• 한국정밀공학회지
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• 제20권6호
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• pp.214-221
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• 2003

Tension mask assembly is positioned right behind the glass-made front panels of CRT type display devices. The frame-supported thin metal sheet contains numerous slits, through which electron beams are focused to enhance definition. Pretension is imposed on the masks, especially for enlarged flat screens, in order to avoid vibration due to acoustic or mechanical impact. High temperature assembly process subsequent to pretensioning, however, degenerates the creep resistance of common mask materials, and if tensile stress is high enough, tension on the mask may be loosened substantially due to creep deformation. In this study, the assembly is modeled as a combined structure of beams and wire array, and a numerical simulation is attempted for pretensioning followed by high temperature process. According to a model study, small amount of creep strain is likely to be generated, but its adverse influence is not negligible. Some structural modification measures to reduce the creep-induced tension loosening are proposed and evaluated. Also, optimal configuration of frame structure is sought for, which maintains high tension of masks and minimizes the possible creep of frame simultaneously.

• 한국신뢰성학회:학술대회논문집
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• 한국신뢰성학회 2004년도 정기학술대회
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• pp.227-232
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• 2004

The failure of geogrids can be defined as an excessive creep strain which causes the collapse of slopes and embankments. In this study, the accelerated creep tests were applied to two different types of polyester geogrids, at 75, 80, 85$^{\circ}C$ by applying 50% load of ultimate tensile strengths using a newly designed test equipment which is allowed the creep testing at higher temperatures. And then the creep curves were shifted and superposed in the time axis by applying time-temperature supposition principles. In predicting the lifetimes of geogrids, the underlying distribution for failure times were determined based on identification of the failure mechanism. The results indicate that the conventional procedures with the newly designed test equipment are shown to be effective in prediction of the lifetimes of geogrids with shorter test times. In addition, the predicted lifetimes of geogrids having different structures at various creep strains give guidelines for users to select the proper geogrids in the fields.

• Nuclear Engineering and Technology
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• 제53권9호
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• pp.2953-2959
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• 2021

A new practical modeling of the Norton's power law creep is proposed and implemented to analyze the high temperature behaviors of Alloy 690 SG tube material. In the model, both the stress exponent n and the rate constant B are simply treated as the temperature dependent parameters. Based on the two-step optimization procedure, the temperature function of the rate constant B(T) was determined for the data set of each B value after fixing the stress exponent n value by using the prior optimized function at each temperature. This procedure could significantly reduce the numerical errors when using the power law creep equations. Based on the better description of the steady-state creep rates, the experimental rupture times could also be well predicted by using the Monkman-Grant relationship. Furthermore, the difference in tensile strengths at high temperatures could be very well estimated by assuming the imaginary creep stress related to the given strain rate after correcting the temperature effects on the elastic modulus.

• Journal of Mechanical Science and Technology
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• 제20권12호
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• pp.2136-2146
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• 2006

A study on a creep-fatigue crack growth behavior has been carried out for a cylindrical structure with weldments by using a structural test and an evaluation according to the assessment procedures. The creep-fatigue crack growth behavior following the creep-fatigue crack initiation has been assessed by using the French A16 procedure and the conservatism for the present structural test has been examined. The structural specimen is a welded cylindrical shell made of 316 L stainless steel (SS) for one half of the cylinder and 304 SS for the other half. In the creep-fatigue test, the hold time under a tensile load which produces the primary nominal stress of 45 MPa was one hour at $600^{\circ}C$ and creep-fatigue loads of 600 cycles were applied. The evaluation results for the creep-fatigue crack propagation were compared with those of the observed images from the structural test. The assessment results for the creep-fatigue crack behavior according to the French Al6 procedure showed that the Al6 is overly conservative for the creep-fatigue crack propagation in the present case with a short hold time of one hour.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1995년도 춘계학술대회논문집
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• pp.238-251
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• 1995

Creep of metals has been explained conventionally by dislocation climb and grain boundary sliding indiffusion controlled process. The reorienations of the atoms in the grain by three dimensional crystallizing $\pi$-bondings are visualized as grain rotatins during slow deformation, fold formatin at triple point, increased crevice dspace between grains. grain boundary sliding, grain boundary micration and formation of cracks at the grain boundaries . And also the rupture time and average creep strain rate are explained by the three-dimensional crystallizing $\pi$- bondings and they can be determined by uniaxial tensile test.

• 대한기계학회논문집A
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• 제24권12호
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• pp.3096-3103
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• 2000

High temperature material properties of a welded joint were experimentally studied. Tensile and creep properties were measured for each part of weld metal. HAZ(heat affected zone) and parent metal at 538$^{\circ}C$. HAZ metal was obtained by a simulated heat treatment. Results showed that the order of tensile strength is weld>HAZ> parent both at 24$^{\circ}C$ and at 538$^{\circ}C$. Creep resistance was also the highest for weld metal and lowest for parent metal. Creep rupture life curves were obtained and converted to Monkman-Grant relation which is useful for life assessment. Use of the data obtained in this study is discussed.

• 대한기계학회논문집A
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• 제25권12호
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• pp.1995-2001
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• 2001

In this study, a small punch creep(SP-Creep) test using miniaturized specimen(10${\times}$10${\times}$0.5mm) is described to develop the new creep test method for high temperature structural materials. The SP-Creep test is applied to 2.25Cr-lMo(STBA24) steel which is widely used as boiler tube material. The test temperatures applied for the creep deformation of miniaturized specimens are between 550∼600$^{\circ}C$. The SP-Creep curves depend definitely on applied load and creep temperature, and show the three stages of creep behavior like in conventional uniaxial tensile creep curves. The load exponent of miniaturized specimen decreases with increasing test temperature, and its behavior is similar to stress exponent behavior of uniaxial creep test. The creep activation energy obtained from the relationship between SP-Creep rate and test temperature decreases as the applied load increases. A predicting equation or SP-Creep rate for 2.25Cr-lMo steel is suggested. and a good agreement between experimental and calculated data has been found.

• 수산해양기술연구
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• 제29권3호
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• pp.191-199
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• 1993

In order to analysis creep characteristics and elastic recovery of the high-tech fibers for fishing gear materials, creep and elasticity tests were carried out on netting twines made of nylon, kevlar 29 and techmilon respectively. After creep tests, the rupture surface of raw materials was observed by scanning electron microscope(SEM). The results obtained are as follows: 1. Netting twines were arranged in order of creep rupture time as follow: techmilon, kevlar 29, nylon. The creep progressive pace was the fastest in techmilon. 2. In order of the creep elongating, netting twines were arranged as follows: nylon, techmilon, kevlar 29. 3. The rupture time T sub(r) decreased almost linearly with the increase of applied load L on the log-log scaled graph. The empirical equations computed for kevlar 29 and techmilon are as follows: T sub(r kevlar 29)=1.9512$\times$1037L super(-15.773). T sub(r techmilon)=2.7146$\times$1016L super(-6.831). 4. It was observed by SEM that creep was progressed in all netting twines. The difference of rupture morphology was recognized clearly in tensile and creep tests. 5. In order of the elastic recovery, netting twines were arranged as follows: techmilon, kevlar 29, nylon.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2010년도 춘계 학술발표회
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• pp.631-642
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• 2010

This study is experimentally investigated for the characteristics of compression and tensile of lightweight air-trapped soils with uniform quality. Previously, EPS blocks are often used as lightweight embankment, but many problems such as the level difference and cracks caused by plastic(creep) deformation occurred in the EPS blocks. So, a new material development is urgent. By means of alternatives, lightweight air-mixed soil using in-situ soils has been developed and applied to fields. In comparison with EPS block, lightweight air-mixed soil have less creep deformation in long-time, but the strength characteristics of them are different depending on soils where they are obtained. Therefore, the quality management of them is very difficult. In this study, therefore, characteristics of lightweight air-trapped soil samples are investigated. To do this, the lightweight air-trapped soils are prepared using a manufactured sand with uniform quality. To found out the compression and tensile characteristics of lightweight air-tapped soils, unconfined compression test and splitting tensile test are conducted on the specimens prepared with different unit weight, cement-sand ratio and air-pore.

• 콘크리트학회논문집
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• 제23권5호
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• pp.581-590
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• 2011

초고성능 시멘트 복합체(ultra-high-performance cementitious conposites, UHPCC)는 우수한 압축강도와 연성을 나타내기 때문에 구조 부재 적용 시 단면을 상당히 감소시키고, 낮은 물-결합재비와 고분말 혼화 재료의 사용으로 높은 수축 변형률이 발생하게 되어 거푸집 및 보강근 등의 구속에 의한 수축 균열의 발생 가능성이 크다. 그러므로 이 연구에서는 UHPCC의 수축을 저감시키기 위한 방법으로 팽창재와 수축 저감제를 조합하여 혼입하고 자유수축과 구속 수축 거동을 평가하여 적합성 여부를 산정하였다. 실험 결과 팽창재와 수축 저감제를 조합하여 혼입한 경우에 약 40~44%의 자유수축 저감 효과를 보였으며, 잔류 인장응력은 약 35%와 47% 감소하였다. 지속적인 구속 하중에 의한 인장 크리프의 발생으로 탄성 수축 응력의 약 61%, 64%가 이완되는 것으로 나타났으며, 따라서 구속 수축 거동을 평가할 때에는 반드시 크리프 효과를 고려해야 한다고 판단되었다. 구속도는 0.78~0.85로 나타났으며 팽창재와 수축 저감제의 혼입에 의한 영향은 미미하였고 콘크리트 링의 두께가 클수록 감소하는 경향을 보였다. 또한, UHPCC의 인장 크리프 변형률을 측정하고 재령에 따라 변하는 구속 하중을 적용한 4-매개 변수 크리프 예측 모델과 비교하였다.