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The mechanical properties and fatigue crack growth rate fracture toughness of permanent mould cast austempered gray cast iron(AGI) were compared to those of sand cast AGI. Specimens prepared for tensile, impact and fatigue test were austenitized at 900℃ and austempered at 270℃, 320℃, 370℃ and 420℃ for 1 hour. The strength, impact and fatigue crack propagation behavior of permanent mold cast AGI were found to be superior to those of sand cast AGI. Maximum values in tensile strength, BHN, Charpy impact energy, were obtained at the austempering temperature of 270℃ Samely, the slowest fatigue crack growth rate was appeared at the austempering temperature of 270℃. But ductility of AGI was not improved by permanent mould casting.
전분 이용이 가능한 산업용 Saccharomyces cerevisiae균주를 개발하기 위해 alcohol dehydrogenase 유전자 프로모터(ADClp)의 조절하에 발현되는 Aspergillus awamori glucoamylase cDNA 유전자(GA1)를 산업용 S. cerevisiae의 염색체에 도입하였다. 산업적 이용에 적합한 효모균주를 얻기 위해 세균 ampicillin 저항성 유전자가 제거되고 GA1 유전자와 선별 표지유전자로 S. cerevisiae aureobasidin A 저항성 유전자(AUR1-C)와 재조합 부위로 Tyretrotransposon $\delta$-서열이 포함된 integrative cassette를제조하였다. 이 $\delta-integrative$ cassette로 형질전환된 산업용 S. cerevisiae는 배지상에 glucoamylase를 생산 분비하였고 전환을 유일한 탄소원으로 하여 생장하였다. 형질전환체를 비선택배지에서 배양했을 매 삽입된 GA1유전자가 100세대까지 안정되게 유지되었다. To construct an amylolytic industrial strain of Saccharomyces cerevisiae, the glucoamylase cDNA gene (GAl) from Aspergillus awamori was expressed under the control of the alcohol dehydrogenase gene promoter (ADC1p) and integrated into the chromosomes of industrial S. cerevisiae. An integrative cassette lacking bacterial ampicillin resistance gene but containing the GA1 gene, $\delta$ sequences of Ty1 retrotransposon as target sites for homologous recombination and S. cerevisiae aureobasidin A resistance gene (AUR1-C) as the selection marker was constructed to obtain a strain eligible for commercial use. Industrial S. cerevisiae transformed with this 15-integrative cassette efficiently secreted glucoamylase into the medium and grew on starch as the sole carbon source. The transformants were mitotically stable for 100 generations in nonselective medium.
6063-T5 alloys are tested in laboratory air, water and 3%% NaCl solution to investigate the effects of corrosive environment on the retardation behavior through single overload fatigue test. Also, the fatigue crack propagation and the crack closure behavior are studied. The results obtained in this experimental study are summarized as follows : 1) Behaviors of fatigue crack growth retardation are observed in water and 3% NaCl solution as they do in air. The number of delay cycles and the size of affected region by single overload decrease greatly in water and 3% NaCl compared with those in air. 2) In fractographic results, the overload marking by single overload appear remarkably in air, but indistinctly in water and 3% NaCl solution. 3) The effect of crack closure on crack propagation is most remarkable in the beginning of crack propagation. With crack propagation, the crack closure level and its effect decrease greatly.
Elastic-plastic fracture toughness(J_(IC)) by ultrasonic method is evaluated in terms of width and thickness. Widths of specimen in 6061-T6 aluminum alloy are 50㎜ and 100㎜, thicknesses of those are 20㎜ and 25㎜, respectively. Elastic-plastic fracture toughness by ultrasonic method is independent of specimen thickness and side groove. Angle beam probe which are placed on the end of the compact specimen detect the maximum crack extension effectively. Comparing with elastic-plastic fracture toughness by ultrasonic method and that of unloading compliance method, J_(IC) of ultrasonic method are underestimated to that of unloading compliance method. Elastic-plastic fracture toughness of width 100㎜ specimen are underestimated to that of width 50㎜ specimen about 20%.
The mechanical properties and fracture toughness of permanent mold cast austempered gray cast iron(AGI) were compared to those of sand cast AGI. The iron was melted to eutectic composition in order to get better castability especially in permanent mold casting. Specimens prepared for tensile, impact and fracture toughness test were austenitized at 900℃ and austempered at 270℃ and 370℃ for 1hour. The strength, impact and fracture toughness of permanent mold cast AGI were found to be superior to those of sand cast AGI. The maximum value of 836㎫ in tensile strength, was obtained at the austempering temperature of 270℃. But ductility of AGI was not improved by permanent mold casting.
Effects of tensile and compression residual stresses in the welded SS41 and Al7075-T6 on fatigue crack propagation behavior are investigated when a crack propagates from residual stresses region. We propose the fatigue crack growth equation on tensile and compression residual stresses in welded metal. The results obtained in this experimental study are summarized as follows : 1) A fatigue crack growth equation which applied fatigue fracture behavior of the welded metal is proposed. (수식) where, α, β, γ and δ are constants, and R_(eff) is effective stress ratio [R_(eff)=(K_(min)+K_(res))/(K_(max)+K_(res)], K_(ef) is critical fatigue stress intensity factor. The constants are obtained from nonlinear least square method. The relation between crack length and number of cycles obtained by integrating the fatigue crack growth rate equation is in agreement with the experimental data. 2) The experimental results confirmed that the cause of crack extension and retardation by residual stresses has relation to the phenomenon of crack closure. 3) The relaxing trend of residual stresses by the crack propagation was greater in case of compressive residual stress than that of tensile residual stress in the welded metal.
This study describes the 3D design and analysis of a composite insulation boom of an aerial work platformfor electric work. Structural analysis of the composite insulation boom was performed using ANSYS and thetotal deformation, equivalent stress, and normal elasticity were investigated at 150 mm, 500 mm, 1,000 mm,and 1,500 mm using a mounted strain gauge. The results of the analysis and test were similar in a linearlyincreasing slope and the difference in the strain value was about 10%. This difference depends on thecompressive force in the manufacturing process.
The retardation effect of fatigue crack propagation after cyclic overloading seems to be affected by strain hardening exponent. Namely, for the material with high values of n, the delay effect is found to be severe. We proposeds a modified crack retardation equation which may apply the retardation of fatigue crack growth after a cyclic overloading, as ?? where, ?? is effective stress ratio ??. The constants μ= - 0.5 and λ= 0.6, and the values are found to be identical for materials such as aluminum (A 1060), steel (SS 34), brass (BsS1B) and stainless steel (SUS 304) used in this investigation.