• Journal of Ocean Engineering and Technology
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• v.25 no.3
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• pp.66-72
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• 2011

This is the fourth of a series of companion papers dealing with the mechanical property reductions of various marine structural steels. Even though a reduction of the elastic modulus according to temperature increases has not been obtained from experiments, high temperature experiments from room temperature to $900^{\circ}C$ revealed that initial the yield strength and tensile strength are both seriously degraded. The mechanical properties obtained from high temperature experiments are compared with those from EC3 (Eurocode 3). It is found that the high temperature test results generally comply with the prediction values by EC3. Based on the prediction of EC3, time domain nonlinear finite element analyses were carried out for a blast wall installed on a real FPSO. After applying the reduced mechanical properties, corresponding to $600^{\circ}C$ to the FE model of the blast wall, more than three times the deflections were observed and it was observed that most structural parts experience plastic deformations exceeding the reduced yield strength at the high temperature. It is noted that a protection facility such as PFP (passive fire protection) should be required for structures likely to be directly exposed to fire and explosion accident.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.10
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• pp.1419-1425
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• 2010

Boron steel sheet is suitable for fabricating automobile parts because it is very strong and has low weight. Recently, many car makers are investigating the feasibility of fabricating the chassis part of automobiles using boron steel. In order to use boron steel sheets to fabricate the chassis part of automobiles, much better material property of low cycle fatigue life as well as high formability during hot stamping is required. Therefore, the low-cycle fatigue life of hot-stamped quenched boron steel was investigated in this study. The fatigue life observed at low strain amplitude was longer than that of an as-received boron steel sheet. However, the fatigue life reduced at high strain amplitude because of the low ductility and low fracture toughness of martensite, which was produced as a result of hot stamping.

• Journal of Conservation Science
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• v.19
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• pp.85-98
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• 2006

Rock property of the Seosan Maaesamjonbulsang is composed of biotite granite with medium grained texture. The triad Buddha is highly deteriorated by the joint, fracture, break-out, exfoliation, dew condensation of the surface and discoloration of the secondary pollutant. Host rock of the triad Buddha is divided dozens of rock blocks with various shape, and developed irregular discontinuity planes. Besides, the host rock is promoted biological pollution due to the surrounding crowded vegetation and high humidity environment. As the results of structural stability, it is confirmed that developed discontinuity system in the host rock is exposed instability sloping environment. Therefore, the host rock and surroundings are required maintenance, and required preservation by continuance monitoring for understand behavior of discontinuity system.

• Geomechanics and Engineering
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• v.12 no.1
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• pp.53-69
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• 2017

Rock burst may cause serious casualties and property losses, and how to conduct effective monitoring and warning is the key to avoid this disaster. In this paper, we reviewed both the rock burst mechanism and the principle of using electromagnetic radiation (EMR) from coal rock to monitor and forewarn rock burst, and systematically studied EMR monitored data of 4 rock bursts of Qianqiu Coal Mine, Yima Coal Group, Co. Ltd. Results show that (1) Before rock burst occurrence, there is a breeding process for stress accumulation and energy concentration inside the coal rock mass subject to external stresses, which causes it to crack, emitting a large amount of EMR; when the EMR level reaches a certain intensity, which reveals that deformation and fracture inside the coal rock mass have become serious, rock burst may occur anytime and it's necessary to implement an early warning. (2) Monitored EMR indicators such as its intensity and pulses amount are well and positively correlated before rock bursts occurs, generally showing a rising trend for more than 5 continuous days either slowly or dramatically, and the disaster bursts generally occurs at the lower level within 48 h after reaching its peak intensity. (3) The rank of EMR signals sensitive to rock burst in a descending order is maximum EMR intensity > rate of change in EMR intensity > maximum amount of EMR pulses > rate of change in the amount of EMR pulses.

• Journal of the Korean Ceramic Society
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• v.45 no.2
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• pp.94-98
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• 2008

Silicon nitride($Si_3N_4)$ is one of the most widely used structural ceramic materials. However silicon nitride is difficult to sinter because of its strong covalent bonding characteristics. In this study, $Si_3N_4$ ceramics were fabricated by spark plasma sintering process with $Y_2O_3$ and $Al_2O_3$ addition to improve the sinterability and the mechanical properties and their phase transformation behavior, microstructure and mechanical properties were evaluated. Fully densified $Si_3N_4$ ceramics could be obtained by spark plasma sintering process at a lower temperature than conventional sintering method. The formation of network microstructure was affected by the addition of $Al_2O_3$ because it could accelerate a to ${\alpha}$ to ${\beta}$ phase transformation of $Si_3N_4$. As a result, the mechanical properties depended on amounts of $Al_2O_3$ addition. The hardness value increased with increasing ${\alpha}$-phase fraction, but fracture toughness value increase with increasing ${\beta}$-phase fraction.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.13 no.6
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• pp.309-314
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• 2003

A spark plasma sintering technique has been used for the fabrication of $Al_2O_3$-SiC nanocomposites at the low temperature of $1100^{\circ}C$∼$1500^{\circ}C$. The sintered $Al_2O_3$-SiC composites shows very homogeneous microstructure without any particular abnormal grain growth, indicating that the addition of nano-sized SiC particles is very effective to control grain growth and to induce the residual stress in the $Al_2O_3$ matrix, resulting in the intragranular fracture. These SiC particles are present in the grain boundaries and also intragrain, depending on the sintering condition, and improve remarkably the mechanical properties of $Al_2O_3$-SiC composite through the mechanisms of strengthening and toughening induced by crack diffraction and crack bridging.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.17 no.3
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• pp.128-132
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• 2007

[ $B_4C$ ] ceramics were fabricated by spark plasma sintering process and their sintering behavior, microstructure and mechanical properties were evaluated. Relative density of $B_4C$ ceramics were obtained by spark plasma sintering method reached as high as 99% at lower temperature than conventional sintering method, in addition, without any sintering additives. The mechanical properties of $B_4C$ ceramics was improved by a methanol washing process which can be removed $B_2O_3$ phase from a $B_4C$ powder surface. This improvement results ken the formation of homogeneous microstructure because the grain coarsening was suppressed by the elimination of $B_2O_3$ phase. Particularly, fracture toughness of the sintered specimen using a methanol washed powder improved over 30% compared with the specimen using an as-received commercial powder.

• Journal of the Korean Ceramic Society
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• v.47 no.6
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• pp.560-565
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• 2010

Nano laminated bulk $Ti_3SiC_2$ was synthesized by hot press process using TiCx/Si powder mixture at the temperature range of $1300^{\circ}C\sim1500^{\circ}C$. pure $Ti_3SiC_2$ was synthesized by a hot pressing above $1400^{\circ}C$, while unreacted TiCx were remained in bulk $Ti_3SiC_2$ which synthesized below $1400^{\circ}C$. The sintering density of bulk $Ti_3SiC_2$ were varied with the amount of TiCx. It was found that the mechanical properties and micro structures of bulk $Ti_3SiC_2$ were closely related to the amounts of TiCx which was controlled by the hot pressing temperature. The TiCx increase the flexural strength of bulk $Ti_3SiC_2$, while the fracture toughness and thermal shock resistance of bulk $Ti_3SiC_2$ were decreased with the content of TiCx. The plastic deformations of bulk $Ti_3SiC_2$ were appeared above $1000^{\circ}C$.

• Composites Research
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• v.17 no.6
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• pp.37-43
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• 2004

To determine the effect of chain length and chemical structure of linear amine curing agents on thermal and mechanical properties, a standard bifunctional linear DGEBF epoxy resin was cured with EDA and HMDA having amine group at the both ends of main chain in a stoichiometrically equivalent ratio in condition of preliminary and post cure. From this work, the effect of linear amine curing agents on the thermal and mechanical properties is significantly influenced by numbers of carbon atoms of main chain. In contrast, the results show that the DCEBF/EDA system having two carbons had higher values in the thermal stability, density, shrinkage (%), grass transition temperature, tensile modulus and strength, flexural modulus and strength than the DGEBF/HMDA system having six carbons, whereas the DGEBF/EDA cure system had relatively low values in maximum ekothermic temperature, maximum conversion of epoxide, thermal expansion coefficient than the DGEBF/HDMA cure system. These findings indicate that the packing capability (rigid property) in the EDA structure affects the thermal and mechanical properties predominantly. It shows that flexural fracture properties have a close relation to flexural modulus and strength.

• 한국터널공학회:학술대회논문집
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• 2005.04a
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• pp.348-358
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• 2005

Although the evaluation of the mechanical properties and behavior of discontinuous rock masses is very important for the design of underground openings, it has always been considered the most difficult problem. One of the difficulties in describing the rock mass behavior is assigning the appropriate constitutive model. This limitation may be overcome with the progress in discrete element software such as PFC, which does not need the user to prescribe a constitutive model for rock mass. Instead, the micro-scale properties of the intact rock and joints are defined and the macro-scale response results from those properties and the geometry of the problem. In this paper, a $30m{\times}30m{\times}30m$ jointed rock mass of road tunnel site was analyzed. A discrete fracture network was developed from the joint geometry obtained from core logging and surface survey. Using the discontinuities geometry from the DFN model, PFC simulations were carried out, starting with the intact rock and systematically adding the joints and the stress-strain response was recorded for each case. With the stress-strain response curves, the mechanical properties of discontinuous rock masses were determined and compared to the results of empirical methods such as RMR, Q and GSI. The values of Young's modulus, Poisson's ratio and peak strength are almost similar from PFC model and Empirical methods. As expected, the presence of joints had a pronounced effect on mechanical properties of the rock mass. More importantly, the mechanical response of the PFC model was not determined by a user specified constitutive model.