반복압축하의 응집된 알루미나 분말의 치밀화

김기태,손건석,서정,Kim, K.T.,Son, G.S.,Suh, J. 한국세라믹학회 1992 한국세라믹학회지 Vol.29 No.2

The effects of cyclic stress, frequency and bias-pressure on densification of Al2O3 powder cyclic compaction are investigated. The effect of frequency was not significant on densification of Al2O3 powder under cyclic compaction. The higher the cyclic stress and the lower the bias pressure, the higher densification was achieved. To obtain a higher densification, cyclic compaction was more efficient than 1 stroke compaction. A densification equation was proposed to describe an cyclic time dependent pressure-volume relation for Al2O3 powder under cyclic compaction. This equation was obtained empirically, based on the pressure-volume equation proposed by Cooper and Eaton, the time dependent densification equation by Kim and Suh and experimental data for Al2O3 powder under cyclic compaction. The agreement between the proposed equation and experimental data for Al2O3 powder under cyclic compaction was very good.

반복 일축응력하의 알루미나 파괴거동에 미치는 압축응력의 영향

김기태,서정,백성기,Kim, K.T.,Shu, J.,Baik, S.K. 한국세라믹학회 1991 한국세라믹학회지 Vol.28 No.9

The effect of cyclic compressive stress on fracture responses of Al2O3 was investigated under uniaxial stress cycling. Experimental data were obtained for Al2O3 tension specimens under uniaxial tension-unloading and tension-compression cyclic loading conditions. To investigate the effect of compressive stress on the crack growth, theoretical results from the crack growth rate were compared with measured stress vs. failure relations. At low stress level in tension-compression cycling, residual tensile strains were also observed about failure time.

알루미나 튜브의 인장/비틀림 조합하중하의 파괴거동

김기태,서정,조윤호,Kim, K.T.,Suh, J.,Cho, Y.H. 한국세라믹학회 1991 한국세라믹학회지 Vol.28 No.1

Fracture of Al2O3 tubes for different loading path under combined tension/torsion was investigated. Macroscopic directions of crack propagation agreed well with the maximum principal stress criterion, independent of the loading path. However, fracture strength from the proportional loading test($\tau$/$\sigma$= constant) showed either strengthening or weakening compared to that from uniaxial tension, depending on the ratio $\tau$/$\sigma$. The Weibull theory was capable to predict the strengthening of fracture strength in pure torsion, but not the weakening in the proportional loading condition. The strengthening or weakening of fracture strength in the proportional loading condition was explained by the effect of shear stresses in the plane of randomly oriented microdefects. Finally, a new empirical fracture criterion was proposed. This criterion is based on a mixed mode fracture criterion and experimental data for fracture of Al2O3 tubes under combined tension/torsion. The proposed fracture criterion agreed well with experimental data for both macroscopic directions of crack propagation and fracture strengths.

다공질 금속의 비탄성거동을 위한 특수 구성방정식의 형성 II

김기태,서정,Kim, K.T.,Suh, J. 대한기계학회 1988 대한기계학회논문집 Vol.12 No.1

본 논문에서는 기공의 붕괴에 의한 변형경화 뿐 아니라 모재의 변형경화도 고려하여 더 일반적인 다공질 금속의 특수 구성 방정식 등을 제안하였다. A special set of constitutive equations is formulated to predict elastic-plastic strain hardening responses of porous metals. Including the effect of the material's strain hardening in the yield function, the constitutive equations are capable of showing no dip phenomena in uniaxial strain compression and prediction work-hardening response for plastically precyled porous metal. The proposed constitutive equations are compared with experimental data for porous tungsten.

알루미나 튜브의 복합하중 파괴에 미치는 압축응력의 영향

김기태,서정,Kim, K.T.,Suh, J. 한국세라믹학회 1991 한국세라믹학회지 Vol.28 No.10

Fracture responses of Al2O3 tubes were investigated for various loading paths under combined tension/torsion. The fracture criterion did not depend on loading paths. Fracture angles agreed well with the maximum tensile stress criterion. As the loading condition approaches a shear dominant state, the tensile principal stress at fracture increases compared to the uniaxial fracture strength. By using the Weibull modulus obtained from tension and torsion tests, the Weibull statistical fracture strengths were compared with experimental data. This comparison suggests that fracture may occur at the surface of the specimen when tensile stress is dominant, but within the volume of the specimen when shear stress is dominant. The Weibull fracture strength increased as the loading conition approached a shear dominant state, but underestimated compared to experimental data. Finally, a new fracture criterion was proposed by including the effect of compressive principal stress. The proposed criterion agreed well with experimental data of Al2O3 tubes not only at combined tension/torsion but also at balanced biaxial tension.

CP-Ti 및 Ti-6Al-4V 합금의 전기화학적 특성에 미치는 제조공정의 영향

김기태 ( K. T. Kim ),조현우 ( H. W. Cho ),장현영 ( H. Y. Chang ),김영식 ( Y. S. Kim ) 한국부식방식학회 2018 Corrosion Science and Technology Vol.17 No.1

Ti and its alloys show the excellent corrosion resistance to chloride environments, but they show less corrosion resistance in HCl, H₂SO₄, NaOH, H₃PO₄, and especially HF environments at high temperature and concentration. In this study, we used the commercially pure titanium and Ti-6Al-4V alloy, and evaluated the effect of the manufacturing process on the electrochemical properties. We used commercial products of rolled and forged materials, and made additive manufactured materials by DMT (Directed Metal Tooling) method. We annealed each specimen at 760℃ for one hour and then air cooled. We performed anodic polarization test, AC impedance measurement, and Mott-Schottky plot to evaluate the electrochemical properties. Despite of the difference of its microstructure of CP-Ti and Ti-6Al-4V alloys by the manufacturing process, the anodic polarization behavior was similar in 20% sulfuric acid. However, the addition of 0.1% hydrofluoric acid degraded the electrochemical properties. Among three kinds of the manufacturing process, the electrochemical properties of additive manufactured CP-Ti, and Ti-6Al-4V alloys were the lowest. It is noted that the test materials showed a Warburg impedance in HF acid environments.

36개월간 국내 옥외폭로시험에 따른 아연도강의 표면외관 변화

김기태 ( K. T. Kim ),김영식 ( Y. S. Kim ) 한국부식방식학회 2018 Corrosion Science and Technology Vol.17 No.6

If galvanized steel is exposed to an outdoor environment, atmospheric corrosion will occur with time and red rust will form when the sacrificial protection capacity of zinc reaches its limit. With corrosion, the surface appearance of steel changes, and the properties of the exterior materials degrade. In this study, two kinds of galvanized steel, (GA and GI specimens) were subjected to an outdoor exposure test for 36 months in six regions of Korea. Chrominance (color, chroma, and brightness) and glossiness surface analyses were performed. The color change was not significant, regardless of the exposed area or the specimens tested. With increasing exposure times, the GA specimen became blackened by the formation of zinc oxide, and red coloration was increased by the formation of red rust. As the exposure time of GI specimen increased, the surface proceeded to blacken, but no red rust was formed and the color did not change significantly. Regardless of the outdoor exposure area or the specimen, longer exposure times led to lower glossiness, and this behavior appears to be influenced by the formation of zinc oxide.

8인치 직경의 304L 스테인리스강관의 부식특성에 미치는 제작공정의 영향

김기태 ( K. T. Kim ),허승영 ( S. Y. Hur ),장현영 ( H. Y. Chang ),김영식 ( Y. S. Kim ) 한국부식방식학회 2018 Corrosion Science and Technology Vol.17 No.6

Austenitic stainless steels used in nuclear power plants mainly use pipes made of seamless pipes, which depend on imports. The manufacturing process and high cost are some of the problems associated with seamless pipes. Therefore, in this study, the corrosion characteristics of the seamless pipe and the SAW pipe were assessed to determine the safety and reliability of the SAW pipe in a bid to replace the seamless pipe. Microstructure was analyzed using an optical microscope and the degree of hardness was measured using a Rockwell B scale. Intergranular corrosion resistance was evaluated by ASTM A262 Practice A, C, and E methods. The degree of sensitization was determined using a DL-EPR test. Anodic polarization test was performed in deaerated 1% NaCl solution at 30 ℃ and the U-bend method was used to evaluate the SCC resistance in 0.01 M Na₂S₄O₆ at 340 ℃ and 40% NaOH solution at 290 ℃. Weld metal of the SAW pipe specimen showed relatively high degree of sensitization and intergranular corrosion rate. However, annealing to SAW pipes improved the corrosion properties in comparison to that of the seamless pipe.

적층가공 방식으로 제조된 CP-Ti의 캐비테이션 중 부식에 대한 전기화학적 접근

김기태 ( K. T. Kim ),장현영 ( H. Y. Chang ),김영식 ( Y. S. Kim ) 한국부식방식학회 2018 Corrosion Science and Technology Vol.17 No.6

The effect of passive film on corrosion of metals and alloys in a static corrosive environment has been studied by many researchers and is well known, however few studies have been conducted on the electrochemical measurement of metals and alloys during cavitation corrosion conditions, and there are no test standards for electrochemical measurements ‘During cavitation’ conditions. This study used commercially additive manufactured(AM) pure titanium in tests of anodic polarization, corrosion potential measurements, AC impedance measurements, and repassivation. Tests were performed in 3.5% NaCl solution under three conditions, ‘No cavitation’, ‘After cavitation’, and ‘During cavitation’ condition. When cavitation corrosion occurred, the passive current density was greatly increased, the corrosion potential largely lowered, and the passive film revealed a small polarization resistance. The current fluctuation by the passivation and repassivation phenomena was measured first, and this behavior was repeatedly generated at a very high speed. The electrochemical corrosion mechanism that occurred during cavitation corrosion was based on result of the electrochemical properties ‘No cavitation’, ‘After cavitation’, and ‘During cavitation’ conditions.

채널형 전극 기반 강유전성 액정을 활용한 능동형 광학 셔터

김기태(K. Kim),김기홍(G. H. Kim),최문영(M.-Y. Choi),나준희(J.-H. Na) Korean Society for Precision Engineering 2022 한국정밀공학회 학술발표대회 논문집 Vol.2022 No.5