• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1997년도 가을 학술발표회 논문집
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• pp.539-545
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• 1997

Since the length of massive wall-structure is generally longer and larger than its thickness and a lift height of concrete of placement, cracks induced by hydration heat are governed by outer structural restriction rather than inner one. However, the degree of restriction control is expected to be affected by the sizes of wall thickness, length and a lift height. Thus, this analytical study aims at the development of relationship among those to minimize thermal cracks. In addition, the effect of types of cement on the thermal heats and stresses is evaluated for anti-sulphate and 2blended Portland cements concrete. It was found from analytical study that a lift height of concrete placement is the most important factor controlling thermal cracks, and the increase of lift heights is not always detrimental to structural safety.

• 한국표면공학회지
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• 제31권1호
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• pp.12-23
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• 1998

The Thermal Barrier Coating(TBC) has been used to improve the heat barrier and tribological properties of the aircraft engine and the automobile engine in high temperature. Especially, the high temperature tribological propertied of the cylinder haed and the piston crown of diesel engine was emphasized. Therefore, the purpose of this work was to evaluate the microstructure, tribological propeer in high tempearmal shock resistance and bonding strength of five layer functionally gradient TBC for the applications. The five layerwere composed with 100% ceramic insulating later, 75(ceramic):25 (metal) layer, 50:50 layer, 25:75 layer and 100% metal bonding layer to redude the thermal stress. the YSL and MSL poweders were the insulation ceramics powers. The NiCrAly, Inconel625 and SUS powders were the bonding and mixingg powders for plasma spray process. According to the result of high temperature wear test, the wera resistance of YSZ/NiCrAlY siytem was most out standing at 600 and $800^{\circ}C$. At $400^{\circ}C$, the wear resistance of YSZ/Inconel system was better than others. Wear volume at other temperature because of the low temperature degration of zirconia. The thermal shock mechanism of 5 later is the vertical crack gegration in insulating layer. this means that the initial cracks were generated in the top layer, and then developed into the composite layers during thermal shock test. Finally, these cracks werereached to the interface of coating and substrate and also, these vertioal cracks join with the horizontal cracks of the each layers. The bonding strength of YSZ/NiCrAlY and YSZ/Inconel 5 layer system is better than other 5layer systems. The theramal shock resistance of thermal barrier coating s with 5 layer system is better than that of 3 layers and 2 layers.

• 한국세라믹학회지
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• 제54권6호
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• pp.492-498
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• 2017

A thermal barrier coating (TBC) with self-healing property for cracks was proposed to improve reliability during gas turbine operation, including structural design. Effect of healing agent on crack propagation behavior in TBCs with and without buffer layer was investigated through furnace cyclic test (FCT). Molybdenum disilicide ($MoSi_2$) was used as the healing agent; it was encapsulated using a mixture of tetraethyl orthosilicate and sodium methoxide. Buffer layers with composition ratios of 90 : 10 and 80 : 20 wt%, using yttria stabilized zirconia and $MoSi_2$, respectively, were prepared by air plasma spray process. After generating artificial cracks in TBC samples by using Vickers indentation, FCTs were conducted at $1100^{\circ}C$ for a dwell time of 40 min., followed by natural air cooling for 20 min. at room temperature. The cracks were healed in the buffer layer with the healing agent of $MoSi_2$, and it was found that the thermal reliability of TBC can be enhanced by introducing the buffer layer with healing agent in the top coat.

• 대한토목학회논문집
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• 제14권4호
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• pp.771-781
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• 1994

매스콘크리트에서 시멘트의 수화반응에 의해 발생되는 열은 약재령 콘크리트(young concrete)에서 내부온도의 증가와 체적변화를 초래하므로 외적인 구속이 존재하면 구조물에는 인장응력이 발생하고 이는 구조물에 균열을 일으킬 수 있다. 따라서 매스콘크리트의 설계와 시공단계에서 균열을 제어하기 위해서는 온도응력을 정확하게 예측하는 것이 매우 중요하다. 또한 약재령 콘크리트에서는 크리이프 효과가 노재령 콘크리트(old concrete)의 경우에 비하여 현저하게 크므로 이를 반드시 고려하여야 할 것이다. 이 논문은 매스콘크리트에서의 온도에 관한 시간이력을 구하고 크리이프와 각 요소의 온도를 고려한 탄성계수를 적용하여 온도응력을 구하는 유한요소 프로그램을 개발하는데 그 목적이 있다. 개발된 프로그램의 해석결과를 문헌의 자료들과 비교해 볼 때 온도균열의 제어에 가장 중요한 내부에서의 온도이력과 응력이 실험결과와 잘 부합되고 있다. 따라서, 이 논문의 해석방법은 매스콘크리트 구조물의 설계와 시공단계에서 균열의 제어를 위해 유용하게 사용될 수 있을 것이다.

• Computers and Concrete
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• 제22권1호
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• pp.93-100
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• 2018

It is seldom possible that geotechnical materials like rocks and concretes found without joints, cracks, or discontinuities. Thereby, the impact of micro-cracks on the mechanical properties of them is to be considered. In the present study, the effect of micro-crack on the failure mechanism of rock specimens under uniaxial compression was investigated experimentally. For this purpose, thermal stress was used to induce micro-cracks in the specimens. Several cylindrical and disk shape specimens were drilled from granite collected from Zanjan granite mine, Iran. Some of the prepared specimens were kept in room temperature and the others were heated by a laboratory furnace to different temperature levels (200, 400, 600, 800 and 1000 degree Celsius). During the experimental tests, Acoustic Emission (AE) sensors were used to monitor specimen failure at the different loading sequences. Also, Scanning Electron Microscope (SEM) was used to distinguish the induced micro-crack by heating in the specimens. The fractographic analysis revealed that the thin sections heated to $800^{\circ}C$ and $1000^{\circ}C$ contain some induced micro-fractures, but in the thin sections heated to $200^{\circ}C$, $400^{\circ}C$ and $600^{\circ}C$ have not been observed any micro-fracture. In the next, a comprehensive experimental investigation was made to evaluate mechanical properties of heated and unheated specimens. Results of experimental tests showed that induced micro-cracks significantly influence on the failure mode of specimens. The specimens kept at room temperature failed in the splitting mode, while the failure mode of specimens heated to $800^{\circ}C$ are shearing and the specimens heated to $1000^{\circ}C$ failed in the spalling mode. On the basis of AE monitoring, it is found that with increasing of the micro-crack density, the ratio of the number of shear cracks to the number of tensile cracks increases, under loading sequences.

• 한국건설순환자원학회논문집
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• 제2권3호
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• pp.233-238
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• 2014

합벽, 옹벽, 교량 주탑과 같이 세장한 매스콘크리트 구조물의 경우 온도균열 제어를 위해 저발열 콘크리트 배합의 적용 및 균열유발줄눈의 설치 등이 주로 적용되고 있다. 하지만 이러한 대책이 적용되어도 세장한 매스 구조물들은 내부구속에 비해 외부구속이 크기 때문에 완벽한 균열제어가 불가능하고, 특히 균열이 발생하게 되면 거의 관통을 하기 때문에 온도균열제어를 위한 특별한 관리가 필요하다. 합벽, 옹벽, 교량 주탑과 같이 세장한 매스콘크리트 구조물의 적극적인 온도균열 제어를 위하여 기존의 파이프쿨링 공법을 개선시킨 연직파이프쿨링 공법을 개발하였으며, 현장 우수조/중수조 합벽 콘크리트에 적용이 되었다. 그 결과 합벽의 중심부 최고 온도를 10도 이상 낮츨 수 있는 것으로 나타났으며, 균열은 50% 이상 감소시킬 수 있는 것으로 나타났다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2002년도 봄 학술발표회 논문집
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• pp.29-34
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• 2002

The heat of hydration of cement causes the internal temperature rise at early age, paticular in massive concrete structures. As the results of the temperature rise and restraint condition, the thermal stress may induce cracks in concrete. The prediction of the thermal stress is important in design and construction stages in order to control the cracks in mass concrete. It is poor economy to analysis for prediction of the thermal stress on each design or construction. In this study, the hydration heat and thermal stress analysis is performed by ABAQUS program, as a results of thermal analysis, the formula of size-placing depth relationship is proposed.

• 비파괴검사학회지
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• 제32권3호
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• pp.296-301
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• 2012

This paper describes an analytic method for infrared thermography to detect surface cracks in thin plates. Traditional thermographic method uses the spatial contrast of a thermal field, which is often corrupted by noise in the experiment induced mainly by emissivity variations of target surfaces. This study developed a robust analytic approach to crack detection for thermography using the holomorphic function of a temperature field in thin plate under steady-state thermal conditions. The holomorphic function of a simple temperature field was derived for 2-D heat flow in the plate from Cauchy-Riemann conditions, and applied to define a contour integral that varies depending on the existence and strength of singularity in the domain of integration. It was found that the contour integral at each point of thermal image reduced the noise and temperature variation due to heat conduction, so that it provided a clearer image of the singularity such as cracks.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2000년도 가을 학술발표회 논문집(II)
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• pp.1163-1168
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• 2000

The method for preventing thermal cracks is necessary in mass concrete structures. So various experiments were carried out for the controls of thermal cracks and we substituted fly ash for a quarter of cement quantity in order to decrease hydration heat. The maximum block size is determined by numerical analysis as well. Hydration heat and thermal stress were measured through various gauges and analysis considering the steps of concrete placement were carried out. It was found from this study that the appropriate block size was able to be determined properly by numerical analysis.

• 대한기계학회논문집
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• 제12권2호
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• pp.286-294
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• 1988

본 논문에서는 Bogdanoff가 제시한 열응력 관계식을 이용하여 균일 열유동을 받는 무한체내에 존재하는 균열에 대해서 균열표면에 자유트랙션조건과 열경계 조건을 부여하여 TSIF를 구하고자 한다. 다루고자 하는 커스프 균열의 형태는 하이포사이클로이드형(hypocycloid type), 대칭익형(symmetric airfoil type), 그리고 대칭입술형(symmetric lip type)이다.