• Water Engineering Research
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• 제3권3호
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• pp.163-176
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• 2002

In this study, laboratory experiments have been performed to investigate characteristics of the velocity fields and turbulence for non-buoyant plane jet in the vicinity of the jet nozzle using PIV system. The experimental results show that, in the transition region, the lateral velocity profile is in good agreement with Gaussian distribution. However, the coefficient of Gaussian distribution, $\K_{u,}$, decreases with longitudinal distance in the transition region. The existing theoretical equation for the centerline velocity tends to overestimate the measured data in the transition region. A new equation for the centerline velocity derived by incorporating varying $k_{u}$ gives better agreement with the measured data than the previous equation. The results of the turbulence characteristics show peak values are concentrated on the shear layers. The Reynolds shear stress profile shows the positive peak in the upper layer and negative peak in the lower layer. The turbulent kinetic energy also provides double peaks at the shear layers. The peak of the Reynolds shear stress and the turbulent kinetic energy increases until x/B=8, and then it decreases afterwards.s.

• Structural Engineering and Mechanics
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• 제78권2호
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• pp.163-174
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• 2021

Track-bridge interaction has become an essential part in the design of bridges and rails in terms of modern railways. As a unique ballastless slab track, the longitudinal continuous slab track (LCST) or referred to as the China railway track system Type-II (CRTS II) slab track, demonstrates a complex force mechanism. Therefore, a comprehensive track-bridge interaction study between multi-span simply supported beam bridges and the LCST is presented in this work. In specific, we have developed an integrated finite element model to investigate the overall interaction effects of the LCST-bridge system subjected to the actions of temperature changes, traffic loads, and braking forces. In that place, the deformation patterns of the track and bridge, and the distributions of longitudinal forces and the interfacial shear stress are studied. Our results show that the additional rail stress has been reduced under various loads and the rail's deformation has become much smoother after the transition of the two continuous structural layers of the LCST. However, the influence of the temperature difference of bridges is significant and cannot be ignored as this action can bend the bridge like the traffic load. The uniform temperature change causes the tensile stress of the concrete track structure and further induce cracks in them. Additionally, the influences of the friction coefficient of the sliding layer and the interfacial bond characteristics on the LCST's performance are discussed. The systematic study presented in this work may have some potential impacts on the understanding of the overall mechanical behavior of the LCST-bridge system.

• 한국터널지하공간학회 논문집
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• 제21권6호
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• pp.825-835
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• 2019

터널 설계 시 조사를 통하여 지반을 여러 개의 등급으로 분류한 후 지보패턴표에 따라 해당 지보시스템을 적용하게 된다. 그러나 암반 등급을 기준으로 한 단순한 패턴 적용방식은 암반 등급 경계부에서 발생하는 종방향 응력 전이를 고려하지 못한다. 본 연구는 NATM 터널의 종방향 암반 등급 변화부에서 응력 변화를 추정하기 위한 3차원 수치해석을 실시하였으며 Influence Line (영향선) 및 Trend Line (경향선)을 이용하여 하중전이 영향권을 파악하고자 하였다. 연구 결과 터널 굴착방향으로 암반 등급이 하향 변화하면 등급변화 경계를 중심으로 강성이 약한 암반에서 강성이 큰 암반 방향으로 0.35~0.7D 범위에서 응력 전이가 발생하며 이를 고려하여 암반 등급의 종방향 하향 변화부에서는 연구결과의 0.35~0.7D 영향권을 감안하여 안전측으로 1.0D 정도의 하향패턴을 추가 적용하는 것이 필요하다.

• 오토저널
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• 제12권1호
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• pp.26-32
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• 1990

This study investigated a method for the determination of the J-integral for a tough glassy polymer such as polycarbonate plates by using the method of caustics. Comparing the values of J-integral determined by a numerical analysis and by the method of caustics, the method of caustics was found to be an effective experimental technique for the determination of the J-integral. The ratio between two J-integrals determined by the method of caustics and by finite element method converged into 1 within the limit of low load. However, it was noticed that the greater the plastic zone at the crack tip was, the lower the J-integral obtained by the reflect method of caustics. This difference may be deduced from the damage at the crack tip such as craze appeared in the polycarbonate plate. It was confirmed that the ratio of longitudinal diameter( $D_{l}$ ) to transverse diameter ( $D_{t}$) of caustics generally converged into 1 at the low load. The transition of the state of stress at the vicinity of a crack tip from plane strain to plane stress was deduced by noticing that the longitudinal diameter( $D_{l}$ ) grew faster than the transverse diameter( $D_{t}$) of caustics within the higher load range.

• International Journal of Railway
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• 제5권4호
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• pp.163-166
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• 2012

Many different types of floating slab track have been developed and installed around the world to reduce vibrations and noise originating in the surrounding environment. The main objective of this study is to examine the influence of slab length on behavior of floating slab track based on rail-slab-isolator interaction. The floating slab track is modeled by the connection between rail, slab, isolator, and slab mat in the transition zone. All elements were assembled in a simplified two-dimensional (2D) finite element model (FEM). The maximum length of FST is then investigated based on the maximum additional rail stress criterion as described in UIC 774-3R since no fully accepted design criteria for the slab length in FST systems currently exist.

• International Journal of Korean Welding Society
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• 제1권1호
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• pp.51-57
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• 2001

Effect of the joint details on the stress distribution over a slot structure has been studied in order to improve its fatigue life using a finite element analysis. The joint details of interest are the radius and height of scallop at the stiffener as well as the mis-alignment between the stiffener and longitudinal member. For a slot structure currently used, the stiffener heel is subjected to the maximum stress for a given external load, where is a potential fatigue crack initiation site. The stresses at the stiffener heel and toe decrease both by increasing the scallop radius and more significantly by increasing the mis-alignment while no notable effect of the scallop height on it is appreciated. A proper combination of these factors makes it possible to reduce the stresses at the stiffener heel and In, theoretically, more than 50%. This is attributed to the modification of the stress distribution over the slot structure including the transition of the maximum stressed region from the stiffener heel to the slot surface of the transverse web. Such then results in a g[eat improvement of the fatigue life of the slot structure.

• Structural Engineering and Mechanics
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• 제56권2호
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• pp.275-291
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• 2015

Adherence between reinforcement and the surrounding concrete is usually ignored in finite element analysis (FEA) of reinforced concrete (RC) members. However, load transition between the reinforcement and surrounding concrete effects RC members' behavior a great deal. In this study, the effects of bond-slip on the FEA of RC members are examined. In the analyses, three types of bond-slip modeling methods (perfect bond, contact elements and spring elements) and three types of reinforcement modeling methods (smeared, one dimensional line and three dimensional solid elements) were used. Bond-slip behavior between the reinforcement and surrounding concrete was simulated with cohesive zone materials (CZM) for the first time. The bond-slip relationship was identified experimentally using a beam bending test as suggested by RILEM. The results obtained from FEA were compared with the results of four RC beams that were tested experimentally. Results showed that, in FE analyses, because of the perfect bond occurrence between the reinforcement and surrounding concrete, unrealistic strains occurred in the longitudinal reinforcement. This situation greatly affected the load deflection relationship because the longitudinal reinforcements dominated the failure mode. In addition to the spring elements, the combination of a bonded contact option with CZM also gave closer results to the experimental models. However, modeling of the bond-slip relationship with a contact element was quite difficult and time consuming. Therefore bond-slip modeling is more suitable with spring elements.

• 가정과삶의질연구
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• 제33권1호
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• pp.1-17
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• 2015

This study investigated how variables of internalizing and externalizing problematic behaviors change according to gender and as time passes by and how the variables influence adolescent problematic behaviors. The variables selected for the analysis are personal variables, parent variables, peer and school variables, and community variables. longitudinal data collected for 4 years from the Korea Youth Panel Survey(KYPS) were utilized for the analysis. Data were collected initially from 2,707 fifth grade elementary students in 2005 and they were traced until 2008. The final respondents were 2,448 students. The findings are as follows. Frist, the statistical significance was found in changes of problematic behavioral variables in terms of the gender difference. Second, variables, such as self-esteem and self-control are negatively correlated to the problematic behaviors and stress level is strongly positively correlated to the behaviors. Third, the study pressure and peer attachment level are correlated to the initial value of internalizing problematic behaviors. In conclusion, given that more statistical significances were found at initial values than the change rates among variables, early intervention is important in addressing adolescent problematic behaviors.

• 한국산학기술학회논문지
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• 제21권3호
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• pp.164-172
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• 2020

본 연구의 목적은 다문화 가정 청소년의 심리적 특성인 자아존중감, 문화적응스트레스, 우울, 사회적 위축을 활용하여 초등학교와 중학교 시기의 잠재프로파일 및 전이 과정을 탐색하고, 전이 과정에 영향을 미치는 변인을 알아보는 것이다. 이를 위해 다문화청소년패널조사의 2차년도(2012년)와 6차년도(2016년) 자료의 1,188명을 분석 대상으로 하였으며, 잠재프로파일분석과 다항로지스틱 회귀분석을 사용하였다. 분석 결과, 첫째, 초등학교 시기(2차년도 자료)와 중학교 시기(6차년도 자료)의 잠재프로파일분석 결과, 두 시점 모두 '안정형 집단', '사회적 위축형 집단', '불안정형 집단'의 세 개의 잠재집단이 확인되었다. 둘째, 잠재집단의 전이 과정을 확인한 결과, 심리적 특성의 안정적인 양상도 나타나지 만, 긍정적 변화 및 부정적 변화 또한 나타나는 것이 확인되었다. 셋째, 방임적 양육태도, 부모효능감, 가족의지지, 친구의지지가 잠재집단 구분에 영향을 미치는 것으로 확인되었으며, 특히 친구의지지 수준이 높을수록 불안정형 집단에 계속 머무르기보다는 안정형 집단으로 전이될 확률이 높은 것으로 확인되었다. 본 연구결과는 적절한 지원이 이루어질 경우 다문화 가정 청소년이 더 안정적인 심리적 특성을 갖는 방향으로 변화할 수 있음을 시사한다.

• 한국산학기술학회논문지
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• 제21권7호
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• pp.370-379
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• 2020

본 연구의 목적은 분할함수 성장모형에 기초하여 초등학교 시기와 중학교 시기의 학교급 전환기를 거치는 과정에서 다문화 청소년의 학업성취 변화양상이 어떻게 나타나는지 확인하고, 이에 영향을 미치는 부모, 개인, 친구 변인을 탐색하는 것이다. 다문화청소년패널연구의 1차년도(2011년, 초4)부터 6차년도(2016년, 중3)까지의 6년 동안의 자료를 사용하여 분할함수 성장모형 분석을 하였으며, 분석 결과는 다음과 같다. 첫째, 학업성취는 초등학교 6학년 시기를 기준으로 하여 초등학교 시기(초4-초6), 중학교 시기(초6-중3)로 변화율을 다르게 추정하는 분할함수 성장모형이 무변화, 선형변화, 이차변화 성장모형보다 타당한 것으로 나타났다. 둘째, 무조건 분할함수 성장모형 분석 결과, 다문화 청소년의 학업성취는 초등학교 시기와 중학교 시기 모두 감소하는 특징이 있는 것으로 확인되었으며, 중학교 시기의 감소폭이 더 큰 것으로 나타났다. 셋째, 조건 분할함수 성장모형 분석 결과, 학업성취 초기치(초4)는 부모효능감, 자아존중감, 친구의 지지 수준이 높을수록, 방임적 양육태도, 문화적응스트레스 수준이 낮을수록 높은 것으로 나타났다. 변화율 1(초등학교 시기)에는 초등학교 4학년 시기의 자아존중감과 친구의 지지가 학업성취의 감소 패턴을 억제하는 효과가 있는 것으로 나타났으며, 변화율 2(중학교 시기)에는 초등학교 4학년 시기의 문화적응스트레스가 중학교 시기의 학업성취 감소 패턴을 증가시키는 효과가 있는 것으로 나타났다.