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The 2024-T8, 7075-T6, and 7075-T73 alloys were studied in laboratory air and 3.5 percent sodium chloride (NaCl) solution to investigate the effects of a corrosive environment on the retardation behavior of aluminum alloys. Single overload cycles at overload ratios of 1.5, 2.0, and 2.5 were used. The single overloads were found to cause a decrease in constant-amplitude crack-growth in saltwater just as they did in air. Also, the number of delay cycles increased with each increase in overload ratio (OLR) in saltwater just as they did in air. However, the number of delay cycles was larger in air than in 3½ percent saltwater, and the difference was greater for the 7075-T6 alloy than it was for the 2024-T8 and 7075-T73 alloys. The results indicated that a given microstructure and its susceptibility to environmental attack are important in determining its fatigue behavior under variable amplitude loading in a corrosive environment. An alloy which shows superior retardation behavior in air can be inferior in a corrosive environment such as saltwater as was found for the 7075-T6 and 2024-T8 alloys. The difference in the retardation behavior of these two alloys in air was small, while in saltwater, the 2024-T8 alloy was generally superior. Environmental attack similar to that which was observed in the constant-amplitude saltwater fatigue specimens was found by fractographic examination of selected retardation specimens. However, these observations indicated that the general nature of the fractographic features in relation to the OLR and the applied baseline stress-intensity factor is similar in both air and saltwater, except that, at the lower OLR and low K values, the overload markings were not visible in the specimens tested in saltwater. This lack of overload marking was more common in the 7075-T6 alloy because of its high susceptibility to environmental attack.
corrosion fatigue, fatigue-crack growth, retardation, fractography, stress corrosion, microstructure, aluminum alloys
Engineering specialist, Metallics Research Department, Aircraft Division, Northrop Corporation, Hawthorne, Calif