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Environmental fatigue of an Al-Li-Cu alloy. I - Intrinsic crack propagation kinetics in hydrogenous environmentsDeleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant delta K and K(sub max) loading. The da/dN are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness of aqueous 1 percent NaCl with anodic polarization, pure water vapor, moist air, and NaCl with cathodic polarization. While da/dN depends on delta K(sup 4.0) for the inert gases, water vapor and chloride induced multiple power-laws, and a transition growth rate 'plateau'. Environmental effects are strongest at low delta K. Crack tip damage is ascribed to hydrogen embrittlement because of the following: (1) accelerated da/dN due to part-per-million levels of H2O without condensation; (2) impeded molecular flow model predictions of the measured water vapor pressure dependence of da/dN as affected by mean crack opening; (3) the lack of an effect of film-forming O2; (4) the likelihood for crack tip hydrogen production in NaCl; and (5) the environmental and delta K-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3, and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.
Document ID
19910070423
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Piascik, Robert S.
(NASA Langley Research Center Hampton, VA, United States)
Gangloff, Richard P.
(Virginia, University Charlottesville, United States)
Date Acquired
August 14, 2013
Publication Date
October 1, 1991
Publication Information
Publication: Metallurgical Transactions A - Physical Metallurgy and Materials Science
Volume: 22A
ISSN: 0360-2133
Subject Category
Metallic Materials
Accession Number
91A55046
Funding Number(s)
CONTRACT_GRANT: NAG1-745
Distribution Limits
Public
Copyright
Other

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