• Journal of Ocean Engineering and Technology
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• v.27 no.1
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• pp.37-42
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• 2013

The presence of a crack can increase the local stress or strain, which can cause inelastic deformation and significantly reduce the life of a component or structure. Therefore, in this study, the fatigue crack growth (FCG) behaviors of friction stir welded Al 2024-T3 and Al 7075-T6 specimens were examined, with fatigue cracks growing parallel to the dynamically recrystallized zone at variable ${\Delta}K$ values and an R ratio of 0.3. In addition, the FCG values of the base metal Al 2024-T3 and Al 7075-T6 were tested under the same conditions and parameters as comparative groups. The results showed that compared with the base metal Al 2024 specimen, which had the best fatigue property, the welded specimen had only 88% of the fatigue cycles.

• Korean Journal of Materials Research
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• v.4 no.1
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• pp.24-30
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• 1994

Effect of low temperature retrogression on RRA treatment were investigated in the thermomechanically treated 7075 Al alloy. The complete dissolution of GP zones did not occur during retrogression at $170^{\circ}C$ in T6 material. llrop in strength during initial stage of retrogression was due to the partial dissolution of GP zones. And the strength increased with the formation of $\mu '$ and decreased again with the growth of $\mu '$ and/or formation of $\mu '$ When RRA treatment was applied at the minimum or the secondary peak (maximum) in the hardness curve of retrogression treatment, SCC property was improved markedly without reduction of the strength in comparision with that of T6 materials. And the rhanges in the matrix were not sensitive with time during retrogression at low temperature of $170^{\circ}C$ that the strength and SCC properties were similar at both points in the hardness curve of retrogression treatment.

• Journal of Electrochemical Science and Technology
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• v.13 no.2
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• pp.213-226
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• 2022

In the present study, the corrosion behavior of aluminum Al-7075 tempered (T-6 condition) alloy was evaluated by immersion testing and electrochemical testing in 1.75% and 3.5% NaCl environment at acidic, neutral and basic pH. The data obtained by both immersion tests and electrochemical corrosion tests (potentiodynamic polarization and electrochemical impedance spectroscopy tests) present that the corrosion rate of the alloy specimens is minimum for the pH=7 condition of the solution due to the formation of dense and well adherent thin protective oxide layer. Whereas the solutions with acidic and alkaline pH cause shift in the corrosion behavior of aluminum alloy to more active domains aggravated by the constant flux of acidic and alkaline ions (Cl^- and OH^-) in the media which anodically dissolve the Al matrix in comparison to precipitated intermetallic phases (cathodic in nature) formed due to T6 treatment. Consequently, the pitting behavior of the alloy, as observed by cyclic polarization tests, shifts to more active regions when pH of the solutions changes from neutral to alkaline environment due to localized dissolution of the matrix in alkaline environment that ingress by diffusion through the pores in the oxide film. Microscopic analysis also strengthens the results obtained by immersion corrosion testing and electrochemical corrosion testing as the study examines the corrosion behavior of this alloy under a systematic evaluation in marine environment.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2005.06a
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• pp.348-358
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• 2005

This paper showed mechanical properties and behaviors of macrostructures for specimens welded by F.S.W according to welding conditions and tool with 6.35$mm_t$ aluminum 7075-T651alloy plate. It resulted in defect-free weld zone in case tool rotation speed was 800rpm, 1250rpm and 1600rpm respectively that transition speed was changed to 15mm/min, 61mm/min and 124mm/min with tool's pin diameter 4${\Phi}$mm and 6${\Phi}$mm. The optimum mechanical property, ultimate stress,${\sigma}_Y$=470Mpa was obtained at the condition of 124mm/min of travel speed with 800rpm of tool rotation speed using full screw type pin, shoulder dia. 20${\Phi}$mm, pin dia.6${\Phi}$mm and pin length 6mm. The full-screw type and the half-screw type pin showed the similar behaviors of weldability. It is found that the size of nugget was depended on tool transition speed and tool dimension by macrostructures of the cross section of weld zone.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.1
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• pp.30-41
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• 2006

This paper shows mechanical Properties and behaviors of macrostructures for specimens welded by F.S.W according to welding conditions and tool dimensions with $6.35mm_t$ aluminum 7075-T651 alloy plate. It apparently results in defect-free weld zone in case transition speed was changed to 15mm/min 61mm/min and 124mm/min under conditions of tool rotation speed such as 800rpm. 1250rpm and 1600rpm respectively with tool's Pin diameter 40mm and 60mm. The optimum mechanical property, ultimate stress,${\sigma}_Y=470Mpa$ is obtained at the condition of 124mm/min of travel speed with 800rpm of tool rotation speed using full screw type pin. shoulder dia. $20{\phi}mm$ pin dia. $6{\phi}mm$ and pin length 6mm. The full-screw type and the half-screw type pin shows the similar behaviors of weldability. It is found that the size of nugget is depended on tool transition speed and tool dimension by macrostructures of the cross section of weld zone.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2002.10a
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• pp.335-340
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• 2002

The shot peening process is most often used to improve fatigue properties of metal parts. In order to achieve optimum, repeatable and reliable fatigue enhancement from the shot peening process, the important shot peening parameters must be controlled. In this paper, the optimum shot peening condition is investigated. Rotate bending fatigue test was accomplished to investigate the effects of shot peening on the fatigue strength. Experimental results show that the fatigue strength was tremendously increased by optimum peening condition. But the fatigue strength was decreased by under peening or over peening.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.7
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• pp.2159-2166
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• 1996

The stochastic properties of variation in fatigue crack growth are important in reliability and stability of structures. In this study,the stochastic model for the variation of fatigue crack growth rate was proposed in consideration of nonhomogeneity of materials. For this model, experiments were ocnducted on 7075-T6 aluminum alloy under the constant stress intensity factor range. The variation of fatigue crack growth rate was expressed by random variables Z and r based on the variation of material coefficients C and m in the paris-Erodogan's equation. The distribution of fatigue life with respect to the stress intensity factor range was evaluated by the stochastic Markov chain model based on the Paris-Erdogan's equation. The merit of proposed model is that only a small number of test are required to determine this this function, and fatigue crack growth life is easily predicted at the given stress intensity factor range.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1995.11a
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• pp.195-203
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• 1995

본 연구는 강도와 인성이 우수한 7175Al 단조재의 국산화 개발을 위하여 기존 7XXX합금의 제조공정(일반공정)을 적용한 7075-T6/T73, 7050-774 및 7175-T74 단조재와 중간열처리, 용체화처리 등을 고온처리하는 특수제조 공정을 적용한 7175-T74 단조재의 강도, 파괴인성($K_IC$) 및 피로특성을 비교 평가하였다. 특수공정을 적용한 7175S-T74 단조재는 일반공정의 7075-T73, 7050-T74 및 7175-T74재보다 강도, 파괴 인성 및 피로특성이 향상되었다. 이것은 주조시 응고과정에서 형성되는 2차상입자의 감소와 특수공정의 적용효과에 기인하는 것으로 판단된다.

• Journal of Ocean Engineering and Technology
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• v.6 no.1
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• pp.131-139
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• 1992

The understanding and appllication of fatigue crack propagation mechanism in variable amplitude loading is very important for life prediction of the air travel structures. Particularly, the retardation and arrest behavior of fatigue crack propagation by single tension overloading is essential to the understanding and appllication of fatigue crack propagation mechanism in variable amplitude loading. Numerous studies of the retardation behavior have been performed, however investigations of the arrest behavior have not been enough yet. As for the arrest behavior, Willenborg had reported that the overload shut-off ratio $[R_{so}=(K_{OL})/K_{max})_{crack arrest}]$ had been the material constant, but recently several investigators have reported that the overload shut-off ratio depends upon the stress ratio. In this study, authors have investigated the effect of stress ratio on the threshold overload shut-off ratio to generate arrest of fatigue crack growth in high tensile aluminum alloy 7075-T735 which have used in material for air travel structures, It has been $-0.4\leqqR\leqq0.4$ till now, the region of stress ratio investigated. The threshold overload shut-off ratio has decreased as stress ratio has increased in overall region of -$-0.4\leqqR\leqq0.4$ and the linearity has been seen in this material. Moreover, the experimental equation between $R_{so}$ and R has been made; The relation has been $R_{so}=-R+2.6$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.4
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• pp.834-842
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• 1998

This paper examines the crack growth behavior of 7075-T651 and 5052-H32 aluminum alloy under high-low block loading condition. The cantilever beam type specimen with a chevron notch is used in this study. The crack growth and closure ae investigated by compliance method. The applied stress ratios are R=0.15, 0.0, -0.15 and R=-0.15, 0.0, 0.15. The crack growth rate was found to increase as the load amplitude increases. However,${\bigtriangleup}K_eff$ was almost independent on the stress ratio. The experimental constants of 7075-T651 and 5052-H32 in Paris law were c`=1-1.3${\times}{10^-7},m`=3~3.2 and c`=4~6{\times}{10^-9}, m`=4.3-4.8$, respectively. $K_op$ of 7075-T651 and 5052-H32 becomes smaller as the stress ratio decreases. It seems that the crack closure affects $K_op$.