• 한국자동차공학회논문집
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• 제3권1호
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• pp.1-12
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• 1995

Isothermal and thermal-mechanical fatigue characteristics of 12Cr heat resisting steel used for high temperature applications were investigated including hold time effects. Isothermal low cycle fatigue test at $600^{\circ}C$ and in-phase, out-of-phase thermal-mechanical fatigue test at 350 to $600^{\circ}C$ were conducted using smooth cylindrical hollow specimen under strain-control with total strain ranges from 0.006 to 0.015. Regardless of thermal-mechanical and isothermal fatigue tests, cyclic softening behavior was observed and much more pronounced in the thermal-mechanical fatigue tests with hold times due to the stress relaxation during the hold time. The phase difference between temperature and strain in thermal-mechanical fatigue tests resulted in significantly shorter fatigue life for out-of-phase compared to in-phase. The differences in fatigue lives were dependent upon the magnitudes of plastic strain ranges and mean stresses. During the hold time in the strain-controlled fatigue tests, the increase in the plastic strain range and the stress relaxation were observed. It appeared that the increase in plastic strain range per cycle and the introduction of creep damage made important contributions to the reduction of thermal-mechanical fatigue life with hold time, and the life reduction tendency was more remarkable in the in-phase than in the out-of-phase thermal-mechanical fatigue. Isothermal fatigue tests performed under the combination of fast and slow strain rates at $600^{\circ}C$ showed that the fatigue life decreased as the strain rate and frequency decreased,especially for the low strain ranges.

• 대한기계학회논문집A
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• 제34권9호
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• pp.1265-1272
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• 2010

최근 들어 화력발전소는 잦은 기동과 부하 변동 하에서 안정적인 운전 및 관리에 대한 요구가 높아지고 있다. 특히 터빈, 보일러와 같이 고온 고압의 조건에서 운전되는 발전 설비의 경우 크리프 및 피로 손상의 영향으로 설비의 수명이 감소하게 된다. 보다 안전한 발전소 운영을 위해 설비의 정확한 수명평가가 중요하며 현재까지 다양한 방법이 개발되어 적용되고 있다. 그러나 현재까지는 표준화된 가이드라인이나 절차 없이 정성적/준정량적 분석에 의해 주요 설비의 수명을 평가하고 있다. 본 연구에서는 크리프 및 피로 손상기구에 근거하여 국내 화력발전 주요 설비의 수명 소비율을 평가하는 표준화된 기준을 개발하였고 실제 설비에 적용하여 평가 기준의 활용성을 검증하였다. 본 기준은 2010 전력기술기준에 수록될 예정으로 수명평가의 정확성 향상과 수명관리 표준화에 기여할 것이다.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2004년도 추계학술발표회 발표논문집
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• pp.1291-1292
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• 2004

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2005년도 추계학술발표회
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• pp.101-102
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• 2005

• 콘크리트학회논문집
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• 제13권5호
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• pp.415-422
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• 2001

본 연구에서는 반복하중을 받는 철근콘크리트보의 피로현상에 피로의 지속으로 인한 크리프 특성을 고려함으로서 실제의 반복사용하중상태에서 크리프와 피로가 공존하는 것을 반영하여 모델을 구성하였다. 반복크리프 모델에서, 철근 콘크리트 보의 휨손상 진행에 따른 압축영역의 손상속도를 정확히 대표할 수 있는 반복크리프 지수(n)의 특성을 도입하여 평가하였다. 이 지수는 크리프특성의 지수 n$_1$과 피로특성의 지수 n$_2$로 구분하였다. 모델의 적정성을 평가하기 위하여 철근콘크리트보의 피로실험을 수행하였으며, n$_2$의 경우 보통강도에서 고강도까지 콘크리트의 강도와 하중크기를 주요변수로 실험하였다. 본 실험결과로부터 반복하중에 의한 크리프모델의 지수를 결정하였다. 본 연구에서 제시된 반복크리프모델은 철근콘크리트 보의 피로손상의 누적과 처짐거동을 잘 설명할 수 있는 것으로 나타났으며, 앞으로 이들 부재의 피로거동해석에 유용하게 적용될 수 있을 것으로 사료된다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.1740-1743
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• 2007

In this study, two types of fatigue tests were conducted. First, cyclic bending tests were performed using the micro-bending tester. A four-point bending test method was adopted, because it induces uniform stress fields within a loading span. Second, thermal fatigue tests were conducted using a pseudo power cycling machine which was newly developed for a realistic testing condition. The pseudo-power cycling method makes up for the weak points in a power cycling and a chamber cycling method. Two compositions of solder are tested in all test condition, one is lead-free solder (95.5Sn4.0Ag0.5Cu) and the other is eutectic lead-contained solder (63Sn37Pb). In the cyclic bending test, the solder that exhibits a good reliability can be reversed depending on the load conditions. The lead-contained solders have a longer fatigue life in the region where the applied load is high. On the contrary, the lead-free solder sustained more cyclic loads in the small load region. A similar trend was detected at the thermal cycling test. A three-dimensional finite element analysis model was constructed. A finite element analysis using ABAQUS was performed to extract the applied stress and strain in the solder joints. A constitutive model which includes both creep and plasticity was employed. Thermal fatigue was occurred due to the creep. And plastic deformation is main damage for bending failure. From the inelastic energy dissipation per cycle versus fatigue life curve, it can be found that the bending fatigue life is longer than the thermal fatigue life.

• 동력기계공학회지
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• 제16권1호
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• pp.58-64
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• 2012

Power plant boiler is one of the most important utilities providing steam to turbine in thermal power plant. It is composed of thousands of boiler tubes for high efficient heat transfer. Boiler tube material is used in such high temperature and pressure as $540^{\circ}C$, $170kg/mm^2$. The boiler tube material is needed to resist corrosion damage, creep damage and fatigue damage. 2.25%Cr-1Mo steel is used for conventional boiler tubes. In these days steam temperature and pressure of the power plant became higher for high plant efficiency. So, the material property of boiler tube must be upgraded to meet the plant property. Several boiler tube material was developed to meet such condition. X20CrMoV12.1 steel is also developed in early 1980's and used for superheater and reheater tubes in supercritical boilers. The material has martensitic structure, which is difficult to evaluate the material degradation. Boiler tube material at severe condition was tested to evaluate long term and short term degradation and creep. Through long term and high temperature degradation test, lath structure was decreased and recrystallization has been proceeded by sub-crystal. And in this research the effect of temperature and stress on boiler tube characteristic,for example, deformation by creep was changed rapidly at relatively high temperature and stress because creep was affected easily by temperature and stress.

• 한국안전학회지
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• 제24권2호
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• pp.17-22
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• 2009

Thermal stress and elastic creeping stress analysis was conducted by finite element method to simulate start-up process of a boiler header of 500MW standard fossil power plant. Start-up temperature and operating pressure history were simplified from the real field data and they were used for the thermal stress analysis. Two kinds of thermal stress analysis were considered. In the first case only temperature increase was considered and in the second case both of temperature and operating pressure histories were considered. In the first analysis peak stress was occurred during the temperature increase from the room temperature. Hence cracking or fracture may occur at the temperature far below the operating maximum temperature. In the results of the second analysis von Mises stress appeared to be higher after the second temperature increase. This is due to internal pressure increase not due to the thermal stress. When the stress components of radial(r), hoop($\theta$) and longitudinal(z) stress were investigated, compression hoop stress was occurred at inner surface of the stub tube when the temperature increased from room temperature to elevated temperature. Then it was changed to tension hoop stress and increased because of the operating pressure. It was expected that frequent start-up and shut-down operations could cause thermal fatigue damage and cracking at the stub tube hole in the header. Elastic-creeping analysis was also carried out to investigate the stress relaxation due to creep and stabilized stress after considerable elapsed time. The results could be used for assessing the creep damage and the residual life of the boiler header during the long-tenn service.

• 한국도로학회논문집
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• 제7권2호
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• pp.69-76
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• 2005

반복적인 교통하중은 아스팔트 포장에 손상을 누적시키는 반면, 휴식기 동안 아스팔트 혼합물에는 축적된 손상의 회복현상이 지속적으로 일어난다. 이러한 손상회복 현상은 아스팔트 포장의 피로수명을 증가시킨다. 본 연구에서는 간접인장시험을 이용하여 아스팔트 혼합물의 손상회복특성을 측정할 수 있는 시험방법을 개발하고, 이를 통해 일단 아스팔트와 개질 아스팔트를 포함하는 총 네 가지 아스팔트 혼합물의 손상회복특성을 평가하였다. 본 연구에서 개발한 손상회복시험은 반복재하시험과 일련의 $M_R$시험으로 구성된다. 반복재하시험과 $M_R$ 시험 동안 회복변형 (resilient deformation)을 측정하고 응력과 변형률을 이용하여 DCSE (Dissipated Creep Strain Energy)를 구하고 이를 통해 아스팔트 혼합물의 손상회복특성을 평가할 수 있는 파라메터로서 $DCSE/DCSE_{applied}$를 이용한 정규화된 손상회복속도 (normalized healing rate)를 제안하였다. 실험결과, 아스팔트 혼합물의 손상회복속도는 온도가 증가함에 따라 지수 함수적으로 증가함을 알 수 있었으며, 아스팔트 혼합물의 손상회복특성은 폴리머 개질과 같은 아스팔트 바인더의 특성보다는 아스팔트 함량과 같은 혼합물의 구조적인 특성에 더 많은 영향을 받음을 알 수 있었다.

• 마이크로전자및패키징학회지
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• 제17권3호
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• pp.33-41
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• 2010

Crack propagation mechanisms of Sn-3.0Ag-0.5Cu solder were studied in strain controlled push-pull creepfatigue conditions using the fast-fast (pp) and the slow-fast (cp) strain waveforms at 313 K. Transgranular cracking was found in the pp strain waveform which led to the cycle-dominant crack propagation and intergranular cracking in the cp strain waveform that led to the time-dominant crack propagation. The time-dominant crack propagation rate was faster than the cycle-dominant crack propagation rate when compared with J-integral range which resulted from the creep damage at the crack tip in the cp strain waveform. Clear recrystallization around the crack was found in the pp and the cp strain waveforms, but the recrystallized grain size in the cp strain waveform was smaller than that in the pp strain waveform. The cycle-dominant crack propagated in the normal direction to the specimen axis macroscopically, but the time-dominant crack propagated in the shear direction which was discussed in relation with shear micro cracks formed at the crack tip.