Effect of Low Temperature on Fatigue Crack Formation and Microstructure-Scale Growth from Corrosion Damage in Al-Zn-Mg-C
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PROGNOSIS of component fatigue performance (initiation and microstructurally small crack growth) at progressively smaller size scales requires a predictive model of cracking informed by understanding of the interaction of loading, crack-formation feature, local microstructure, and environment.[1] Failure analyses[2–4] suggest that the interaction of ground-based corrosion with fatigue in flight is of particular importance to airframe prognosis.[5–7] Micro-mechanical and continuum models of fatigue life in Al alloys have been put forth for cracks emanating from microstructural constituents (~25 lm)[1,8–10] or localized corrosion topography (~250 to 1,000 lm).[11–22] Recent work established the foundation for such modeling including formation feature morphology, relevance of linear elastic fracture mechanics (LEFM), fatigue crack formation life (Ni), and microstructure-scale crack (MSC) propagation rates JAMES T. BURNS, formerly Air Force Research Laboratory Materials and Manufacturing Directorate (AFRL/RX), Wright Patterson Air Force Base, OH 45433, is now Research Assistant Professor with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904. Contact e-mail: [email protected] RICHARD P. GANGLOFF, Professor, is with the Department of Materials Science and Engineering, University of Virginia. Manuscript submitted November 28, 2011. Article published online September 29, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
(da/dN).[11,12,23,24] This prior research focused on fatigue of an Al-Zn-Mg-Cu alloy in ambient temperature moist air. However, flight environments range from salt spray at near sea-level to low water vapor pressures and cold temperatures at higher altitudes.[25–27] Applying fracture mechanics models with ambient temperature-moist air fatigue properties to cracking in cold environments leads to highly conservative life predictions.[28,29] As such a better empirical and mechanistic understanding of low temperature fatigue is needed. Extensive experiments and theory establish that the deleterious effect of moist environment on fatigue in Al alloys is due to hydrogen embrittlement following crack tip production and uptake of atomic hydrogen (H) from either chemical reaction of water molecules on aluminum, or coupled anodic and cathodic electrochemical processes at the crack tip.[30–32] A large body of work debated the atomistic mechanisms of H-enhanced fatigue.[30,33–41] For precipitation hardened Al stressed in a ambient temperature humid environment, H-enhanced fatigue propagation in the mid to low Paris regime results in facet-like features on the crack surface which are parallel to {100}, {110} and high index {hkl} planes.[33,42,43] Gupta and Agnew reported similar features, with identical crystallographic characteristics, on a fatigue crack surface emerging from constituent particle clusters in 7075-T651 stressed in moist air at 296 K and 223 K (23 C and 50 C).[44] Such features are not produced by fatigue in ultra-high vacuum, which often VOLU
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