The objective of this project to develop an accelerated environmental fatigue crack test method which can be used to better understand the effects of in-flight and nocturnal/diurnal wetting and drying due to natural temperature and relative humidity changes through a 24-hour day. This method could hopefully be of use for evaluating newer aluminum alloys and corrosion coatings with respect to their effect on environmentally assisted fatigue. This method measures changes in the fatigue crack growth rate to determine the effect of simulated atmospheric environment. Complex wetting and drying effects are known to occur under atmospheric corrosion conditions, it is thought that these conditions also accelerate environmentally assisted cracking.
Age-hardenable aluminum alloys are used for many aerospace applications, where high strength and low density are required. In service, these alloys must withstand both demanding mechanical loads and corrosive environmental conditions. For components made of these alloys, problems may occur when mechanical stresses are present with a corrosive environment. Several reviews of aircraft structural failures and teardown inspections have concluded that fatigue and corrosion dominate aerospace component failures [1,2]. Additionally, 78% of corrosion damage sites identified during aircraft teardowns were found to have initiated fatigue cracks [2]. These findings demonstrate the importance of quantifying and understanding the combined effects of corrosion and mechanical stresses, a phenomenon referred to as environment assisted cracking (EAC).
EAC is a material degradation process that occurs because of the interaction of mechanical stresses and a corrosive environment that localizes within a crack. When cyclic fatigue loads are encountered, EAC is termed corrosion fatigue (CF). The precise mechanism by which EAC occurs in aerospace aluminum alloys is still debated, but it is likely either hydrogen environment assisted cracking (HEAC aka hydrogen environment embrittlement, HEE) or film rupture - anodic dissolution or some combination of these two [3,4,5,6]. It is generally accepted the chemical and electrochemical conditions of the bulk environment are not maintained at the crack tip, and it is the stress state and chemical/electrochemical conditions local to the crack tip that control EAC. As the Department of Defense (DoD) extends the service life of their aircraft or acquires new aircraft, understanding combined atmospheric environmental mechanical effects is paramount.
