• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.345-350
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• 2001

In order to perform elastic-plastic fracture mechanical analyses, fracture resistance curves for concerned materials are required. 1T-CT specimen was used to obtain fracture resistance curves. But the fracture resistance curve by the 1T-CT specimen was very conservative to evaluate the integrity of the structure. And fracture resistance curve was affected by the specimen geometry and crack plane orientation. The objective of this paper is to be certain the conservativeness of the fracture resistance curve by the 1T-CT specimen and to provide the additional safety margin. For these, the fracture tests using the real pipe specimen and standard 1T-CT specimen test were performed. 4-point bending jig was manufactured for pipe test and direct current potential drop method was used to measure the crack extension and length for pipe test. From the pipe and the 1T-CT specimen test results, it was observed that the J-integral of the 1T-CT specimen test at the crack initiation point was very small compare to that of the pipe specimen test.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.1062-1067
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• 2003

In this study, corrosion fatigue characteristics of CF8M and CF8A steel were investigated on the simulated PWR condition(Temp.:$316^{\circ}C$, Pres.: 15:MPa). To make the simulated PWR condition. the special test machine consisted of INSTRON, Autoclave, LOOP and Measurement system was developed. As ${\Delta}K$ is ranged from 11 to $20MPa{\sqrt{m}}$, Crack growth rate of PWR condition is faster than air condition. Above $20MPa{\sqrt{m}}$, the crack growth rate of PWR and air condition is similar. Corrosion fatigue characteristics regardless of the ferrite contents($10{\sim}25wt.%$) is not different. After the test, the fracture surface of specimens was examined. It was difficult to verify the fracture modes such as striation, intergranular crack and cleavage and so on. As the ferrite content of CF8M is increased, the more particles covered fracture surface were peeled.

• Carbon letters
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• 제7권1호
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• pp.19-26
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• 2006

Three point bending tests of single edge notched beam (SENB) specimens were carried out to evaluate the fracture behavior of the fine-grain isotropic nuclear grade graphite, IG-11. To measure the crack initiation point and the subsequent crack growth, the direct current potential drop (DCPD) method and a traveling microscope were used. The effects of test variables like initial crack length, specimen thickness, notch type and loading rate on the measured fracture toughness, $K_Q$, were investigated. Based on the test results, the ranges of the test variables to measure the reliable fracture toughness value were proposed. During the crack growth, the rising R-curve behavior was observed in IG-11 graphite when the superficial crack length measured on the specimen surface was used. The increase of crack growth resistance was discussed in terms of crack bridging, crack meandering, crack branching, microcracking and crack deflection, which increase the surface energy and friction force.

• 대한기계학회논문집A
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• 제29권1호
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• pp.107-114
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• 2005

In order to analyze the elastic-plastic fracture behavior of a structure, the fracture resistance curve of the material should be known first. The standard CT specimen was used to obtain the fracture resistance curves of a piping system. However, it is known that the fracture resistance curve by the standard CT specimen is very conservative to evaluate the integrity of a structure. Also the fracture resistance curve is effected by the specimen geometry and the dimensions because of the constraint effect. The objective of this paper is to be certain the conservativeness of the fracture resistance curve by the standard CT specimen and to provide an additional safety margin. For these, the fracture tests using a real pipe specimen and the standard CT specimen test were performed. A 4-point bending jig was manufactured for the pipe test and the direct current potential drop method was used to measure the crack extension and the length for the pipe test. Also finite element analyses were performed with a CT specimen and a pipe in order to prove the additional safety margin. From the result of tests and analyses of the pipe and the standard CT specimen, it was observed that the fracture analysis with the standard CT specimen is conservative and the additional safety margin was proved.

• Corrosion Science and Technology
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• 제18권1호
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• pp.33-38
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• 2019

Alloy 600/182 with excellent mechanical/chemical properties have been utilized for nuclear power plants. Although both alloys are known to have superior corrosion resistance, stress corrosion cracking failure has been an issue in primary water environment of nuclear power plants. Therefore, primary water stress corrosion crack (PWSCC) growth rate tests were conducted to investigate crack growth properties of Alloy 600/182. To investigate PWSCC growth rate, test facilities including water chemistry loop, autoclave, and loading system were constructed. In PWSCC crack growth rate tests, half compact-tension specimens were manufactured. These specimens were then placed inside of the autoclave connected to the loop to provide primary water environment. Tested conditions were set at temperature of $360^{\circ}C$ and pressure of 20MPa. Real time crack growth rates of specimens inside the autoclave were measured by Direct Current potential drop (DCPD) method. To confirm inter-granular (IG) crack as a characteristic of PWSCC, fracture surfaces of tested specimens were observed by SEM. Finally, crack growth rate was derived in a specific stress intensity factor (K) range and similarity with overseas database was identified.

• Nuclear Engineering and Technology
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• 제38권7호
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• pp.639-656
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• 2006

In order to apply the leak before break (LBB) concept to nuclear piping systems, the dynamic strain aging effect of low carbon steel materials has to be taken into account, in compliance with the requirements of the Korean Standard Review Guide (KSRG) 3.6.3-1. For this goal, J-R tests are needed for a range of various temperatures and loading rates, including dynamic loading conditions. In the dynamic loading J-R test, the unloading compliance method can not be applied to measure the crack growth and direct current potential drop (DCPD) method; this method also has a problem defining the crack initiation point. The normalization method is known as a very useful method to determine the J-R curve under dynamic loading because it does not need additional equipment or complicated loading sequences such as electric current or unloading. This method was accepted by the American Society for Testing and Materials (ASTM) as a standard test method E1820 A15 in 2001. However, it has not yet been clearly verified yet if the normalization method is sufficiently reliable to be applied to LBB. In this study, the basic background of the J-integral, LBB and dynamic loading J-R test are explained, and the current status for dynamic loading J-R test methods are reviewed from the view point of LBB for nuclear piping. In particular, the theoretical and historical background of the normalization method which has received attention recently, is summarized. Recent studies for this method are introduced and future works are suggested that may improve the reliability of LBB for nuclear piping.

• 한국재료학회:학술대회논문집
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• 한국재료학회 2011년도 추계학술발표대회
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• pp.39.1-39.1
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• 2011

가압경수로의 압력경계기기는 약 $300^{\circ}C$, 150기압의 고온고압수환경에서 가동되고 있다. 특히 가압기 밀림관은 고온수와, 저온수가 교차하는 부분으로 열성층 형성으로 열적, 기계적 피로 및 수화학환경이 더해진 부식피로 등에 의하여 손상을 받는다. PWR 원전에서 수화학환경은 대표적으로 용존산소(DO) 5ppb, pH 6~8, 용존수소(DH) <30 cc/kg, 온도 $316^{\circ}C$의 환경을 유지하게 된다. 가압기 밀림관에는 오스테나이트계 스테인리스강이 사용되는데, 오스테나이트계 스테인리스강은 고온 수화학환경에 민감한 것으로 알려져 있다. 따라서 오스테나이트계 스테인리강을 공기중에서의 기계적특성 및 피로특성을 향상시키기 위하여 질소를 첨가한 스테인리스강을 제조하여 PWR 원전환경에서의 피로균열성장특성을 평가하였다. 실험에 사용된 재료는 PWR 원전 가압기 밀림관 소재인 Type 347 스테인리스강에 0.0005 wt%가 첨가된 상용재와 0.11 wt% 질소가 첨가된 재료이다. 사용된 시편형상은 두께 5 mm, 폭 25.4 mm의 CT 시편이다. 수화학환경은 150기압, 온도 $316^{\circ}C$, 용존산소(DO) 5ppb, 용존수소(DH) 30 cc/Kg, pH는 약 7로 유지 하였으며, 응력비 0.1, 하중 반복속도 10Hz의 기계적 조건에서 하중제어로 시험을 진행하였다. 균열길이는 직류전위차법(Direct Current Potential Drop: DCPD)을 이용하여 측정하였다. 질소함량이 증가할수록 동일 사이클에서 균열길이가 늦게 성장하였고, 피로균열성장속도도 약간 늦어지는 것으로 나타났다. 각 스테인리스강의 피로파면 관찰결과 상용재는 약 1 ${\mu}m$의 산화물들이 생성되는 반면 질소첨가 스테인리스강은 약 0.1 ${\mu}m$정도 산화물이 생성되었다. 산화막의 두께도 질소가 첨가됨으로써 상용재에 비해 얇게 생성되었다. 따라서 질소가 첨가됨으로써 부식환경에서 내산화성이 향상되었으며, 이는 피로균열성장특성에 영향을 미치는 것으로 판단된다.

• Nuclear Engineering and Technology
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• 제51권1호
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• pp.228-237
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• 2019

The crack growth responses of as-received and as-welded Alloy 600/182 and Alloy 690/152 welds to constant loading were measured by a direct current potential drop method using compact tension specimens in primary water at $325^{\circ}C$ simulating the normal operating conditions of a nuclear power plant. The as-received Alloy 600 showed crack growth rates (CGRs) between $9.6{\times}10^{-9}mm/s$ and $3.8{\times}10^{-8}mm/s$, and the as-welded Alloy 182 had CGRs between $7.9{\times}10^{-8}mm/s$ and $7.5{\times}10^{-7}mm/s$ within the range of the applied loadings. These results indicate that Alloys 600 and 182 are susceptible to cracking. The average CGR of the as-welded Alloy 152 was found to be $2.8{\times}10^{-9}mm/s$. Therefore, Alloy 152 was proven to be highly resistant to cracking. The as-received Alloy 690 showed no crack growth even with an inhomogeneous banded microstructure. The cracking mode of Alloys 600 and 182 was an intergranular cracking; however, Alloy 152 was revealed to have a mixed (intergranular + transgranular) cracking mode. It appears that the Cr concentration and the microstructural features significantly affect the cracking resistance and the cracking behavior of Ni-base alloys in PWR primary water.