• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.309-312
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• 2001

FEM simulating system of the cross-rolling texture formation offers a systematic and efficient way of exploring the relationship between the process variables and the state of plastic anisotropy of sheet product. Cross-rolled sheets possess higher average plastic strain ratios and lower planer anisotropy than those of the straight-rolled sheets. The employed model is a finite-element polycrystal model which each element used in FEM is assumed to be a crystal having different orientation by Takahashi. Texture development, deformation textures due to cross-rolling are predicted for face-centered cubic sheet metal. Crystal orientations are assigned on the basis of the pole figures obtained by X-ray diffraction. Development of anisotropy during cross rolling of an fcc sheet material is predicted theoretically with respected to flow stress and R-value in tensile test.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.29 no.5
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• pp.192-202
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• 2019

Physicochemical properties and sintering characteristics of pottery stone (Taebaek, Haenam, Aphae, Haengnam) were evaluated as a raw material for porcelain products. Due to acid leaching procedure, the concentration of iron oxide ($Fe_2O_3$) was decreased to < 1.0 wt%, which affects the whiteness of sintered samples. Mean particle size of acid leached samples is $5.7{\sim}10{\mu}m$ with narrow particle size distribution (PSD), which is lower than that of the pristine ($8{\sim}18{\mu}m$) with broad PSD. According to phase analysis by X-ray diffraction, most of pottery stones (PS) have Quartz phase as a main phase with Pyrophyllite as a second phase, however, Haenam PS shows halloysite phase. The absorption rate was in order of Taebaek (A, B, C)~Aphae (A, B) < Taebaek (Special A) < Haengnam < Haenam, and the samples sintered in reductive atmosphere showed lower absorption rate. This result might be due to the concentration of feldspar contained in PS, working as a flux in sintering process. Comparing the color of the sintered samples, the whiteness of refined PS (Taebaek special A, Haenam, Hangnam) is higher than acid leached PS (Taebaek A/B/C, Aphae A/B). The whiteness (L*) for refined PS is 95~97 %, which is higher than acid leached (82~96 %). This might be due to lower iron oxide concentration of the refined PS (0.11~0.58 %) than those of the acid leached PS (0.41~1.91 %) even though most of iron oxide was removed by acid leaching.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.8
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• pp.818-823
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• 2012

In order to predict necking behaviour of aluminium sheets, a crystal plasticity model is introduced in the finite element analysis of tensile test. Due to the computational limits of time and memory, only a small part of tensile specimen is subjected to the analysis. Grains having different orientations are subjected to numerical tensile tests and each grain is discretized by many elements. In order to predict the sudden drop of load carrying capacity after necking, a well-known Cockcroft-Latham damage model is introduced. The mismatch of grain orientation causes stress concentration at several points and damage is evolved at these points. This phenomenon is similar to void nucleation. In the same way, void growth and void coalescence behaviours are well predicted in the analysis. For the comparison of prediction capability of necking, same model is subjected to finite element analysis using uniform material properties of polycrystal with and without damage. As a result, it is shown that the crystal plasticity model can be used in prediction of necking and fracture behavior of materials accurately.

• Journal of the Korean Society for Precision Engineering
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• v.15 no.5
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• pp.167-175
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• 1998

A mathematical formulation is presented to model anisotropy from the deformation textures developed in a forming process. In this work, a micro-mechanical-based polycrystalline analysis is implemented into a consistent finite element method for the anisotropic, viscoplastic deformation of polycrystalline metals. As suggested by Taylor, the deformation of each grain in an aggregate is assumed to be same as the macroscopic deformation of an aggregate or a macro-continuum point. Algorithms are developed to represent the plastic anisotropy, such as the anisotropic yield surface and R-value, from the predicted deformation texture. As applications, the evolution of texture in rolling, upsetting and drawing/extrusion processes are simulated and the corresponding changes of mechanical properties such as yield surface and R-value are predicted.

• Journal of the Korean Ceramic Society
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• v.35 no.11
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• pp.1203-1211
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• 1998

The microwave sintered BaTiO3 samples were obtained by using the microwave sintering device which can precisely control the sintering temperature and the sinter time by using IR optical thermometer and PID temperature controller. During microwave sintering the internal temperature of samples were mesur-ed by the optical fiber thermometer to compare the sintering behaviors between microwave- and con-ventionally sintered ones. The former showed the faster rate of grain growth with sintering time and the larger grain size than the latter. Also they showed the similar pattern of dielectric properties with tem-perature changes from 2$0^{\circ}C$ to 16$0^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.40 no.12
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• pp.1177-1182
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• 2003

The porous sensing materials for BaTiO$_3$ gas sensors were fabricated by adding the graphite powders. The crystalline structure and microstructure of the porous BaTiO$_3$-based ceramics were studied. All the sintered bodies showed a tetragonal perovskite structure. The porosity increased with increasing graphite contents. This is mainly due to an enhanced evolution of CO and $CO_2$ gases resulting from the exothermic reactions of graphite and oxygen during the sintering. It was found that the discrepancy in the resistivities measured in air and CO atmospheres at high temperatures (>∼20$0^{\circ}C$) became remarkable with increasing temperature. The sensitivity of CO gas increased with porosity, since the reactions between CO gas and $O_2$$^{[-10]}$ and between CO gas and $O^{[-10]}$ are active due to the formation of many reaction sites. The porous BaTiO$_3$-based ceramics could be promising as a sensing material for CO gas sensors.

• Transactions of Materials Processing
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• v.16 no.4 s.94
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• pp.276-281
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• 2007

The hardening and the constitutive equation based on the crystal plasticity are introduced for the numerical simulation of hemispherical sheet metal forming. For calculating the deformation and the stress of the crystal, Taylor's model of the crystalline aggregate is employed. The hardening is evaluated by using the Taylor factor, the critical resolved shear stress of the slip system, and the sum of the crystallographic shears. During the hemispherical forming process, the texture of the sheet metal is evolved by the plastic deformation of the crystal. By calculating the Euler angles of the BCC sheet, the texture evolution of the sheet is traced during the forming process. Deformation texture of the BCC sheet is represented by using the pole figure. The comparison of the strain distribution and punch force in the hemispherical forming process between the prediction using crystal plasticity and experiment shows the verification of the crystal plasticity-based formulation and the accuracy of the hardening and constitutive equation obtained from the crystal plasticity.

• Korean Journal of Materials Research
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• v.6 no.12
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• pp.1179-1185
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• 1996

고온용 세라믹 쉘 몰드를 용융 알루미나와 콜로이달 실리카를 사용하여 제조한 후, 세라믹 쉘 몰드의 강도에 영향을 주는 요인을 분석하였다. 세라믹 쉘 몰드의 강도는 바인더의 실리카 입자의 크기가 작을수록 크며 또한 실리카 농도에 비례하여 증가함을 보이며 저온에서의 강도 발현은 콜로이달 실리카의 필름 형성에 의한 입자들의 결합임을 알 수 있었다. 세라믹 쉘 몰드의 소성 강도는 부착 스타코의 크기가 작을수록 크며 소성 온도에 비례하여 증가함을 보였다. 고온에서의 강도 발현은 알루미나 입자와 콜로이달 실리카 바인더의 결합뿐만 아니라 알루미나 입자사이의 결합도 영향을 줌을 알 수 있었다. 쉘 몰드의 결정상은 130$0^{\circ}C$이하에서 소성한 경우 $\alpha$-알루미나만 존재함을 보여 실리카는 비정질로 계속 남아 있음을 알 수 있고 140$0^{\circ}C$ 이상에서 소성한 경우 뮬라이트가 생성됨을 보였다.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.21 no.1
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• pp.34-40
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• 2011

The bulk density, water absorption and microstructure of the artificial aggregates were controlled as a function of sintering temperature (1100 and $1200^{\circ}C$) and time (10~60 min) in the fabrication process of the artificial aggregates by the direct sintering process using dredged soil, the inorganic wastes. Also, the physical properties of the artificial aggregates fabricated according to the different sintering methods such as the direct sintering method used in this study and the increasing temperature sintering method used in the previous report, were compared and analysed. The bulk density of aggregates sintered at $1200^{\circ}C$ by the direct sintering method showed below 1.0, and the thickness of a shell and the pore size of the black core were increased with sintering temperature. Also, in the same sintering temperature, the area of black core was decreased, the thickness of shell was increased and the water absorption was decreased with sintering time. The black core of artificial aggregates of bulk density below 1.0 had the similar microstructure, regardless of sintering methods. In contrast, the shell of aggregates fabricated by the increasing temperature sintering method showed more dense microstructure than that by direct sintering method, hence the water absorption of aggregate sintered using direct sintering was relatively high. Thus, the direct sintering method is suitable for fabrication of artificial aggregates in ceramic carriers or absorbents applications.

• 기계와재료
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• v.23 no.3
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• pp.82-95
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• 2011

다결정압연판재는 열가공공정을 거쳐서 생산되므로 공정의 특성을 반영하는 미세조직/집합조직 특성을 가지게 된다. 결정립들은 특정방향으로 배향하고 결정립의 형상과 크기도 변화한다. 이러한 변화는 거시적으로 다결정판재의 이방성으로 귀결이 되는데, 근본적으로 판재를 구성하는 단결정들의 기계적 물성이 각각의 방향별로 이방성을 띄기 때문이다. 본 자료에서는 압연공정시 다결정판재의 수직/평면이방성의 발생원인을 집합조직과 결정소성학을 이용하여 제시하고자 하였다.