• Water Engineering Research
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• v.3 no.3
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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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• v.78 no.2
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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.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.6
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• pp.825-835
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• 2019

When the tunnel is designed, the ground is classified into several grades and the corresponding support system is applied according to the support pattern table. However, a simple pattern application based on rock grade does not take into account the longitudinal stress transitions occurring at rock grade boundaries. In this study, three-dimensional numerical analysis was performed to estimate the stress change in the longitudinal rock grade change of NATM tunnel, and the influence zone of load transfer was investigated using the influence line and trend line. As a result, the downward change of rock grade in the direction of tunnel excavation occurs in the range of 0.35~0.7D from low-strength rock to high-strength rock around the grade change boundary. It is necessary to apply a downward pattern of about 1.0D to the safety direction in consideration of the influence range of 0.35D to 0.7D.

• Journal of the korean Society of Automotive Engineers
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• v.12 no.1
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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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• v.5 no.4
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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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• v.1 no.1
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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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• v.56 no.2
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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.

• Journal of Families and Better Life
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• v.33 no.1
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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.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.3
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• pp.164-172
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• 2020

This study investigated the longitudinal transition of adolescents' psychological character profiles, which includes self-esteem, acculturative stress, depression, and social withdrawal, and the predictive effects of parental efficacy, families' support, and friends' support in multicultural families. The sample consisted of 1,188 adolescents in elementary school, who were re-examined in three years, when they were in middle school from the part of waves 2 and 6 of the Multicultural Adolescents Panel Study. A latent profile analysis identified three distinct profiles of psychological character among adolescents in elementary school and middle school: stable, social withdrawal, and unstable. As compared to elementary school, adolescents' psychological characteristics stayed stable and played critical roles on the latent classes. Parental efficacy, and supports from family and friends played critical roles on the latent classes. Specifically, as adolescents perceived full of supports from friends, they were more likely to make transitions to the stable group. Implications for intervention in multicultural families are discussed.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.7
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• pp.370-379
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• 2020

The purpose of this study is to estimate the longitudinal changes in the academic achievement of Korean adolescents who transition from elementary to middle school and examine the individual and social factors of the changes using piecewise growth modeling. For this study, the Multicultural Adolescents Panel Study (MAPS) data from 1st to 6th waves (4th-9th grade) were used. The results are as follows. First, the results showed that the piecewise growth function was the most appropriate method to explain changes in the academic achievement of Korean adolescents with respect to the time of vacation. Second, adolescents' academic achievement declined constantly and was greater for middle school students. Third, parental efficacy, self-esteem, and support from friends had a significant positive effect, while uninvolved parenting and acculturation stress had a significant negative effect on the initial value of academic achievement. Self-esteem and support from friends in the 4th grade prohibit its decline in the first change rate of academic achievements. Meanwhile, acculturation stress for 4th-grade students accelerates the decline of academic achievement.