• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• 제30권12호
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• pp.762-767
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• 2017

Mullite ($3Al_2O_3{\cdot}2SiO_2$) has emerged as a promising candidate for high-temperature structural materials due to its erosion resistance, chemical and thermal stabilities, relatively low thermal expansion coefficient, excellent thermal shock and creep resistances, and low dielectric constant. However, since the pure mullite sintering temperature is as high as $1,600{\sim}1,700^{\circ}C$, there is an increasing need for a sintering additive capable of improving the strength characteristics while lowering the sintering temperature. Herein we have tried to obtain the optimal sintering additive composition by adding MgO, $Cr_2O_3$, and $Y_2O_3$ to mullite, followed by sintering at $1,325{\sim}1,550^{\circ}C$ for 2 h. With additives of 2 wt% of MgO, 2 wt% of $Cr_2O_3$, 4 wt% of $Y_2O_3$, A density of $3.23g/cm^3$ was obtained for the sintered body at $1,350^{\circ}C$ upon using 2 wt% MgO, 2 wt% $Cr_2O_3$, and 4 wt% $Y_2O_3$ as additives. The three-point flexural strength of that was 275 MPa and the coefficient of thermal expansion (CTE) was $4.15ppm/^{\circ}C$.

• Tunnel and Underground Space
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• 제16권4호
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• pp.288-295
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• 2006

In this study, dynamic behavior of the vehicles is analyzed, while the track is subjected to lateral vibrations due to earthquake and blasting load. A computer program(WERIA, Wheel Rail Interaction Analysis) is used, which can simulate dynamic responses of vehicles subjected to lateral vibrations. The analysis considers two types of vehicles: I.e. power cars of KTX and Busan subway train. It can also consider the interaction with sub-structures such as tracks and soil. The creep force module is considered, and the running safety of railway vehicles subjected to earthquake and blasting loading is studied. Based on the results of this study, the running safety of the vehicles can be confirmed against lateral vibration.

• Structural Engineering and Mechanics
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• 제46권1호
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• pp.53-73
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• 2013

Prediction of prestressed concrete girder integral abutment bridge (IAB) load effect requires understanding of the inherent uncertainties as it relates to thermal loading, time-dependent effects, bridge material properties and soil properties. In addition, complex inelastic and hysteretic behavior must be considered over an extended, 75-year bridge life. The present study establishes IAB displacement and internal force statistics based on available material property and soil property statistical models and Monte Carlo simulations. Numerical models within the simulation were developed to evaluate the 75-year bridge displacements and internal forces based on 2D numerical models that were calibrated against four field monitored IABs. The considered input uncertainties include both resistance and load variables. Material variables are: (1) concrete elastic modulus; (2) backfill stiffness; and (3) lateral pile soil stiffness. Thermal, time dependent, and soil loading variables are: (1) superstructure temperature fluctuation; (2) superstructure concrete thermal expansion coefficient; (3) superstructure temperature gradient; (4) concrete creep and shrinkage; (5) bridge construction timeline; and (6) backfill pressure on backwall and abutment. IAB displacement and internal force statistics were established for: (1) bridge axial force; (2) bridge bending moment; (3) pile lateral force; (4) pile moment; (5) pile head/abutment displacement; (6) compressive stress at the top fiber at the mid-span of the exterior span; and (7) tensile stress at the bottom fiber at the mid-span of the exterior span. These established IAB displacement and internal force statistics provide a basis for future reliability-based design criteria development.

• Journal of the Korean Society for Railway
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• 제17권1호
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• pp.14-18
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• 2014

Wheel/rail contact is a significant problem in railway dynamics. In this paper, the wheel/rail contact is examined analytically and numerically as a contact problem between two cylinders where torque and friction have effect. Furthermore, the contact of a real wheel and rail is investigated numerically where the normal and shear force act. This study demonstrates that the wheel/rail contact is a process that generates traction force through creep where rolling and sliding occurs simultaneously depending on the shape of the wheel and rail, and the friction coefficient between them.

• Journal of Korea Foundry Society
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• 제18권4호
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• pp.364-372
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• 1998

Effects of Si and Ca additions on the mechanical properties of AM60 based Mg alloys have been investigated. Hardness of the AM60 based Mg alloys reached a maximum value after aging for approximately 33 hours but the amount of hardness increase was negligible. The poor age hardening response of the alloys was due to low Al content, which implies that Al content must be >6 wt.% to observe age hardening effect. The tensile and yield strength increased with increasing Al, Si, and Ca content but elongation decreased with increasing Al and Si content. The best mechanical properties obtained in AM 40-2.5Si-0.2Ca alloy after T4 heat treatment were as follows; tensile strength 193.4 MPa, yield strength 79.2 MPa, and elongation 11.2%. High temperature property obtained from creep test was also improved by introducing $Mg_2Si$ which has high hardness, high melting temperature and low thermal expansion coefficient.

• Proceedings of the KSR Conference
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• 한국철도학회 2009년도 춘계학술대회 논문집 특별세미나,특별/일반세션
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• pp.225-230
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• 2009

From the view point of railway vehicle dynamics, the interaction between wheel and rail have an huge effect on the behavior of the vehicle. This phenomenon is an unique motion, only for railway vehicles. Furthermore, close investigation of the backgrounds of the interaction is the key to estimate the dynamic behavior of the vehicle, successfully. To evaluate the model including flexible bodies such as car body and catenary system of the next generation express train, it is necessary to develop proper dynamic solver including a wheel rail contact module. In this study, wheel-rail contact module is developed using the general purpose dynamic solver. First of all, the procedure for calculation of the wheel-rail contact force has been established. Generally, yaw angle of the wheelset is ignored. Sets of information are summarized as tables and splined for further uses. With this information, normal force and creep coefficient can be extracted and used for FASTSIM algorithm, which has been shown good reliability over years. Normal force and longitudinal, lateral force at the contact surface are also calculated. Those data are verified by commercial railway simulation program 'VAMPIRE'. This procedure and program can offer a basic process for estimation of the dynamic behavior and wear of the wheel-rail system, even while running on the curved rail. Finally, multi-dimensional inspection tool will be developed including the prediction of the derailment.

• Journal of Korean Society of Forest Science
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• 제108권2호
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• pp.216-223
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• 2019

This study was conducted to measure the changes in the geological and soil properties following slow-moving landslide events in Yangbuk-myun and Gyungju-si, Gyeongsangbuk-do, South Korea. The geological characteristics of the study site comprised black shale in the Gyeongsang nodal group formed in the Cretaceous period and quartz feldspar carcinoma in the east side with conglomerate in the Yeonil group formed in the Quaternary period. The study site exhibited the geologic characteristics of a slow-moving landslide with severely weathered rocks. The maximum collapsing depth of the slow-moving landslide was 12.0 m with colluvial deposits. The strike and joint aspects in the slope areas of the slow-moving landslides were $N46^{\circ}E$ in lower slope and $N62^{\circ}E$ in upper slope, respectively. Soil hardness of ${\leq}20cm$ deep was not measured because of the completely disturbed soil resulting from soil creeping. Soil from 25 to 90 cm deep was 1.4-4.7 times softer in the slow-moving landslide areas than in the undisturbed or natural forests. Soil bulk density was $1.24-1.29g/cm^3$ in land creep areas. Soil bulk in both areas was 1.6 times denser than that in the natural forest. The soil pore space was 51.5-53.3% in the land creep areas. The values are 1.3-1.4 times lower than those within the natural forest. Black shale areas showed the lowest coefficient of permeability (8.75 E-06 cm/s) and mesopore ratio (pF 2.7: 9.8%) compared with those resulting from other study areas.

• Proceedings of the Korean Geotechical Society Conference
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• 한국지반공학회 2003년도 봄 학술발표회 논문집
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• pp.257-264
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• 2003

Tunnelling in water bearing soils influences the ground water regime. It has been indicated in the literature that the existence of ground water above a tunnel influences tunnel stability and the settlement profile. Only limited research, however, has been done on ground water movements around tunnels and their influence on tunnel performance. Time dependent soil behaviour can be caused by the changes of pore water pressure and/or the viscous properties of soil(creep) under the stress change resulting from the advance of the tunnel face. De Moor(1989) demonstrated that the time dependent deformations due to tunnelling are mainly the results of pore pressure dissipation and should be interpreted in terms of effective stress changes. Drainage into tunnels is governed by the permeability of the soil, the length of the drainage path and the hydraulic boundary conditions. The potential effect of lime dependent settlement in a shallow tunnel is likely to occur rapidly due to the short drainage path and possibly high coefficient of consolidation. Existing 2D modelling methods are not applicable to these tunnelling problems, as it is difficult to define empirical parameters. In this paper the time-based 2D modelling method is adopted to account for the three dimensional effect and time dependent behaviour during tunnel construction. The effect of coupling between the unloading procedure and consolidation during excavation is profoundly investigated with the method. It is pointed out that realistic modelling can be achieved by defining a proper permeability at the excavation boundary and prescribing appropriate time for excavation Some guidelines for the numerical modelling of drained and undrained excavation has been suggested using characteristic time factor. It is highlighted that certain range of the factor shows combined effect between the unloading procedure due to excavation and consolidation during construction.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 한국전기전자재료학회 2004년도 하계학술대회 논문집 Vol.5 No.2
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• pp.1262-1265
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• 2004

송전용량 증가를 위해 개발되어 최근 본격적으로 사용되고 있는 STACIR/AW 송전선은 송전용량의 증가에 따라 그 운전환경도 변화하여 연속사용온도의 경우, 기존 ACSR 전선의 90t에 비해 높은 $210^{\circ}C$로 규정 되어 있을 만큼 고온에서 운전되고 있다. 따라서 STACIR/AW 송전선은 이도설계와 그 운용에 있어서 운전 온도 상승에 따른 각별한 주의관리가 필요하다 실제 STACIR/AW송전선은 그 설계단계에서도 이와 같은 고온운전 환경을 고려하여 고온에서도 소정강도를 유지하는 내열 Al도체와 이도제어를 위한 낮은 열팽창 특성의 INVAR합금(Fe-35Ni계 합금)을 강선으로 하는 특화된 재료로 구성되어 있다. 그러나 이와 같은 재료 설계적 보완책에도 불구하고 실제 송전선은 전선의 자중, 철탑 간에 형성된 가설장력과 같은 다양한 응력이 고온환경에서 부하되는 복합 열화 상태에 노출되어 있고, 이것은 재료학적인 관점에서 크릴 변형 발생의 가능성을 높이고 있으나 이것에 대한 연구 또는 실험결과는 크게 미미한 실정이다. 본 연구에서는 STACIR/AW $410mm^2$ 송전선과 그 구성소재를 대상으로 $200^{\circ}C$, $300^{\circ}C$에서 장시간 열화한 후, 구성소재의 탄성계수, 열팽창계수 및 STACIR/AW전선의 크림변형 거동을 조사하여 열화에 노출된 STACIR/AW 송전선의 이도변화 거동을 규명하고자 하였다.

• Proceedings of the KIEE Conference
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• 대한전기학회 1999년도 추계학술대회 논문집 학회본부 C
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• pp.1003-1007
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• 1999

This paper is reported for the stability of ZnO-$Pr_{6}O_{11}-CoO-Er_{2}O_{3}$ based ceramic varistors with $Er_{2}O_{3}$ added in the addition range 0.0 to 2.0 mol%. The varistors sintered at $130^{\circ}C$ exhibited abrupt positive current creep phenomena, which accompany thermal run away within short times, even under weak d.c. stress. As a result, these varistors were completely degraded. On the contrary, the stability of varistors sintered at $1350^{\circ}C$ was far better than that of $1300^{\circ}C$. In particular, the varistor containing 0.5 mol% $Er_{2}O_{3}$ showed a excellent stability, which the variation rate of the varistor voltage, the nonlinear coefficient, and leakage current is below 1%, 2%, and 3.5%, respectively, even under more severe d.c. stress, such as ($0.8V_{1mA}/90^{\circ}C/12h$) + ($0.85V_{1mA}/115^{\circ}C/12h$) + ($0.9V_{1mA}/120^{\circ}C/12h$) + ($0.9V_{1mA}/150^{\circ}C/12h$). Consequently, it is estimated that the basic composition of ZnO-$Pr_{6}O_{11}-CoO-Er_{2}O_{3}$ based varistor contain 0.5 mol% $Er_{2}O_{3}$ will be used to the fabrication of the varistors for high performance and stability in a forthcoming.