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크리프 역전 변수 도입에 의한 9Cr강의 크리프 피로 균열성장 거동의 평가
마영화,백운봉,윤기봉,Ma, Young-Wha,Baek, Un-Bong,Yoon, Kee-Bong 대한기계학회 2002 大韓機械學會論文集A Vol.26 No.7
Creep-fatigue crack growth models have been proposed employing $(C_t)_{avg}$ as a crack tip parameter characterizing the time-dependent crack growth. The basic assumptions made in these previous models were ideal creep reversal conditions such as no creep reversal and complete creep reversal condition. Due to this assumption, the applicability of the models was limited since they did not consider partial creep reversal condition which is usually observed in many engineering metals at high temperature. In this paper the creep reversal parameter, Temperature;$C_R$, which was defined by Grover, is critically evaluated to quantity the extent of partial creep reversal at the crack tip. This approach does not rely on any simplifying assumptions regarding the extent of the amount of creep reversal during the unloading part of a trapezoidal fatigue cycles. It is shown that the $(C_t)_{avg}$ value calculated for 9Cr steel agrees well with the experimentally measured one. It is argued that the extent of improvement is not significant when the result is compared with that of the conventional model which has an assumption of full creep reversal behavior.
초기청소년이 지각한 부모-자녀간 의사소통유형과 학교생활적응과의 관계에서 분노의 매개효과
마영화,박성희 한국청소년학회 2019 청소년학연구 Vol.26 No.11
The purpose of this study is to establish a model that displays how anger plays a role as a mediating factor in regard to the influence of a parent-child communication type perceived by early adolescents on school life adaptation and to verify the model based on empirical research. For this, data was collected from 324 early adolescents of grade 5 and 6 at elementary schools located in Seoul. It was taken for analysis, using Pearson’s correlation, and structural equation modeling in SPSS 22.0 and AMOS 22.0. The results of the study is as follows. First, the open parent-child communication type exerts direct influence on school life adaptation. However, problematic communication type does not exert direct influence. Second, the anger of early adolescents had negative effect on school life adaptation. Third, the open parent-child communication type had negative effect on anger of early adolescents and problematic communication type had positive effect on anger of early adolescents. Fourth, anger of early adolescents was affect to have partial mediating effect among open communication type and school life adaptation. However, the anger of early adolescents was affect to have completely mediating effect between the problematic parent-child communication type and school life adaptation. Based on the result of this study, implications and suggestions are discussed. 본 연구는 초기청소년이 지각한 부모-자녀간 의사소통유형이 학교생활적응에 미치는 영향에 대해 분노가 매개하는 연구모형을 설정하고, 실증적 자료를 통해 모형의 적합도를 검증하는데 목적이 있다. 이를 위해 서울시에 소재한 초등학교에 재학 중인 초기청소년(초등학교 5, 6학년) 324명을 대상으로 자료를 수집하였다. 수집된 자료는 SPSS 22.0와 AMOS 22.0 프로그램을 사용하여 상관분석과 구조방정식을 실시하였다. 연구 결과는 다음과 같다; 첫째, 부모-자녀간 개방형 의사소통은 학교생활적응에 직접적 영향을 미쳤으나, 문제형 의사소통은 학교생활적응에 직접적 영향을 미치지 않았다. 둘째, 초기청소년의 분노는 학교생활에 부적영향을 미치는 것으로 나타났다. 셋째, 부모-자녀간 개방형 의사소통 유형은 청소년의 분노에 부적영향을 미치고 문제형 의사소통은 정적영향을 미치는 것으로 나타났다. 넷째, 초기 청소년의 분노는 부모-자녀간의 의사소통 유형 중 개방형 의사소통과 학교생활적응과의 관계에서 부분매개 효과가 있는 것으로 나타났다. 반면, 문제형 의사소통유형과 학교생활적응의 관계에서는 완전매개 효과가 있는 것으로 나타났다. 마지막으로 이러한 연구결과를 바탕으로 연구의 시사점 및 제언을 논의하였다.
온도에 따른 일방향응고 니켈기 초내열합금의 저주기 피로수명 변화
마영화,송전영,구지호,석진익 대한기계학회 2019 大韓機械學會論文集A Vol.43 No.5
The high-temperature parts of a gas-turbine engine eventually fail, due to the accumulated damage caused by the thermal and mechanical cyclic stresses during start-up and shut-down. To prevent such failures, methods for predicting the engine life under various damage mechanisms0 have been studied. In this paper, an experimental study on low-cycle-fatigue (LCF) life assessment was carried out at elevated temperatures. The results showed that the LCF life is on the order of 800>650>950℃. This tendency was consistent with that of the yield-strength variation with temperature. The plastic strain in the tensile behavior was the lowest at 800℃. Thus, it was confirmed that the plastic strain affected the LCF life. Fracture-surface and microstructural observations did not exhibit characteristics that could explain the LCF life tendency with temperature. In the future, additional analyses will be conducted to find the metallurgical factors affecting LCF life. 가스터빈 엔진의 고온부품은 잦은 기동정지에 따른 부하변동으로 인해 열적 및 기계적 반복응력에 기인한 손상이 누적되어 파손이 발생한다. 이에 파손을 방지하기 위한 다양한 손상기구들에 대한 수명예측 방법들이 연구 되고 있다. 본 논문에서는 운전조건에 기반하여 650, 800, 950℃ 온도 하에서 저주기피로 수명예측에 관한 실험연구를 수행하였다. 실험 결과, 저주기 피로수명은 800 > 650 > 950℃ 순으로 나타났다. 이 경향은 온도에 따른 항복강도 변화 경향과 일치하였다. 반면 인장거동 특성에 대한 고찰 결과에서는 800℃에서의 소성변형률이 가장 적게 나타났다. 결국 소성변형률이 저주기 피로수명에 영향을 준 것으로 판단된다. 파단면 및 미세조직 관찰에서는 온도에 따른 저주기 피로수명 경향을 설명할만한 특이점이 발견되지 않았다. 향후 피로수명에 영향을 주는 금속학적 인자를 찾기 위해 r' 의 크기 및 형상, 공정, 탄화물, 결정립 크기, 전위 등 추가적인 분석이 수행될 예정이다.
고온피로균열성장 수명평가 모델 개선을 위한 크리프-역전 변수 도입
마영화(Young-Wha Ma),백운봉(Un-Bong Baek),윤기봉(Kee-Bong Yoon) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8
Crack growth at elevated temperatures becomes an important consideration in estimating the remaining life, in determining the operating conditions and in deciding the inspection criterion and intervals. Fracture mechanics approaches that have been developed over the past few decades have proven useful in providing a rigorous framework for accomplishing these goals. In this paper, for improvement of creep-fatigue crack growth behavior model, it was performed to study for creep-reversal parameter measurement. Because conventional models to evaluate values of (Ct)avg become unuseful for creep-ductile materials with expensive ranges of the effect of cyclic plasticity for creep deformation. creep-fatigue crack growth tests for CR(creep-reversal parameter) measurements were tested by using trapezoidal loading waveforms with hold times of 0, 60, 600 and 3600 seconds. Measurements of the load-line deflection change and degree of reversal during hold periods were made using an elevated temperature clip-on capacitance gage. These reversed deflection changes were then used to calculate the experimental values of CR without any assumptions with regards to the creep deformation behavior of the materials or effects of cyclic plasticity. thus, The experimentally defined creep-reversal parameter, CR, will provide a direct means to accurate calculating (Ct)avg.
횡방향으로 등방성인 크리프 재료에서 고온파괴매개변수 C<SUB>t</SUB>
마영화(Young Wha Ma),윤기봉(Kee Bong Yoon) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11
High temperature crack growth life assessment procedures need to be extended for anisotropic creeping materials such as directionally solidified Ni-based superalloys because they have been developed for isotropic materials. In this study, the previously proposed estimation equation of C<SUB>t</SUB> for the transversely isotropic secondary-creeping materials was applied for the creep crack growth rate data of a directionally solidified Ni-based superalloy which is one of the transversely isotropic materials. Effectiveness of the proposed equation was shown and its limitation was discussed.