• 한국세라믹학회지
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• 제35권6호
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• pp.551-558
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• 1998

Detection of microcrack in {{{{ {Al }_{2 } {O }_{3 } }} ceramics were studided by AE(acoustic emission) technique with 4-point bending test in order to evaluate the fracture process and formation of microcrack. Fully-dense alu-mina ceramics having a different grain size were fabricated by varing the hot-pressing temperature. The grain size of alumina increased with increasing the hot-pressing temperature whereas the bending strength decreasd. The microcracks were observed by SEM and TEM. The generation of AE event increased with increasing the applied load and many AE event was generated at maximum applied load. Alumina with smaller grain size shows the generation of many AE event resulting in an increase of microcrack formation. An intergranular fracture is predominantly observed in fine-grained alumina whereas intragranular fracture occurs predominantly in coarse-grained alumina,. Analysis of micorstructure and AE prove that primary mi-crocracks occur within grain-boundaries of alumina. The larger microcracking were formed by the growth and/or coalesence of primary microcracks. Then the materials become to fracuture by main crack gen-eration at the maximum applied load.

• Geomechanics and Engineering
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• 제20권3호
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• pp.243-254
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• 2020

Due to the permanent damage to structures during earthquakes, soil liquefaction is an important issue in geotechnical earthquake engineering that needs to be investigated. Typical examples of soil liquefaction have been observed in many earthquakes, particularly in Alaska, Niigata (1964), San Fernando (1971), Loma Prieta (1989), Kobe (1995) and Izmit (1999) earthquakes. In this study, liquefaction behavior of uniform sands of different grain sizes was investigated by using the energy-based method. For this purpose, a total of 36 deformation-controlled tests were conducted on water-saturated samples in undrained conditions by using the cyclic simple shear test method and considering the relative density, effective stress and mean grain size parameters that affect the cumulative liquefaction energy. The results showed that as the mean grain size decreases, the liquefaction potential of the sand increases. In addition, with increasing effective stress and relative density, the resistance of sand against liquefaction decreases. Multiple regression analysis was performed on the test results and separate correlations were proposed for the samples with mean grain size of 0.11-0.26 mm and for the ones with 0.45-0.85 mm. The recommended relationships were compared to the ones existing in the literature and compatible results were obtained.

• Asian-Australasian Journal of Animal Sciences
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• 제31권1호
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• pp.80-85
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• 2018

Objective: This study was conducted to investigate the effect of corn grain particle size on ruminant fermentation and blood metabolites in Holstein steers fed total mixed ration (TMR) as a basal diet to explain fundamental data of corn grain for cattle in Korea. Methods: Four ruminally cannulated Holstein steers (body weight $592{\pm}29.9kg$) fed TMR as a basal diet were housed individually in an auto temperature and humidity modulated chamber ($24^{\circ}C$ and 60% for 22 h/d). Treatments in a $4{\times}4$ Latin square design were TMR only (control), TMR with whole corn grain (WC), coarsely ground corn grain (CC), and finely ground corn grain (FC), respectively. The corn feeds substituted for 20% energy intake of TMR intake. To measure the ruminal pH, ammonia N, and volatile fatty acids (VFA), ruminal digesta was sampled through ruminal cannula at 1 h intervals after the morning feeding to determine ruminal fermentation characteristics. Blood was sampled via the jugular vein after the ruminal digesta sampling. Results: There was no difference in dry matter (DM) intake between different corn particle size because the DM intake was restricted to 1.66% of body weight. Different corn particle size did not change mean ammonia N and total VFA concentrations whereas lower (p<0.05) ruminal pH and a ratio of acetate to propionate, and higher (p<0.05) propionate concentration were noted when the steers consumed CC compared with WC and FC. Concentration of blood metabolites were not affected by different particle size of corn grain except for blood triglyceride concentration, which was significantly (p<0.05) increased by FC. Conclusion: Results indicate that feeding CC may increase feed digestion in the rumen, whereas the FC group seemed to obtain inadequate corn retention time for microbial degradation in the rumen.

• 한국결정성장학회지
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• 제29권6호
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• pp.251-256
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• 2019

ZnO를 통상적인 소결온도 이상의 온도 1385℃에서 소결하는 과정에서 mm 크기로 거대 성장된 입자를 갖는 세라믹스를 제조하였다. 1400℃에서 8시간 소결하는 경우 성장에 참여하지 않은 입자의 크기는 30~40 ㎛이고 거대 성장된 입자는 1,000 ㎛에 달하여 부피비 최소 10,000배 이상의 급속한 성장이 이루어졌다. 이러한 급속한 성장의 원인으로 일차 입자 크기분포, 성형압 불균일 또는 불순물의 합입등을 고려하였으며, 이들 중 일차입자 크기 분포일 것으로 추정되나 확실한 증거를 확보하지 못하였다. 미세구조 관찰을 통해 거대입자 성장은 주변의 입자를 통째로 합치는 과정을 통해 성장하는 것으로 추정된다.

• 대한지리학회지
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• 제45권5호
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• pp.553-572
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• 2010

퇴적물이나 토양의 입도분석은 가장 중요하고 기초적인 지구과학적 연구방법 중 하나이다. 본 연구는 뢰스-고토양 퇴적물을 대상으로 10가지 전처리 과정에 따른 입도분석 결과의 차이를 비교하였다. 퇴적물의 분산에 있어 확산제의 효과가 가장 크지만, 확산제와 염산의 처리 순서에 따라 큰 차이를 보인다. 이는 염산의 염소 이온과 확산제의 나트륨 이온 사이의 정전기적 인력으로 인해, 나트륨 이온이 효과적으로 퇴적물의 칼슘 이온을 치환하지 못하기 때문으로 생각된다. 또한 과산화수소와 염산은 끓는 물보다는 효과적이지만 퇴적물의 분산에 큰 영향을 미치지 않는다. 따라서 연구 목적 및 시료 특성에 맞는 적절한 방법을 선정해야 신뢰할 수 있는 입도분석 결과를 얻을 수 있을 것이다.

• 한국지반공학회논문집
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• 제29권7호
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• pp.47-55
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• 2013

조립재료의 요소시험에서 장비에 허용되는 조립재료의 입자크기가 실제 현장에서 사용되는 입자크기에 비해 작은 경우가 종종 발생하기 때문에, 이러한 경우에는 시험에 사용되는 재료의 최대입경을 시험장치의 가용 크기까지 축소시켜 시험을 수행하게 된다. 본 연구에서는 이처럼 최대입경 크기를 축소시켜 입도를 조정했을 때, 저변형률에서의 탄성계수에 미치는 영향을 평가하기 위해 대형반복삼축압축시험을 수행하였다. 본 연구 대상재료의 시험 결과, 입도 조정된 시편들은 원입도 시편의 탄성계수를 과소평가하는 경향이 있었으며, 그 차이는 저변형률에서 더 크게 발생되었다.

• The Journal of Advanced Prosthodontics
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• 제5권2호
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• pp.161-166
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• 2013

PURPOSE. This study aimed to identify the effects of the sintering conditions of dental zirconia on the grain size and translucency. MATERIALS AND METHODS. Ten specimens of each of two commercial brands of zirconia (Lava and KaVo) were made and sintered under five different conditions. Microwave sintering (MS) and conventional sintering (CS) methods were used to fabricate zirconia specimens. The dwelling time was 20 minutes for MS and 20 minutes, 2, 10, and 40 hours for CS. The density and the grain size of the sintered zirconia blocks were measured. Total transmission measurements were taken using a spectrophotometer. Two-way analysis of variance model was used for the analysis and performed at a type-one error rate of 0.05. RESULTS. There was no significant difference in density between brands and sintering conditions. The mean grain size increased according to sintering conditions as follows: MS-20 min, CS-20 min, CS-2 hr, CS-10 hr, and CS-40 hr for both brands. The mean grain size ranged from 347-1,512 nm for Lava and 373-1,481 nm for KaVo. The mean light transmittance values of Lava and KaVo were 28.39-34.48% and 28.09-30.50%, respectively. CONCLUSION. Different sintering conditions resulted in differences in grain size and light transmittance. To obtain more translucent dental zirconia restorations, shorter sintering times should be considered.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2006년도 춘계학술대회 논문집
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• pp.57-58
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• 2006

The effects of lowering ECAP temperature during ECAP process and Post-ECAP annealing on microstructure, texture and mechanical properties of the AZ31 alloys have been investigated in the present study. The as-extruded materials were ECAP processed to 2 passes at 553K prior to subsequent pressing up to 6 passes at 523K or 493K. When this method of lowering ECAP temperature during ECAP was used, the rods could be successfully deformed up to 6 passes without any surface cracking. Grain refinement during ECAP process at 553K might have helped the material to endure further straining at lower deformation temperatures probably by increasing the strain accommodation effect by grain boundary sliding, causing stress relaxation. Texture modification during ECAP has a great influence on the strength of Mg alloys because HCP metals have limited number of slip systems. As slip is most prone to take place on basal planes in Mg at room temperature, the rotation of high fraction of basal planes to the directions favorable for slip as in ECAP decreases the yield stress appreciably. The strength of AZ31 Mg alloys increases with decrease of grain size if the texture is constant though ECAP deformation history is different. A standard positive strength dependence on the grain size for Mg alloys with the similar texture (Fig. 1) supports that the softening of ECAPed Mg alloys (a negative slope) typically observed despite the significant grain refinement is due to the texture modification where the rotation of basal planes occurs towards the orientation for easier slip. It could be predicted that if the original fiber texture is restored after ECAP treatment yielding marked grain refinement, yield stress as high as 500 MPa will be obtained at the grain size of ${\sim}1{\mu}m$. Differential speed rolling (DSR) with a high speed ratio between the upper and lower rolls was applied to alter the microstructure and texture of the AZ31 sheets. Significant grain refinement took place during the rolling owing to introduction of large shear deformation. Grain size as small as $1.4{\mu}m$ could be obtained at 423K after DSR. There was a good correlation between the (0002) pole intensity and tensile elongation. This result indicates that tensile ductility improvement in the asymmetrically rolled AZ31 Mg alloys is closely related to the weakening of basal texture during DSR. Further basal texture weakening occurred during annealing after DSR. According to Hall-Petch relation shown in Fig. 1, the strength of the asymmetrically rolled AZ31 is lower than that of the symmetrically rolled one when compared at the same grain size. This result was attributed to weakening of fiber texture during DSR. The DSRed AZ31, however, shows higher strength than the ECAPed AZ31 where texture has been completely replaced by a new texture associated with high Schmid factors.

• Applied Microscopy
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• 제48권4호
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• pp.132-133
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• 2018

One of the most important factors determining the properties of a material is its grain size. However, unclear grain boundaries in the image hinder an accurate measurement of grain size. We demonstrate that grain boundaries existing in the images obtained by scanning transmission electron microscopy (STEM) can be clearly distinguished by applying a Sobel filter to a tilting series of STEM images of a hydrogenation-disproportionation-desorption-recombination processed Nd2Fe14B magnet sample.

• International Journal of Korean Welding Society
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• 제2권1호
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• pp.25-28
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• 2002

In the coarse grain HAZ adjacent to the fusion line, most of the TiN particles in conventional Ti added steel are dissolved and austenite grain growth is easily occurred during welding process. To avoid this difficulty, thermal stability of TiN particle is improved by increasing the nitrogen content in steel. In this study, the effect of hlgh nitrogen TiN particle on preventing austenite grain growth in HAZ was investigated. Increased thermal stability of TiN particle is helpful for preventing the austenite grain growth by pinning effect. High nitrogen TiN particle in simulated HAZ were not dissolved even at high temperature such as 1400'E and prevented the austenite grain growth in simulated HAZ. Owing to small austenite grain size in HAZ the width of coarse grain HAZ in high nitrogen TiN steel was decreased to 1/10 of conventional TiN steel. Even high heat input welding, the microstructure of coarse grain HAZ consisted of fine polygonal ferrite and pearlite and toughness of coarse grain HAZ was significantly improved.