• 비파괴검사학회지
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• 제37권2호
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• pp.91-98
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• 2017

Recently, composite materials have been mainly used in the main wings, ailerons, and fuselages of aircraft and rotor blades of helicopters. Composite materials used in rapid moving structures are subject to impact by hail, lightning, and bird strike. Such an impact can destroy fiber tissues in the composite materials as well as deform the composite materials, resulting in various problems such as weakened rigidity of the composite structure and penetration of water into tiny cracks. In this study, experiments were conducted using a 2 kW halogen lamp which is most frequently used as a light source, a 2 kW near-infrared lamp, which is used for heating to a high temperature, and a 6 kW xenon flash lamp which emits a large amount of energy for a moment. CFRP composite sandwich panels using Nomex honeycomb core were used as the specimens. Experiments were carried out under impact damages of 1, 4 and 8 J. It was found that the detection of defects was fast when the xenon flash lamp was used. The detection of damaged regions was excellent when the halogen lamp was used. Furthermore, the near-infrared lamp is an effective technology for showing the surface of a test object.

• 한국재료학회지
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• 제15권2호
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• pp.93-96
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• 2005

The arc melting was employed to prepare undoped $CoSb_3$ compounds and their thermoelectric properties were investigated. Specimen annealed at $400^{\circ}C$ for 24 hrs showed sound microstructure. However, considerable voids and cracks were found after annealing at above $500^{\circ}C$. It seems to be attributed to the phase dissociation and thermal expansion due to phase transitions during annealing and cooling. Single phase $\delta-CoSb_3$ was successfully obtained by annealing at $400^{\circ}C$ for 24 hrs. In the case of increasing annealing temperature, phase decompositions occurred. Undoped $CoSb_3$ showed p-type conduction and intrinsic semiconducting behavior at all temperatures examined. Thermoelectric properties were remarkably improved by annealing and they were closely related to phase transitions.

• 청정기술
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• 제23권1호
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• pp.90-94
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• 2017

현재 풍력발전은 우리나라에서 가장 주목 받고 있는 신재생에너지 분야로 많은 연구가 진행 중이다. 풍력발전을 구성하는 요소 중 핵심 요소인 블레이드에 손상이 발생할 경우에 발전 효율에 직접적인 영향을 미치므로 효율적인 유지보수를 위해 초기에 결함을 측정하는 기술이 매우 중요한 상태이다. 그러나 기존의 초음파 비파괴 검사 및 열화상 비파괴 검사는 소요시간이 길고 실시간 모니터링이 어려우므로 초기 결함 측정이 불가능하다. 기존의 문제를 보완하고자 본 논문에서는 표면파를 이용한 블레이드 표면상태 실시간 모니터링에 관한 연구를 수행하였다. 압전센서 기반의 시스템을 구성하여 블레이드 표면샘플에 대해 공정 변수 별 기초 성능 실험을 하였고, 소형 블레이드에 대해 공정변수 별 실험을 통해 블레이드의 크랙, 벗겨짐, 장애물 등을 실시간으로 모니터링이 가능한지 연구 하였다.

• 한국지반환경공학회 논문집
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• 제18권1호
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• pp.5-12
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• 2017

한랭지역에 건설된 터널 라이닝은 터널주변 지하수의 결빙 등으로 인한 균열, 누수 및 백태 등이 빈번하게 발생하게 된다. 이러한 터널의 동결피해는 동절기 외기온도에 의해 터널내부 라이닝 및 배면의 지반온도의 하강으로 발생하게 되므로 시공 시 단열재를 설치하여 동결로 인한 피해를 저감하거나 기 시공된 터널에서는 단열재를 설치하거나 발열체 등을 설치하여 동결로 인한 피해를 저감할 수 있다. 본 연구에서는 대기온도의 변화에 의한 라이닝 배면의 지반온도 변화와 발열체가 설치되었을 경우 라이닝 및 배면 지반의 온도변화를 분석함으로써 동결피해 저감을 위한 대응방안 시 나타나는 효과를 분석하였다.

• Computers and Concrete
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• 제19권5호
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• pp.443-449
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• 2017

Taking high geothermal tunnels as background, the deformation of concrete for shotcrete use was studied by simulating hot-humid and hot-dry environments in a laboratory. The research is made up by two parts, one is the influence of two kinds of high geothermal environments on the deformation of shotcrete, and the other is the shrinkage inhibited effect of fiber materials (steel fibers, polypropylene fibers, and the mixture of both) on the concrete in hot-dry environments. The research results show that: (1) in hot and humid environments, wet expansion and thermal expansion happened on concrete, but the deformation is smooth throughout the whole curing age. (2) In hot and dry environments, the concrete suffers from shrinkage. The deformation obeys linear relationship with the natural logarithm of curing age in the first 28 days, and it becomes stable after the $28^{th}$ day. (3) The shrinkage of concrete in a hot and dry environment can be inhibited by adding fiber materials especially steel fibers, and it also obeys linear relationship with the natural logarithm of curing age before it becomes stable. However, compared with no-fiber condition, it takes 14 days, half of 28 days, to make the shrinkage become stable, and the shrinkage ratio of concrete at 180-day age decreases by 63.2% as well. (4) According to submicroscopic and microscopic analysis, there is great bond strength at the interface between steel fiber and concrete. The fiber meshes are formed in concrete by disorderly distributed fibers, which not only can effectively restrain the shrinkage, but also prevent the micro and macro cracks from extending.

• 비파괴검사학회지
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• 제23권1호
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• pp.38-44
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• 2003

원자력발전소의 안전성등급 기기에 적용되는 비파괴검사는 실제 결함을 실현한 시험편을 사용하여 결함탐지능력을 검증하도록 하는 기량검증이 요구되고 있다. 가동중인 원전에서 발생 가능한 균열으로는 기계적 피로균열, 열 피로균열 및 입계부식균열 등이 있으나 본 연구에서는 기계적 피로균열을 대상으로 하였다. 인장 피로하중을 사용하여 기계적 피로결함을 제조하기 위해서 시험편을 설계하였고 원하는 피로결함 파면의 조도를 얻기 위해서 인가하중의 크기 및 사이클 수를 조절하여 피로결함을 발생시켰다. 발생된 결함에 대한 정확한 크기와 위치에 대한 물리적 정보를 얻은 후에 결함이 설계된 크기와 위치에 존재하도록 기밀용접을 실시하였다. 기밀용접 후 잔여 용접 흠은 가스 텅스텐 아크용접 및 플럭스 코어드 아크용접으로 채워졌다. 최종 완성된 피로결함 시험편을 방사선투과검사 및 초음파탐상검사를 통하여 검사한 결과, 설계된 길이와 깊이로 피로결함이 형성되었음을 확인하였다.

• 한국화재소방학회논문지
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• 제18권3호
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• pp.51-56
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• 2004

철근콘크리트구조물에 화재가 발생하게 되면 콘크리트내의 시멘트 수화물과 골재의 상반된 거동으로 조직이 연화되며 열응력 등으로 균열이 발생하여 부분적 혹은 전체구조시스템에 심각한 영향을 끼칠 수 있는 손상이 발생하여, 철근 콘크리트 구조물의 내구성을 현저히 저하시키게 된다. 콘크리트의 화재피해 상황은 콘크리트에 사용된 혼화재료 및 골재 등 사용재료의 영향을 받게 되며 설계기준 강도에 따라 그 피해 상황 역시 다르게 나타나기 때문에 화재피해를 입은 콘크리트 구조물의 열화 진단은 화재에 의한 열화 메커니즘을 바탕으로 이루어져야 하며, 이를 위해서는 고온에 노출된 콘크리트의 공학적 특성에 관한 기초자료가 반드시 필요하다. 그러므로 본 연구에서는 설계기준 강도 및 사용재료에 따라 콘크리트를 제조 하여 고온환경에 노출시켜 폭열상태, 초음파속도, 압축강도 등을 측정하여 화재로 열화된 콘크리트 구조물의 재사용 여부 및 피해 등급 결정을 위한 정밀 진단과 보수ㆍ보강공법의 선정을 위한 기초 자료로 제시하고자 한다.

• 소성∙가공
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• 제17권4호
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• pp.240-248
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• 2008

The effect of alloying elements(Mn, S, Mo, B) on the high temperature deformation behavior of Fe-29%Ni-17%Co (Kovar) alloy were investigated. And the effect of high temperature oxidation on the hot ductility was also studied. The hot ductility of Kovar alloy was drastically increased with the addition of Mn and lowering of S content. It has been found that the brittle intergranular fracture at high temperature cracking is closely associated with the FeS sulfide along the grain boundary. When Mn was added, the type of sulfide was changed to MnS from FeS and ductile intergranular fracture and transgranular fracture were promoted. The formation of oxide layer was found to have minimized the hot ductility of the Kovar alloy significantly. Grain boundary micro-cracks in the internal oxide region were noted following deformation due to high temperature, one of which acting as a notch that caused the poor hot workability of the oxidized specimen. The addition of Mo to the Kovar alloy could also retard the decrease in the hot ductility of the oxidized specimen through the prevention of notching due to internal oxidation. Hot ductility was remarkably improved by the addition of Boron. The improvement of hot ductility results from the grain boundary migration mainly due to the dynamic recrystallization at lower temperature range ($900{\sim}1000^{\circ}C$).

• 한국레이저가공학회지
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• 제11권4호
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• pp.29-34
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• 2008

Ultraviolet laser micromachining has increasingly been applied to the electronics industry where precision machining of high-density, multi-layer, and multi material components is in a strong demand. Due to the ever-decreasing size of electronic products such as cellular phones, MP3 players, digital cameras, etc., flexible printed circuit board (FPCB), multi-layered with polymers and metals, tends to be thicker. In present, multi-layered FPCBs are being mechanically cut with a punching die. The mechanical cutting of FPCBs causes such defects as burr on layer edges, cracks in terminals, delamination and chipping of layers. In this study, the laser cutting mechanism of FPCB was examined to solve problems related to surface debris and short-circuiting that can be caused by the photo-thermal effect. The laser cutting of PI and FCCL, which are base materials of FPCB, was carried out using a pico-second laser(355nm, 532nm) and nano-second UV laser with adjusting variables such as the average/peak power, scanning speed, cycles, gas and materials. Points which special attention should be paid are that a fast scanning speed, low repetition rate and high peak power are required for precision machining.

• 한국재료학회지
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• 제30권3호
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• pp.142-148
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• 2020

Graphene has attracted the interest of many researchers due to various its advantages such as high mobility, high transparency, and strong mechanical strength. However, large-area graphene is grown at high temperatures of about 1,000 ℃ and must be transferred to various substrates for various applications. As a result, transferred graphene shows many defects such as wrinkles/ripples and cracks that happen during the transfer process. In this study, we address transfer-free, large-scale, and high-quality monolayer graphene. Monolayer graphene was grown at low temperatures on Ti (10nm)-buffered Si (001) and PET substrates via plasma-assisted thermal chemical vapor deposition (PATCVD). The graphene area is small at low mTorr range of operating pressure, while 4 × 4 ㎠ scale graphene is grown at high working pressures from 1.5 to 1.8 Torr. Four-inch wafer scale graphene growth is achieved at growth conditions of 1.8 Torr working pressure and 150 ℃ growth temperature. The monolayer graphene that is grown directly on the Ti-buffer layer reveals a transparency of 97.4 % at a wavelength of 550 nm, a carrier mobility of about 7,000 ㎠/V×s, and a sheet resistance of 98 W/□. Transfer-free, large-scale, high-quality monolayer graphene can be applied to flexible and stretchable electronic devices.