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입계 cavitation 손상기구에 기초하는 오스테나이트계 스테인리스강의 크리프-피로 수명예측법과 입계 탄화물 특성 변화에 의한 수명 향상에 관한 연구
洪鉉郁,南壽祐,尹暎喆,洪鎭完 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.4
It is understood that grain boundary cavitation is one of the detrimental processes for the degradation of austenitic stainless steels that reduces the creep-fatigue life at high temperatures. A new damage function based on a model for the creep-fatigue life prediction in terms of nucleation and growth of grain boundary cavities is proposed for austenitic stainless steel. This damage function is a combination of the fatigue and creep terms related to the cavitational damage (cavity nucleation and growth) in the life prediction equation and is found to be generally applicable to all the materials in which failure is controlled by the grain boundary cavitational damage. The cavity nucleation factor, P', which is introduced in the creep-fatigue life model, is found to be closely related with the characteristics of grain boundary carbides acting as cavity nucleation sites. The modification of carbide characteristics through grain boundary serration is successfully made by the special heat treatment so that the modified carbides are favorable for cavitation resistance, resulting in a lowered P' value of material. It is observed that the creep-fatigue life is remarkably improved by the modification of carbide characteristics through grain boundary serration.
홍현욱,최용석,신상민,강창완,Hong, Hyun-Uk,Choi, Yong-Seok,Shin, Sang-Min,Ka, Chang-Wan 한국통계학회 2010 응용통계연구 Vol.23 No.5
정준상관 행렬도는 두 변수군 사이에 연관성이 있는 데이터 행렬을 시각적으로 묘사하고 데이터가 가진 패턴을 찾는데 유용하고, 분석의 더욱 정형화된 방법으로써 결과를 보여주기에도 유용하다. 그럼에도 불구하고, 자료에 결측값이 존재하는 경우에 대부분의 행렬도는 바르게 적용되지 않는다. 이 문제를 해결하기 위해, 결측률에 따라 중앙값과 평균, EM알고리즘, MCMC대체법을 사용해서 결측 자료를 추정한다. 완전하지 않은 자료의 행렬도의 결측값을 추정하더라도, 대체법과 결측률에 따라 행렬도의 모양이 달라진다. 따라서 Shin 둥 (2008)에서 제안한 RMS(root mean square)와 원 행렬도와 추정된 행렬도간의 형상 변동을 측정하고 비교하기 위한 PS(Procrustes statistic)를 사용한다. Canonical correlation biplot is a useful biplot for giving a graphical description of the data matrix which consists of the association between two sets of variables, for detecting patterns and displaying results found by more formal methods of analysis. Nevertheless, when some values are missing in data, most biplots are not directly applicable. To solve this problem, we estimate the missing data using the median, mean, EM algorithm and MCMC imputation methods according to missing rates. Even though we estimate the missing values of biplot of incomplete data, we have different shapes of biplots according to the imputation methods and missing rates. Therefore we use a RMS(root mean square) which was proposed by Shin et al. (2007) and PS(procrustes statistic) for measuring and comparing the shape variability between the original biplots and the estimated biplots.
홍현욱,김태영,김태훈,문준오,이창훈,정준호 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
Recently, there has been a tendency for the buildings to be constructed in the form of high-rise and multipurposes. In addition, to secure the safety design of structures from natural disasters such as earthquakes and fire, the building standards have become more stringent. The present study aimed to design low carbon ferritic and bainitic steels with different contents of Mo, Ti, and Nb for both seismic and fire-resistant qualifications. The hot-rolled steel containing 0.3 wt% Mo-0.02 wt% Nb (‘A’ hereinafter) was primarily strengthened by bainitic transformation, whilst the steel with 0.2 wt% Mo-0.13 wt% Ti (‘B’ hereinafter) consisted of ferrite with a high density of nano-sized (Ti,Mo)-rich MX precipitates. The bainitic microstructure (‘A’ steel) was quite favorable to high-temperature strength and thermal stability. The yield strength of ‘A’ steel at both room and 600 °C temperatures increased consistently with increasing thermal exposure time (600 °C/200-1000 h), since the precipitation of NbC particles occurred while maintaining bainitic ferrite platelets with a high density of dislocations during exposure. During low cycle fatigue tests at room temperature, the main different feature between the two steels is that the ‘A’ steel showed cyclic softening while cyclic hardening was evident in the ‘B’ steel. The bainitic microstructure showed a better fatigue life due to increased ductility manifested by cyclic softening, by which dislocation cell was developed.