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반복 하중 하에서 에너지 기반 손상해석 기법을 적용한 재질열화된 CF8A 재료의 파괴인성 예측 기법
윤교근(Gyo-Geun Youn),남현석(Hyun-Suk Nam),전준영(Jun-Young Jeon),김윤재(Yun-Jae Kim),김진원(Jin-Won Kim) 대한기계학회 2017 대한기계학회 춘추학술대회 Vol.2017 No.11
This paper suggests a method to predict thermal ageing effect of CF8A material under cyclic loading effect. To achieve the goal, FE damage model based on multi-axial ductility theory is used. The model describes the local failure by fracture strain energy and stress-triaxiality. The fracture strain energy of aged CF8A can be determined from tensile test and that of aged CF8A can be assessed adopting thermal ageing constant “C”. To simulate the material behaviour under cyclic loading, combined hardening model is adopted and to predict fracture toughness under cyclic loading condition, two assumptions are made. One is about energy conservation and the other is about crack opening. From the assumptions, same fracture criteria are used to predict fracture toughness under cyclic loading condition.
취화 상수를 적용한 열취화 및 수소취화 재료의 파괴인성 정량화
윤교근(Gyo-Geun Youn),김윤재(Yun-Jae Kim),김진원(Jin-Weon Kim),Poh-Sang Lam,Yasufumi Miura 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4
원전 기기는 가동 조건 하에서 다양한 재질 열화 현상을 겪게 되며, 그 중 대표적인 현상이 열취화와 수소취화다. 열취화는 주조스테인리스강 및 용접스테인리스강이 가동 조건인 고온에 노출되면, δ-페라이트의 상변화에 의해 발생한다. 수소취화는 금속이 수소 환경에 노출되어 있을 때, 금속 내부에 수소가 확산 침입함으로써 발생한다. 비록 열취화와 수소취화의 발생 메커니즘은 다를지라도, 재료의 연성 및 파괴인성이 감소하는 현상은 동일하다. 기존 연구에서 열취화된 주조스테인리스강의 파괴인성을 예측하기 위해 취화 상수 개념을 제안한 바 있으며, 본 연구에서는 열취화와 수소취화 모두 재료의 연성 및 파괴인성이 감소하는 현상에 착안하여 용접 스테인리스강과 수소취화된 스테인리스강에 취화 상수 개념을 도입하여 파괴 인성을 예측할 수 있는 방법론을 제안하였다. 본 연구에는 열취화된 GTAW (gas tungsten arc welding) 316L stainless steel 및 수소취화된 21-6-9 stainless steel 재료가 사용되었으며, 취화 상수 개념을 적용하여 파괴인성을 예측한 결과가 실험 결과와 잘 일치함을 확인하였다. 이를 통해 적은 취화 실험 데이터로도 열취화 및 수소취화된 재료의 파괴 인성을 예측할 수 있음을 보였다. Nuclear power plant undergoes material degradation under operating condition, and thermal aging and hydrogen embrittlement are the representative material degradation. Thermal aging occurs due to a phase change of δ-ferrite when cast stainless steels or welded stainless steels are exposed to high temperature. Since operating temperature is about 280-320oC, it is high enough to cause thermal aging. Hydrogen embrittlement occurs when the metal is exposed to a hydrogen environment, as hydrogen diffuses inside the metal. Although the mechanisms of thermal aging and hydrogen embrittlement are different, the phenomenon of reducing the ductility and fracture toughness of the material is the same. In our previous study, the concept of embrittlement constant was introduced to predict the fracture toughness of thermally aged cast stainless steel. In this study, by focusing on the phenomenon that the ductility and fracture toughness decreases due to both thermal aging and hydrogen embrittlement, a simple method based on embrittlement constant was proposed to predict the fracture toughness of thermally aged weld stainless steel and hydrogen embrittled stainless steel. Thermally aged GTAW (gas tungsten arc weld) 316L stainless steels and hydrogen embrittled 21-6-9 stainless steels were considered in this study. It was shown that the result of predicting fracture toughness by applying the concept of embrittlement constant was in good agreement with the experimental data.
서준민,윤교근,이현재,오영진,김윤재,Seo, Jun-Min,Youn, Gyo-Geun,Lee, Hyun-Jae,Oh, Young-Jin,Kim, Yun-Jae 한국압력기기공학회 2018 한국압력기기공학회 논문집 Vol.14 No.1
When designing high temperature piping system, creep phenomena must be considered. Since ASME code does not provide detailed methods of design by rule (DBR) for high temperature piping, Finite element analysis should be performed. However, In the case of piping system with frequent design changes, creep analysis of the entire piping system for every change is ineffective and practically impossible. Therefore, based on elastic and elastic-plastic analysis, which takes a relatively short time, the creep stress is predicted by using elastic follow-up factor method provided in R5 code and plastic-creep analogy presented by Hoff. The predicted creep stress for a virtual piping system was compared with the creep analysis result and the two results showed similar stress relaxation tendency in time.