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Environment-Assisted Cracking and Corrosion Fatigue of Aircraft Aluminum Alloys in Corrosive Atmospheres

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Environment-assisted cracking and corrosion fatigue of aluminum alloys in corrosive atmospheres present significant maintenance and safety issues for aircraft. It is well known that these cracking phenomena result from the combined effects of environment mechanical loads and material properties. The service life of an aircraft structure is dependent on various stages of degradation associated with the deposition of aggressive contaminants formation of corrosion damage crack nucleation and crack propagation. Each of these stages affect the service life of a structure and are dependent on a large number of time dependent factors including environmental severity mechanical loading and protective properties of coatings. This work focuses on quantifying in situ the influence of environmental conditions including cyclic humidity and salt chemistry on the incubation nucleation and growth of cracks under both static and dynamic loads. Two test systems are used in atmospheric corrosion tests to continuously measure crack length and estimate crack growth rates. An instrumented double cantilever beam sample and loading fixture is used to obtain short crack measurements and a hydraulically actuated four point bend sample is used for long crack measurements. The atmospheric testing demonstrates strong hysteresis for aluminum alloy (UNS A97075) crack growth rate with respect to humidity during wetting and drying. The humidity dependence of crack growth rate is a strong function of the hygroscopic properties of the salts and salt mixtures. The maximum crack growth rates are observed below the deliquescence relative humidity during the drying stage of an atmospheric test cycle. Other factors that influence initiation time and crack growth rate including galvanic couples crevices and protective coatings are being investigated.

Product Number: 51319-13350-SG
Author: Brandi Clark
Publication Date: 2019
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Naval Aviation Environmental Severity Correlation

Product Number: 51322-17808-SG
Author: Christine Sanders, Raymond Santucci, Jr
Publication Date: 2022
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The exposure environment of an engineering material quite often has a large impact on how that material behaves over time. Environments are distinguished by differences in meteorological patterns, geography, salinity, Ultraviolet (UV) radiation, etc1-3. Thus, the degradation of various materials scales proportionately to the characteristics of the exposure site, with more severe sites leading to worse degradation. Developing an understanding of how the local environment impacts the corrosion rates of metals and the deterioration of anti-corrosion coatings is critical for informing asset maintenance schedules and lifetime predictions4. 
Effect of Dynamic Humidity on Environmental Cracking of an Aerospace Aluminum Alloy
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Effect Of Relative Humidity And Galvanic Couples On Stress Corrosion Cracking Of High-Strength Aluminum Alloys

Product Number: 51322-18015-SG
Author: L. Agnew, B. Clark, V. Avance, F. Friedersdorf
Publication Date: 2022
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High-strength aerospace aluminum alloys, such as AA7075-T651, are susceptible to environmental assisted cracking (EAC) under the right combinations of stress, environment, and microstructure.  EAC presents a serious risk to structures and equipment operated in corrosive conditions.  Studies of EAC in aluminum alloys have highlighted the importance of both anodic dissolution and hydrogen embrittlement to EAC initiation and propagation.1–4  The EAC response of alloys under variable atmospheric conditions is of particular importance for assessing material performance for aerospace applications. 
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