• Corrosion Science and Technology
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• 제6권4호
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• pp.206-210
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• 2007

Aerospace aluminum alloys such as Al alloy 2024-T3 and 7075-T6 are subject to localized corrosion due the existence of intermetallics containing Cu, Mg or Zn. Chromate is currently widely used in the aerospace industry as the corrosion inhibitor for these alloys. However, chromate needs to be replaced due to its strong carcinogenicity. In this study, an extensive pigment screening has been performed to find replacements for chromates. Different categories of inhibitors were evaluated by immersion tests, DC polarization tests and other methods. Phosphates, zinc salts, cerium salts, vanadates and benzotriazole were found to be effective inhibitors for AA7075. Among those inhibitors, zinc phosphate was found to be the most effective in our novel, silane-based, one-step aqueous primer system. The performance of this primer is comparable to that of currently used chromate primers in accelerated corrosion tests, while it is completely chromate-free and its VOC is about 80% less than that of current primers. Studies by SEM/EDS showed that the unique structure of the superprimer accounts for the strong anti-corrosion performance of the zinc phosphate pigment. The self-assembled stratified double-layer structure of the superprimer is characterized by a less-penetrable hydrophobic layer at the top and a hydrophilic layer accommodating the inhibitors underneath. The top layer functions as the physical barrier against water ingress, while the lower layer functions as a reservoirfor the inhibitor, which is leached out only if the coating is damaged by a scratch or scribe. The presence of a silane in the primer further improves the adhesion and anti-corrosion performance of the primer.

• 한국해양공학회지
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• 제6권1호
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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 $R_{so}=-R+2.6$.

• 대한기계학회논문집A
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• 제36권11호
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• pp.1381-1389
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• 2012

본 논문에서는 쇼트피닝 가공한 알루미늄 7075-T6 재료의 부식피로향상에 대한 연구를 하였다. 사용된 알루미늄 금속은 상대적으로 가볍고 강한 재료의 특성으로 항공기 주요부품의 재료로 사용되고 있으며, 많은 연구를 통해 알루미늄을 사용하면 약 50%의 무게 감소효과를 낼 수 있다고 한다. 재료의 부식은 인장환경에서 재료의 파단시점을 앞당기고 구조물의 수명을 심각하게 감소시킨다. 따라서 알루미늄 금속의 부식 환경에서의 재료의 부식저항을 향상 시킬 수 있는 연구가 요구된다. 쇼트피닝 가공은 재료 표면에 압축잔류응력을 인가하여 재료의 피로수명을 늘리고 나아가 재료의 신뢰성을 확보하는 기술로 반복하중을 받는 많은 부품들에 이미 적용되어 사용되고 있다. 부식 환경에서는 가공 후 거칠어진 표면으로 인해 공식부식을 야기하지만, 피닝가공에 의한 표면 직하에 압축잔류응력이 부식을 억제하여 부식저항을 증가시킨다. 본 연구에서의 쇼트피닝가공된 시험편에 대한 실험결과와 선행연구에서의 쇼트피닝 가공하지 않은 알루미늄 합금 재료의 부식피로특성을 비교하였다. 실험 결과 쇼트피닝 가공은 알루미늄합금의 피로수명향상에 영향을 미치고, 압축잔류응력은 부식피로수명을 증가시킴을 알 수 있었다.

• 대한기계학회논문집A
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• 제36권4호
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• pp.421-428
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• 2012

본 논문에서는 알루미늄 7075-T6 재료의 부식 기간별 표면 산화막 형성이 피로거동에 미치는 영향에 대해 연구하였다. 사용된 알루미늄 재료는 부식에 대한 부동태화 현상으로 부식저항 특성이 높은 금속으로 알려져 있다. 알루미늄 합금은 다른 재료에 비해 가볍고 강한 재료 특성 때문에 항공기 부품 산업과 같은 다양한 산업분야에서 널리 사용되고 있다. 때문에 부식에 대한 재료의 피로거동 특성과 부식에 대한 부동태화 특성에 대한 연구가 요구된다. 4절점 회전 굽힘 시험기를 사용하여 부식 기간별 재료의 피로거동 특성을 확인하였고, 표면 거칠기를 측정하여 부식 기간별 알루미늄 재료의 표면 부식 정도를 평가하였다. 또한 전자주사현미경을 통해 파단면 분석 및 재료 표면에 형성된 산화막을 측정하였다. 실험결과 초기 부식 4주 동안은 부식이 활발히 진행되어 피로수명은 크게 감소하고 표면 거칠기는 증가하였다. 하지만 4주 이후부터 재료의 부동태화 현상으로 부식 반응이 둔화되는 경향을 보였다. 전자주사현미경을 통한 분석에서도 표면 산화막의 성장이 4주 이후부터 둔화되어 재료 표면의 산화막이 보호층 역할을 하여 더 이상의 부식진행을 방지한다는 결론은 얻었다.

• Structural Engineering and Mechanics
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• 제18권3호
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• pp.277-286
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• 2004

Defective metallic components and structures are being repaired with bonded composite patches to improve overall mechanical and fatigue properties. In this study, fatigue crack growth tests were conducted on pre-cracked 7075/T6 Aluminum substrates with and without bonded Boron/epoxy patches. A considerable increase in the fatigue life and a decrease in the stress intensity factor (SIF) were observed as the number of patch plies increased. The experimental results demonstrate that the patch configurations and patch thickness can enhance fatigue life by order of magnitude. Quantitative comparisons between analytical and experimental data were made, and the analytical model based on a modified Rose's analytical solution appears to best estimate the fatigue life.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회A
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• pp.1601-1606
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• 2007

In order for high strength aluminum alloys to be used in transportation systems and the aerospace industry, excellent mechanical and physical properties are required. In particular, excellent anti-abrasion property is indispensable for parts that require driving force. In general, surface treatment technologies such as high frequency heat treatment, gas solid carburizing, surface rolling, shot peening are used as ways of improving anti-abrasion property. Among various surface treatment technologies, this research chose shot peening processing for Al7075-T6, which is well known as representative high-strength alloy steel. Wear characteristics were compared and analyzed after shot peening processing with shot ball velocities of 40m/s and 70m/s in order to investigate the effects of shot peening processing on wear characteristics.

• 한국정밀공학회지
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• 제7권4호
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• pp.55-64
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• 1990

The fatigue crack growth behavior of physically-short cracks(0.2${\Delta}K$ with $da/dN<1{\times}10^{-7}m/cycle$. The transition crack lengths where similtude with ${\Delta}K$ existed was between 1 and 2mm. The effective stress intensity factor range based on COD measurements gave better correlation between the physically-short and long cracks. Thus it can be considered that the crack closure effect is one of the main factors which causes the differences between these two cracks.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.1269-1270
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• 2006

Mg-Zn-RE alloys had a novel lond period stacking ordered (LPO) structure. Their rapidly solidified powder metallurgy (RS P/M) alloys exhibited a combination of high strength and god ductility (tensile yield strength above 550 MPa and elongation above 5%). The LPO Mg-Zn-RE RS P/M alloys had high elevated temperature strength (tensile yield strength above 380 MPa at 473 K) and exhibited a high-strain-rate superplasticity at higher temperatures. In Japan, a national project for developing high strength LPO Mg-Zn-RE RS P/M alloys has started at 2003 for 5 years, which is founded by the Ministry of Economy, Trade and Industry (METI) of Japan. In the national project, project targets in materials performances have been achieved. The developed LPO Mg-Zn-RE RS P/M alloys exhibited higher tensile yield strength, fatigue strength and corrosion resistance than high strength aluminum alloys of extra-super-duralumin (7075-T6).

• 한국해양공학회지
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• 제18권1호
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• pp.28-34
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• 2004

It is well known that tensile peak overloads may significantly delay suubsequent constant amplitude fatigue crack growth in many materials. Since real structures are usually subjected to complex load histories, the ability to predict accurate crack growth under realistic service conditions is of major engineering interest. This paper describes experiments on fatigue track growth in the integrally stiffened panel of 7075-T6 aluminum alloy. The effect of shape parameters and overload position on the fatigue crack growth behavior of integrally stiffened panels are discussed. Based on the experimental results, the following conclusions have been drawn: the overall fatigue crack growth retardation resulting from single overload in the stiffened panels was generally larger in the larger thickness ratio, although the retardation trends, according to the change in overload positions, were similar to those exhibited in the non-stiffened panels.

• 오토저널
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• 제12권2호
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• pp.43-50
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• 1990

The fatigue growth behavior of surface cracks cannot be adequately predicted solely by stress intensity factor analysis. This is caused by different plastic deformation due to variations in the stress field triaxiality along the crack tip. Therefore, a new model which accounts for the crack closure phenomenon is proposed in this paper to predict the fatigue crack growth patterns for surface cracks. Fatigue tests were performed to develop the new model for the prediction and to assess the accuracy of the analysis. The predicted crack growth behavior for PMMA and Aluminum alloy 7075-T6 materials agreed well with the experimental data.