한 내충격 파손해석을 거시해석(global or macro analysis)과 미시해석(fine or micro analysis)의 두 단계로 나누어 수행하였다. 거시해석은 이중근사기법(DAA : Doubly Asymptotic Approximation)을 이용하였다. 심한 충격하중을 받는 구조는 주로 세 가지 파괴모드를 나타내는데 이는 충격후기에 주로 나타나는 동소성좌굴(Dynamic plastic buckling)에 기인하는 소성대변형과 충격초기에 주로 나타나는 인장 파괴(Tensile tearing failure)와 횡전단파괴(Transverse shear failure)가 있다. 본 논문의 미시해석에서는 잠수구조의 종보강재에 충격압력이 가해진 경우에 대하여 응력파(stress wave)의 파급과 이 응력파와 균열과의 상호작용에 의한 동적응력강도계수 $K_I(t)$의 계산함으로써 인장 파괴모드(Tensile tearing failure mode)해석을 수행하였다. 특히, 동적응력강도계수 $K_I(t)$의 계산에 있어서 실험적 방법으로 널리 사용되는 shadow optical method of caustic로부터 개발된 numerical caustic method를 사용하였다. 본 논문의 충격파손해석 수치 예로서 해석모델을 완전잠수주상체로 잡고 거시해석을 수행한 후 이로부터 구한 충격압력을 입력자료로 하여 종보강재에 대하여 미시해석을 수행하였다.

• Advances in concrete construction
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• 제9권3호
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• pp.235-248
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• 2020

Progressive collapse in a structure occurs when load bearing members are failed and the adjoining structural elements cannot resist the redistributed forces and fails subsequently, that leads to complete collapse of structure. Recently, construction using precast concrete technology is adopted increasingly because it offers many advantages like faster construction, less requirement of skilled labours at site, reduced formwork and scaffolding, massive production with reduced amount of construction waste, better quality and better surface finishing as compared to conventional reinforced concrete construction. Connections are the critical elements for any precast structure, because in past, major collapse of precast structure took place because of connection failure. In this study, behavior of four different precast wet connections with U shaped reinforcement bars provided at different locations is evaluated. Reduced 1/3^rd scale precast beam column assemblies having two span beam and three columns with removed middle column are constructed and examined by performing experiments. The response of precast connections is compared with monolithic connection, under column removal scenario. The connection region of test specimens are filled by cast-in-place micro concrete with and without polypropylene fibers. Performance of specimen is evaluated on the basis of ultimate load carrying capacity, maximum deflection at the location of removed middle column, crack formation and failure propagation. Further, Finite element (FE) analysis is carried out for validation of experimental studies and understanding the performance of structural components. Monolithic and precast beam column assemblies are modeled using non-linear Finite Element (FE) analysis based software ABAQUS. Actual experimental conditions are simulated using appropriate boundary and loading conditions. Finite Element simulation results in terms of load versus deflection are compared with that of experimental study. The nonlinear FE analysis results shows good agreement with experimental results.