• 비파괴검사학회지
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• 제20권1호
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• pp.18-26
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• 2000

화력발전소 보일러 고온배관인 주증기관, 헤드 및 증기드럼 등의 설비들은 장시간동안 고온고압의 가혹한 조건으로 운전됨에 따라 크리프손상에 의해 열화되고 있다. 이들 설비의 크리프손상측정에 적용되는 종래의 비파괴기법인 레프리카법, 전기저항법 및 경도법 등은 복잡한 측정절차 및 접근성, 검사결과의 신뢰도 및 측정정도 등 여러가지면에서 단점이 많다. 따라서 본 논문에서는 화력발전소 주요 고온배관에서 발생되는 경년열화인 크리프손상에 대한초음파 측정연구를 수행한 결과로서, 고온배관재료인 Cr-Mo강의 크리프 인공열화재를 대상으로 이들에 대한 크리프손상 상태별 초음과 음속(sound velocity) 및 감쇠(attenuation)의 초음파 신호특성을 평가하였다.

• 한국해양공학회지
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• 제13권3호통권33호
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• pp.92-99
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• 1999

High temperature and pressure materials in power plant are degraded by creep damage, if they are exposed to constant loads for long times, which occurs in load bearing structures of pressurized components operationg at elevated temperatures. Many conventional measurement techniques such as replica method, electric resistance method, and hardness test method for measuring creep damgage have been used. So far, the replica method is mainly used for the inspection of high temperature and pressure components. This technique is, however, restricted to applications at the surface of the testpieces and cannot be used to material inside. In this paper, ultrasonic evaluation for the detection of creep damage in the form of cavaties on grain boundaries or intergranular microcracks were carried out. And the absolute measuring method of quantitative ultrasonic velocity technique for Cr-Mo material degradation was analyzed. As a result of ultrasonic tests for crept for specimens, we founded that the sound velocity was decreased as increase of creep life fraction(${phi}c$) and also, confirmed that hardness was decreased as increase of creep life fraction(${phi}c$).

• 한국압력기기공학회 논문집
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• 제16권1호
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• pp.49-55
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• 2020

Creep rupture data for Alloy 690 steam generator tubes in a pressurized water reactor are essentially needed to demonstrate a severe accident scenario on thermally-induced tube failures caused by hot gases in a damaged reactor core. The rupture data were obtained using the tube specimens under different applied-stress levels at 650℃, 700℃, 750℃, 800℃, and 850℃. Important creep constants were proposed using various creep laws in terms of Norton power law, Monkman-Grant (M-G) relation, damage tolerance factor (λ), and Zener-Hollomon parameter (Z). In addition, a creep activation energy (Q) value for Alloy 690 tube was reasonably determined using experimental data. Creep behaviors such as creep strength, creep rates, rupture elongation showed the results of temperature dependence well. Modified M-G plot improved a correlation of the creep rate and rupture life. Damage tolerance factor for Alloy 690 tubes was found to be λ =2.20 in an average value. Creep activation energy for Alloy 690 tube was optimized for Q=350 (kJ/mol). A plot of Z parameter obeyed a good linearity, and the same creep mechanism was inferred to be operative in the present test conditions.

• Computers and Concrete
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• 제4권2호
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• pp.101-117
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• 2007

A new model predicting the nonlinear basic creep behaviour of concrete structures subjected to high multi-axial stresses is proposed. It combines a model based on the thermodynamic framework of the elasto-plastic continuum damage theory for time-independent material behaviour and a rheological model describing phenomenologically the long-term delayed deformation. Strength increase due to ageing is regarded. The general 3D solution for the creep theory is derived from a rate-type form of the uniaxial formulation by the assumption of associated creep flow and a theorem of energy equivalence. The model is able to reproduce linear primary creep as well as secondary and tertiary creep stages under high compressive stresses. For concrete in tension a simple viscoelastic formulation is applied. The material law is then incorporated into a finite element solution procedure for analysis of reinforced concrete structures. Numerical examples of uniaxial creep tests and concrete members show excellent agreement with experimental results.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집A
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• pp.31-36
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• 2000

High temperature and pressure materials in power plant are degraded by creep damage, if they are exposed to constant loads for long times, which occurs in the load bearing structures of pressurized components operating at elevated temperatures. Many conventional measurement techniques such as replica method, electric resistance method, and hardness test method for measuring creep damage have been used. So far, the replica method is mainly used for the Inspection of High temperature and pressure components. This technique is, however, restricted to applications at the surface of the testpieces and cannot be used to material inside. In this paper, ultrasonic evaluation for the detection of creep damage in the form of cavaties on grain boundaries or integranular microcracks are carried out. And the absolute measuring method of quantitative ultrasonic velocity technique for Cr-Mo material degradation is analyzed. As a result of ultrasonic tests for crept specimens, we find that the sound velocity is decreased as the increase of creep life fraction$({\Phi}_c)$ and also, confirmed that hardness is decreased as the increase of creep life fraction$({\Phi}_c)$ but the coefficient of ultrasonic attenuation is increased as the increase of creep life fraction$({\Phi}_c)$. Finally based on the result in this paper, it can be recognized that the ultrasonic techniques using velocities and attenuation coefficient factor are very useful non-destructive methods to evaluate the degree of material degradation in fossile power plants.

• 한국지반신소재학회논문집
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• 제4권4호
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• pp.23-37
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• 2005

지오그리드의 장기설계 인장강도는 크리프 변형, 시공시 손상 및 환경적 요인(온도, 화학적 손상, 생물학적 손상)에 영향을 받는다. 특히, 크리프 변형 및 시공 시 손상이 가장 크게 영향을 미치는 요인으로서 반영된다. 보강토구조물에 대한 대부분의 현행 설계법에서는, 이들 영향요인을 각각 산정하여 이를 장기인장강도 산정에 반영하고 있다. 즉, 이러한 방법에서는 지오그리드의 시공 중 손상과 크리프 변형의 복합효과가 장기 설계인장강도 산정에 어떤 영향을 미치는 가에 대한 검토가 되어 있지 않다. 본 연구에서는 지오그리드의 시공 중 손상과 크리프 변형의 복합효과가 지오그리드의 장기인장강도 산정에 영향을 미치는 영향을 평가하기 위한 일련의 실험적 연구를 수행하였다. 다양한 지오그리드를 대상으로 성토흙 종류에 따른 현장 내시공성시험을 수행하였고, 지오그리드 원시료와 시공중 손상을 입은 시료를 대상으로한 크리프시험을 수행하였다. 연구결과 두 영향인자의 복합효과를 고려하여 산정한 지오그리드의 인장강도 감소계수가 현행 설계법에 의해 산정된 감소계수보다 작은 것으로 나타났다.

• 비파괴검사학회지
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• 제9권1호
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• pp.98-105
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• 1989

Detection of creep damages in centrifugally cast HK-40 reformer furnace catalyst tube is a common problem for the safety and scheduled operation of chemical plants. This study was focused on the development of an automatic ultrasonic scanning device with which the creep damage of welded zone of HK-40 reformer tubes is detected and evaluated.

• 한국압력기기공학회 논문집
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• 제13권1호
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• pp.61-68
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• 2017

Reactor pressure vessel (RPV) under severe accident conditions accompanied by core melting is exposed to direct high-temperature thermal loads. Understanding the creep behavior of the material is one of the most important factors for evaluating the structural integrity at these conditions. While damage evaluation studies have been conducted on critical structures of nuclear power plants through finite element (FE) analyses considering creep behavior, for accurate creep damage evaluation, constitutive equations considered in the FE analyses may have different results depending on the time hardening and strain hardening models as well as the tertiary creep consideration. The purpose of this study is to evaluate the creep damage under severe accident conditions by using FE method for a representative domestic RPV material, SA508 Gr.3. The effect of material hardening models and constitutive equations which are the main variables were also investigated.

• Geomechanics and Engineering
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• 제18권1호
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• pp.71-83
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• 2019

This investigation presented conventional triaxial and creep-permeability tests on sandstones considering thermally-induced damage (TID). The TID had no visible effects on rock surface color, effective porosity and permeability below $300^{\circ}C$ TID level. The permeability enlarged approximately two orders of magnitude as TID increased to $1000^{\circ}C$ level. TID of $700^{\circ}C$ level was a threshold where the influence of TID on the normalized mass and volume of the specimen can be divided into two linear phases. Moreover, no prominent variations in the deformation moduli and peak strength and strain appeared as TID< $500^{\circ}C$ level. It is interesting that the peak strength increased by 24.3% at $700^{\circ}C$ level but decreased by 11.5% at $1000^{\circ}C$ level. The time-related deformation and steady-state creep rate had positive correlations with creep loading and the TID level, whereas the instantaneous modulus showed the opposite. The strain rates under creep failure stresses raised 1-4 orders of magnitude than those at low-stress levels. The permeability was not only dependent on the TID level but also dependent on creep deformation. The TID resulted in large deformation and complexity of failure pattern for the sandstone.

• 대한기계학회논문집A
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• 제35권4호
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• pp.447-452
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• 2011

일반적으로 화력발전 터빈 로터나 케이싱과 같이 고온 고압에서 운전되는 설비의 경우 크리프 및 피로 손상이 주된 손상기구로 알려져 있다. 터빈 설비의 수명을 정확하게 예측하기 위해서 크리프 및 피로 손상 기구를 복합적으로 고려해야 하지만 500MW 급 대용량 터빈 설비의 경우 기저 부하를 담당하기 때문에 기동횟수가 많지 않고 고온에서 장시간 동안 안정적인 운전을 하므로 잔여수명을 결정할 때 크리프 수명을 주로 사용한다. 국내에서 20 년 넘게 운전되고 있는 대용량 터빈의 경우 대부분 니켈, 크롬, 몰리브덴, 바나듐 성분이 포함한 재료로 되어 있다. 본 논문에서는 대용량 터빈의 크리프 수명을 예측하기 위하여 1Cr1Mo1/4V 터빈 로터강에 대한 크리프 손상 모델을 제안하고자 한다.