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.

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.

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.

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.

적층가공된 티타늄 합금의 전기화학적 특성에 미치는 불산의 영향

김기태 ( K. T. Kim ),조현우 ( H. W. Cho ),장현영 ( H. Y. Chang ),김영식 ( Y. S. Kim ) 한국부식방식학회(구 한국부식학회) 2018 Corrosion Science and Technology Vol.17 No.4

In this study, the electrochemical properties of CP-Ti (commercially pure titanium) and Ti-64 (Ti-6Al-4V) were evaluated and the effect of hydrofluoric acid on corrosion resistance and electrochemical properties was elucidated. Additive manufactured materials were made by DMT (Directed Metal Tooling) method. Samples were heat-treated for 1 hour at 760 °C and then air cooled. Surface morphologies were studied by optical microscope and SEM. Electrochemical properties were evaluated by anodic polarization method and AC-impedance measurement. The oxide film formed on the surface was analyzed using an XPS. The addition of HF led to an increase in the passive current density and critical current density and decreased the polarization resistance regardless of the alloys employed. Based on the composition of the oxide film, the compositional difference observed by the addition of HF was little, regardless of the nature of alloys. The Warburg impedance obtained by AC-impedance measurement indicates the dissolution of the constituents of CP-Ti and Ti-64 through a porous oxide film.

자동차 핵심부품 생산기반공정 플랫폼 기술

김기태(K. T. Kim) 한국생산제조학회 2013 한국생산제조시스템학회 학술발표대회 논문집 Vol.2013 No.4

36개월간 국내 옥외폭로시험에 따른 아연도강의 부식거동

김기태 ( K. T. Kim ),김영식 ( Y. S. Kim ) 한국부식방식학회(구 한국부식학회) 2018 Corrosion Science and Technology Vol.17 No.5

Atmospheric corrosion is generally an electrochemical degradation process of metal. It can be caused by various corrosion factors of atmospheric component, weather, and air pollutants. Moisture, particles of sea salts, and sulfur dioxide are major factors in atmospheric corrosion. Galvanizing coating is one of the most efficient ways to protect iron from corrosion by zinc plating on the surface of the iron. Galvanized steels are being widely used in automobiles, building structures, roofing, and other industrial structures due to their high corrosion resistance compared to bare iron. Atmospheric corrosion of galvanized steel has shown complex corrosion behavior depending on coating process, coating thickness, atmospheric environment, and air pollutants. In addition, different types and kinds of corrosion products can be produced depending on the environment. Lifespan of galvanized steels is also affected by the environment. Therefore, the objective of this study was to determine the corrosion behavior of galvanized steel under atmospheric corrosion at six locations in Korea. When the exposure time was increased, content of zinc from GA surface decreased while contents of iron and oxygen tended to increase. On the other hand, content of iron was constant even after 36 months of exposure of GI.

이음매 없는 304L 스테인리스강관의 부식특성에 미치는 열처리의 영향

김기태 ( K. T. Kim ),엄상빈 ( S. B. Um ),김영식 ( Y. S. Kim ) 한국부식방식학회(구 한국부식학회) 2017 Corrosion Science and Technology Vol.16 No.6

Austenitic stainless steels have been widely used for various systems of nuclear power plants. Among these stainless steels, small pipes with diameter less than 14 inch have been produced in the form of seamless pipe. Annealing and cooling process during the manufacturing process can affect corrosion properties of seamless stainless steels. Therefore, 12 inch-diameter of as-received 304L stainless steel pipe was annealed and aged in this study. 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. U-bend method in an autoclave was used to evaluate the SCC resistance in 0.01 M Na₂S₄O₆ or 40% NaOH solution at 340 °C. As-received specimen showed relatively high degree of sensitization and intergranular corrosion rate. Carbon segregation was also observed near grain boundaries. Annealing treatment could give the dissolution of segregated carbon into the matrix. Aging treatment could induce segregation of carbon and finally form carbides. Microstructural analysis confirmed that high intergranular corrosion rate of the as-received seamless pipe was due to micro-galvanic corrosion between carbon segregation and grains.

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

김기태 ( K. T. Kim ),장현영 ( H. Y. Chang ),김영식 ( Y. S. Kim ) 한국부식방식학회(구 한국부식학회) 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.