Abstract
Gaseous environmental effects on intrinsic fatigue crack growth are significant for the Al-Li-Cu alloy 2090, peak aged. For both moderate ΔK-low R and low ΔK-high R regimes, crack growth rates decrease according to the environment order: purified water vapor, moist air, helium and oxygen. Gaseous environmental effects are pronounced near threshold and are not closure dominated. Here, embrittlement by low levels of H2O (ppm) supports hydrogen embrittlement and suggests that molecular transport controlled cracking, established for high ΔK-low R, is modified near threshold. Localized crack tip reaction sites or high R crack opening shape may enable the strong environmental effect at low levels of ΔK. Similar crack growth in He and O2 eliminates the contribution of surface films to fatigue damage in alloy 2090. While 2090 and 7075 exhibit similar environmental trends, the Al-Li-Cu alloy is more resistant to intrinsic corrosion fatigue crack growth.
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