Impact of Korean students’ individual learning time on math performance: differential effect of teachers’ assessment competency

변상민,김나영 서울대학교 교육연구소 2020 Asia Pacific Education Review Vol.21 No.4

As the 2015 revised national curriculum is being implementing in Korea, a teacher’s assessment competency has become a point of focus for the potential enhancement of students’ self-directed learning. Relevantly, a teacher’s competency in student assessment is correlated to his/her individual feedback practices. Although students’ self-directed learning affects academic performance and may vary based on their teachers’ assessment competency, few studies have investigated the effect of a teacher’s assessment competency, specially feedback practices, on student outcomes. Therefore, this study aims to empirically investigate the differential effect of a teacher’s assessment competency, particularly focusing on feedback practices (i.e., formative assessment feedback, homework feedback), on Korean students’ individual learning times and math achievements. For the analysis, a 3-level hierarchical linear modeling (HLM) is employed using data of middle school students (N = 2942) and math teachers (N = 426) in 207 middle schools of the Gyeonggi Education Panel Study (GEPS). The study revealed that a teacher’s formative assessment feedback has a positive differential effect on math performance when controlling for student-, teacher-, and school-level covariates. In addition, the effect of individual learning time on math achievement was significantly dependent on teachers’ homework feedback. The study provides newfound empirical evidence that teacher feedback plays a pivotal role in promoting a student’s self-directed learning and academic achievement. Based on these findings, the implications of improving a teacher’s assessment competency are discussed.

유한 요소법을 이용한 이중 금속봉 압출 금형 형상 최적화 ( Die Shape Optimization in Bimetal Extrusion by the Finite Element Method )

변상민,황상무 대한기계학회 1994 대한기계학회 춘추학술대회 Vol.- No.-

다중센서를 이용하는 쿼드콥터 자율항법 알고리즘

변상민,이경민,윤석훈 한국공학안전보건예술학회 2013 한국공학예술학회 논문지 Vol.5 No.-

본 논문에서는 4개의 로터를 이용하여 구동되는 쿼드콥터의 다중센서 기반 자율항법 알고리즘을 제안한다. 고도 측정을 위한 초음파 센서, 물체인식을 위한 적외선 센서 및 쿼드콥터의 기본자세 제어를 위한 GYRO 센서와 가속도 센서를 이용 한다. 자세 유지를 위해 PID 제어 기법이 사용되며 안정적인 이동 비행을 위하여 지수 가중 이동 평균 필터를 사용을 제안 한다. 테스트베드를 이용한 시험결과에 따르면 제안하는 항법 알고리즘을 이용하여 안정적이고 정확한 쿼드콥터의 비행제 어가 가능하다. In this paper, we propose a multi-sensor based autonomous navigation algorithm for Quadcopter, which is driven by four rotors. Multiple sensors include ultra-sonic sensors for height estimate, ultra-red sensors for identifying objects, and Gyro sensor & accelerometer for the posture control. PID control mechanism is also used, and the use of exponential weighted moving average filter is proposed. According to the test results using a testbed, the proposed algorithm can accurately control a Quadcopter for the stable flight.

원형강관 편심 플랜지 접합부의 인장성능에 관한 연구

변상민,최열,이수범,강상구 대한건축학회지회연합회 2023 대한건축학회연합논문집 Vol.25 No.2

This study is an experimental study to evaluate the tensile performance of an eccentric flange-bolted joint that cuts the one side of flange exposed to the outside to install finishing materials and prevent damage when connecting flanges of steel pipes. The characteristics of tensile behavior according to the number and thickness of flanges (flange thickness: 4.0, 4.5, 6.0, 8.0, 9.0 mm, number of bolts: 3, 4, 5) were investigated. In addition, it was simulated through linear finite element analysis, and as a result of the experiment, when the number of bolts and flange thickness of the eccentric joint increased, the yield strength and maximum strength except for some increased. It was found that the ductility decreased as the flange thickness increased. It was found that the yield strength and maximum strength decreased when the number of EFP bolts was 5 compared to when the number of bolts was 4. When estimating the cause of the decrease, it is considered that the bolt pitch is narrow, so the ductility of the plate is lowered, and the flange does not contribute to the overall tensile joint strength, and the tensile member alone reaches the maximum strength. Based on this study, various theoretical and numerical analysis studies should be conducted, and this series of studies can present economical and efficient guidelines for the installation and external finishing of steel pipe truss structures.

실리콘 강판 압연시 에지크랙 발생에 관한 유한요소해석

변상민,이재현,김상록,장윤찬,나두현,이종빈,이규택,송길호,이성진,Byon, Sang-Min,Lee, Jae-Hyun,Kim, Sang-Rok,Jang, Yun-Chan,Na, Doo-Hyun,Lee, Jong-Bin,Lee, Gyu-Taek,Song, Gil-Ho,Lee, Sung-Jin 한국전산구조공학회 2009 한국전산구조공학회논문집 Vol.22 No.6

본 논문에서는 냉간압연 중에 실리콘 강판의 에지부에 발생하는 크랙을 해석하는 방법을 제시하였다. 본 방법은 손상역학의 개념에 기초하여 소재의 변형 중 발생하는 손상개시와 손상진전 및 파괴의 형태로 순차적으로 해석하는 기법이다. 이를 통해 압연 중에 크랙 발생 유무와 발생 후 크랙의 진전된 길이 및 형상을 예측할 수 있다. 소재의 파괴물성치는 일반 판상시편 및 노치 판상시편의 인장실험을 통해서 도출하였다. 압연중 발생하는 에지크랙 해석결과가 얼마만큼 정확한지 평가하기 위해서 시험압연기를 통해 얻은 압연시편과 직접 형상 비교를 수행하였다. 크랙의 길이 및 방향 측면에서 본 해석기법의 예측정도가 실험에 상당부분 근접함을 알 수 있었다. 마지막으로 실리콘 강판의 실제 압연공정으로 에지크랙 해석을 수행하여 초기크랙 길이 및 압연기 출측 강판 감기 인장력에 따른 크랙 진전 거동에 대해 분석하였다. In this paper an finite element approach for the edge crack analysis of silicon-steel sheet during cold rolling is presented. Based on the damage mechanics, the proposed approach follows the analysis steps which are composed of damage initiation, damage evolution and fracture. Through those steps, we can find out the initiation instant of crack and resulting propagated length and shape of the crack. The material constants related to fracture is experimentally obtained by tension tests using standard sheet-type specimen and notched sheet-type specimen. To evaluate the prediction accuracy, we performed a pilot rolling test with a initially notched sheets. It is shown that the results obtained by the approach converged to the experimental one concerning about the direction and length of propagated crack. The capability of the proposed one is demonstrated through the application to the actual silicon-steel rolling mill.

유한요소법을 이용한 열연공정의 Pass Schedule 최적설계

변상민,황상무 ( S . M . Byon,S . M . Hwang ) 대한금속재료학회 1996 압연기술의 현재와 미래 Vol.- No.-

Experimental Study for Roll Gap Adjustment due to Roll Wear in Single-Stand Rolling and Multi-Stand Rolling Test

변상민,박흥식,이영석 대한기계학회 2008 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.22 No.5

To investigate a correlation between the amount of wear and roll gap (pass height) adjustment, we performed a sin-gle-stand reversible pilot groove rolling test as well as rolling test in an actual rod mill. In case of the pilot rolling test, we designed wear contour (profile) and machined it on the original roll groove (i.e., roll groove with no wear) to make the roll groove worn down. For the actual rod mill test, we developed a measuring device which can detect the actual wear profile. To determine the amount of roll gap adjustment, we propose a model for equivalent roll gap decrement which re-duces the increased exit cross sectional area due to wear. We applied the proposed model to multi-stand rolling (rough-ing train of POSCO No. 2 Rod Mill) as well as single-stand reversible rolling. The wear profile of rolls worn down at each stand in mill yard was measured at different roll tonnage. The pilot hot rolling test shows that variation of exit cross sectional area is almost linearly proportional to roll gap change while the roll gap decreases from reference roll gap (6.5m) to 3.5mm. In an actual rod mill which has con-secutive rolling system, relationship between tonage (total amount of tons that the produced rod weighs) and roll gap shape of incoming workpiece.

H-beam 유니버셜 마무리 압연 패스에서 플랜지 부의 압연하중 예측 근사모델에 관한 연구

변상민,김성기,이영석 대한기계학회 2024 大韓機械學會論文集A Vol.48 No.1

본 논문은 H-beam 유니버셜 마무리 압연기에서 플랜지 부의 압연하중을 예측하기 위한 근사모델을 제시한다. 압연의 기하학적 특성을 모델에 반영하기 위해 판 압연 이론에 기초하여 무차원 특성 함수를 도입하였다. 유한요소해석을 통해 무차원 특성 함수에 미치는 각 압연 형상 변수의 의존성을 파악하였다. 무차원 특성 함수의 경향을 가장 잘 나타낼 수 있는 수학모델을 설정하고 유한요소해석 결과와 가장 잘 일치시킬 수 있도록 매개변수들을 최적화하였다. 플랜지 압하율, 웹 압하율 및 소재 두께에 상대적인 롤 반경 비가 핵심 인자로 파악되었다. 판 압연과는 다르게 H-beam 압연에서는 압하율이 영에 근접할 때 무차원 특성 함수가 가장 큰 값을 가지고 압하율 증가에 따라 감소하는 특성을 나타내었다. 제시된 압연하중 근사모델의 예측치는 유한요소해석 예측치와 6.96%의 평균 편차율을 보였다. This study proposes an approximate model for predicting the roll force of an H-beam flange during universal finishing rolling. In order to reflect the geometric characteristics of rolling to the model, a dimensionless characteristic function was introduced based on the flat rolling theory. The approximate model was formulated by examining the relationship between geometric rolling factors and the dimensionless characteristic function and optimizing function parameters to best match the results of finite element analysis. The results indicated that the thickness reduction ratios of the flange and web, and the roll radius ratio relative to thickness were critical geometric factors. The dimensionless characteristic function exhibited the highest value when the reduction ratio was zero and a decrease when the reduction ratio increased. The predicted value of the proposed model exhibited an average deviation rate of 6.96% compared to the results of finite element analysis.

반복-수평력을 받는 비내력벽체 및 보-기둥 접합부의 내력 실험 연구

변상민,이창대,이호 대한건축학회지회연합회 2012 대한건축학회지회연합회 학술발표대회논문집 Vol.2012 No.1

원형 강관 인장 접합부의 구조성능에 관한 연구

변상민,최열,위옥복,강상구 대한건축학회지회연합회 2023 대한건축학회연합논문집 Vol.25 No.1

This study is an experimental study to investigate the performance of steel pipe flange joints, including theoretical yield and breaking strength formula, material test, and linear finite element analysis results. A total of 12 specimens with the various thickness of the flange plate and the number of joint bolts as variables (flange thickness: 3.2, 4.0, 4.5 mm, number of bolts: 4, 6) were prepared and a unidirectional tensile test was conducted. In order to investigate the performance of the joint, the plate thickness was designed to be thinner than the general case in order to induce flange fracture. When the flange thickness was the same, the yield strength and maximum strength increased as the number of bolts increased. However, when the flange thickness increased when the number of bolts was the same, the yield strength increased, but the joint stiffness increased and the maximum strength showed a tendency to decrease. Flange plate fracture was induced, but in the case of the specimen with 6 bolts, the steel pipe was fractured earlier than flage plate. 본 연구는 원형 강관 플랜지 접합부의 성능을 조사하는 실험적 연구로, 접합부의 이론적 항복 및 파단 강도 산정식과 재료의 적합성을 알아보는 재료실험, 또 실험 결과를 평가하기 위한 선형 유한요소 해석 결과를 포함하고 있다. 플랜지 플레이트의 두께와 접합부 볼트 개수를 변수(플랜지 두께: 3.2, 4.0, 4.5 mm, 볼트 개수: 4, 6개)로 하는 총 12개의 접합부 실험체를 제작하여 일방향 인장실험을 실시하였다. 접합부의 성능을 확인하기 위하여 플랜지 파단을 유도하기 위하여 플레이트 두께는 일반적인 경우보다 얇게 설계하였다. 플랜지 두께가 동일한 경우 볼트 개수 증가에 따라 항복강도 및 최대강도는 증가하였으나, 볼트 개수가 동일한 경우 플랜지 두께가 증가할 때 항복강도는 증가하지만 접합부 강성이 높아져 최대강도는 감소하는 경향을 나타내었다. 플랜지 플레이트 파단을 유도하였으나 볼트가 6개인 실험체의 경우 인장재 파단의 파괴양상을 보였다. 항복 하중에서는 실험체의 변화가 미소하였으나 최대 하중 도달 시 플랜지 플레이트가 우선적으로 인장 및 압축 좌굴 되는 것을 실험을 통해 확인할 수 있었다. 이는 이론적으로 도출한 결과와 유사하게 플랜지 플레이트의 항복 후 파괴되는 것을 확인할 수 있었으며, 고장력볼트, 용접부의 항복 및 파괴는 발생하지 않았다.