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Seulbi Lee,Jaewoong Kim,Jungho Choe,Seong‑Woong Kim,Jae‑Keun Hong,최윤석 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.1
The crack formation mechanisms of Ti–48Al–2Cr–2Nb single tracks processed by laser powder bed fusion were extensivelyinvestigated in a wide range of laser powers and scan speeds. The crack patterns were categorized by their directionalities,which were parallel (longitudinal crack) and/or perpendicular (transverse crack) to the scan direction. For the representativeprocess conditions of the keyhole, transition, and conduction modes, cracking behaviors were characterized by combiningthe fractography and the microstructural analysis. Further, thermal-mechanical finite element method simulations wereperformed to predict the distribution of temperatures and thermal stresses during the melt pool formation. On the basis ofthe combined results, the cracks formed in keyhole, transition, and conduction modes were clarified as a solidification crackand/or a thermal crack. In addition, the formation of these cracks was thoroughly understood in terms of thermal stresses andmicrostructural factors that affect the crack susceptibility. Finally, comprehensive mechanisms responsible for cracking ofTi–48Al–2Cr–2Nb single tracks under laser powder bed fusion were proposed for different process conditions (the keyhole,transition and conduction modes).
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Seulbi Lee,Jaewoong Kim,Do‑Sik Shim,박상후,최윤석 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.5
Single track and multi-layers of medium-carbon steel (AISI H13) powders were additively deposited on gray cast ironusing a directed energy deposition (DED) technique. Multi-scale microstructure characterization using a scanning electronmicroscopy, an electron probe micro-analyzer and a scanning transmission electron microscopy, and the thermal analysisusing finite element method were performed to understand the mechanism for the micro-crack formation, which was foundat the single, double and triple layers of medium-carbon steel powders, except for the single track, deposited on a cast ironsubstrate. The crack formation mechanism was thoroughly investigated by combining the layer-by-layer chemistry variationsand the resulting microstructural evolution upon exposure to thermal cycles during the DED process. It was concluded thatmicro-cracking was accompanied by the initiation of the local graphitization and the formation of plate martensite in thevicinity, which seemed to be kinetically facilitated by an indirect exposure to multiple heating–cooling thermal cycles, notby a direct exposure to a melting-cooling (solidification) cycle.
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전극 개수에 따른 근전도 기반 휴먼-컴퓨터 인터페이스의 정확도에 대한 연구
이슬비(Seulbi Lee),지영준(Youngjoon Chee) 한국HCI학회 2015 한국HCI학회 논문지 Vol.10 No.2
NUI(Natural User Interface)는 사용자의 자연스러운 동작이나 동작 시 발생하는 생체 신호를 해석하여 기계에 명령을 내리는 것을 말한다. 물리적인 변화가 있어야 사용이 가능한 가속도 센서나 영상 기반의 NUI와는 달리 특정 동작과 관련된 근육의 표면 근전도(surface Electromyogram, sEMG)를 측정하면 실제 움직임이 발생하지 않아도(isometric contraction) 동작 의도를 예측할 수 있다. 본 연구에서는 근전도 기반으로 손목 동작 의도를 분류할때 전극 개수에 따른 정확도를 확인하고, 키보드 등에 적용 가능한 인터페이스 기술을 제안한다. 손목의 동작 중 신전(extension, up), 굴곡(flexion, down), 외전(abduction, right), 내전(adduction, left)의 네 가지 동작 의도를 분류하는 실험을 진행하였다. 50ms 간격으로 계산된 제곱평균제곱근(Root Mean Square, RMS)을 특징으로 사용하였고, 동작 의도 인식을 위해 역전파 알고리즘으로 학습한 다층 퍼셉트론 분류기를 사용하였다. 전극 쌍의 개수를 네개(91.9%), 세 개(87.0%), 두 개(78.9%)로 줄여가며 정확도를 확인했다. 전극 쌍의 개수가 네 개에서 두 개로 줄었을 때 정확도는 약 13% 감소하였다. 두 쌍의 전극만 사용하는 경우의 분류 정확도를 높이기 위하여 직전의 RMS를 특징에 추가하였다. 150 ms 이전까지의 정보를 사용하였을 때, 분류 정확도가 78.9%에서 83.6%로 4.6% 증가하였다. 전극 쌍의 개수가 감소함에 따라 정확도는 감소하였지만, 이전 데이터를 함께 사용한 경우 부분적으로 증가 시킬 수 있음을 확인하였다. NUI (Natural User Interface) system interprets the user’s natural movement or the signals from human body to the machine. sEMG (surface electromyogram) can be observed when there is any effort in muscle even without actual movement, which is impossible with camera and accelerometer based NUI system. In sEMG based movement recognition system, the minimal number of electrodes is preferred to minimize the inconvenience. We analyzed the decrease in recognition accuracy as decreasing the number of electrodes. For the four kinds of movement intention without movement, extension (up), flexion (down), abduction (right), and adduction (left), the multilayer perceptron classifier was used with the features of RMS (Root Mean Square) from sEMG. The classification accuracy was 91.9% in four channels, 87.0% in three channels, and 78.9% in two channels. To increase the accuracy in two channels of sEMG, RMSs from previous time epoch (50-200 ms) were used in addition. With the RMSs from 150 ms, the accuracy was increased from 78.9% to 83.6%. The decrease in accuracy with minimal number of electrodes could be compensated partly by utilizing more features in previous RMSs.
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Regression Model to Predict Nitrogen Mineralization Using Soil Parameters for Pepper Upland
Yejin Lee,Seulbi Lee,Jwakyung Sung 한국토양비료학회 2020 한국토양비료학회지 Vol.53 No.4
One of the most important practices for nitrogen (N) management in arable lands is to minimize N loss using soil-customized N supply. Despite that N mineralization of soils is an useful parameter to manage N fertilization, it requires a lot of analysis. To predict N mineralization rate of upland soils, we performed regression analysis between N mineralization parameters (Mineralized N, N0, k) and soil properties. Experimental soils were randomly sampled from 40 pepper cultivation fields before fertilization. The rate of soil N mineralization was analyzed through incubation for 16 weeks at 27℃. N mineralization parameters of soils were estimated from first-order kinetic model, and a stepwise regression was conducted to predict the mineralized N, N0 and k using soil properties. Mineralized N was linearly correlated with EC (R²=0.745) which meant a good indicator to predict N mineralization rate. N0 as potentially mineralizable N was positively correlated with soil organic matter (SOM) and total N, and those represented a linear relationship (y=0.0682x, R²=0.729). SOM, sand, clay and C/N ratio were selected as parameters for prediction of N0 (R²=0.448). In addition, a regression equation for estimation of k was composed of EC, CEC, sand and clay (R²=0.508). In conclusion, we suggest that N mineralization rate by combination with soil properties could be determined with a regression equation. However, model equation represented in this study should be validated and improved through field experiments with different levels of N application and different types of soil property.
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이슬비 ( Seulbi Lee ),오충광 ( Chunggwang Oh ),허재홍 ( Jaehong Heo ) 사단법인 아시아문화학술원 2021 인문사회 21 Vol.12 No.6
이 연구는 한국사회에서 구성원의 정신건강을 증진시키는 것으로 알려져 있는 눈치가 거부민감성을 매개로 정신건강을 증진시키는지 알아보는 데 목적이 있다. 이를 검증하기 위해 이 연구에서는 눈치가 거부민감성을 매개로 정신건강을 증진시킬 것이라는 완전매개모형을 연구모형으로 설정하고 이를 검증하고자 하였다. 이를 위해 대학생 310명(남 129명, 여 181명)을 대상으로 자료를 수집하고 구조방정식 모형으로 이를 검증하였다. 그 결과 연구모형은 자료에 잘 부합하는 것으로 나타났다. 이 결과는 주의를 외부상황으로 돌리고 현실적인 상황해석을 하는 눈치 기능이 주관적 안녕감과 심리문제 같은 정신건강에 도움이 된다는 사실과 눈치를 상담전략으로 활용할 수 있다는 점을 시사하였다. 향후 연구에서는 개인 문화성향에 따른 모형 적합도 차이와 매개변인에 대한 추가연구가 필요할 것으로 보인다. The purpose of this study was to investigate whether Nunchi, known to promote the mental health of members of Korean society, enhances the mental health of Koreans through rejection sensitivity (RS). The proposed model was the fully mediated model that Nunchi would enhance mental health in the mediation of RS. To verify this model, data were collected from 310 colleges students (male 129, female 181) and analyzed by the SEM. Results showed that the model fit the data, suggesting that Nunchi, which diverts attention to external situations and interprets the situation realistically, could be beneficial for the mental health such as subjective wellbeing and psychological problems. And Nunchi could be used as the intervention in psychotherapy. In future research, it is necessary to examine whether the model fit varies with individual cultural orientation. And it seems that additional studies on mediating variables are needed.
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금속 적층제조된 대형 실린더 부품의 열변형에 미치는 적층 높이 및 재료 특성 영향에 관한 해석
이슬비(Seulbi Lee),방태양(Tae Yang Bang),김우종(Woojong Kim),강민철(Mincheol Kang),최윤석(Yoon Suk Choi) 대한용접·접합학회 2021 대한용접·접합학회지 Vol.39 No.3
The laser powder bed fusion technique involves depositing layers by selectively melting a material. This leads to rapid melting and solidification, followed by repeated reheating and remelting. The complex thermal hysteresis and resultant thermal gradient inevitably cause thermal stress and distortion in additively manufactured products. The thermal stress and distortion lead to dimensional inaccuracies, defects such as delamination and cracking, and finally, adverse effects on the product life. In this study, a thermomechanical analysis of a large cylinder (hollow shape, with a height of 750 mm, an outer diameter of 500 mm, and a thickness of 20 mm) was performed using the finite element method to investigate the effects of building height and material properties on thermal distortion. Specifically, the evolution of thermal distortion was systematically interpreted with the thermal hysteresis. Depending on the building height of the cylinder, the thermal hysteresis was influenced by the bed plate and the thermal effect of the predeposited and postdeposited layers. In addition, the thermal distortion tendencies of Ti-6Al-4V and Inconel 718 metal powders were examined based on the material properties, particularly thermal diffusivity. The maximum thermal distortion occurred at approximately 100 mm below the top surface of the cylinder for both materials. Moreover, the thermal distortion of the Ti-6Al-4V was more significant than that of Inconel 718 because of the lower thermal diffusivity of the former.
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