STP1401

    Environmentally Influenced Near-Threshold Fatigue Crack Growth in 7075-T651 Aluminum Alloy

    Published: Jan 2000


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    Abstract

    The near-threshold fatigue crack growth behavior of the 7075-T651 aluminum alloy was studied in laboratory air, vacuum, and an aqueous 3.5% NaCl solution. Results indicate that a rising stress ratio (R) enhanced the near-threshold fatigue crack growth by increasing the crack growth rate (da/dN) and decreasing the threshold stress intensity range (ΔKth) in both laboratory air and aqueous 3.5% NaCl solution. However, the reverse was observed in vacuum. It was also noticed that the near-threshold fatigue crack growth resistance was greatest in vacuum, intermediate in aqueous 3.5% NaCl solution, and lowest in laboratory air. Conversely, the crack growth rate at given values of AK were shown to be greatest in laboratory air, less in 3.5% NaCl solution, and lowest in vacuum. In both laboratory air and aqueous 3.5% NaCl solution, ΔKth, initially decreased with increasing R until a critical stress ratio of Rc = 0.5 was reached, which it then leveled off or decreased slightly. The ΔKth values for these two environments appear to converge at a higher R. On the other hand, in vacuum, the ΔKth, increased linearly with increasing R. In addition, at lower R, a greater resistance to near-threshold fatigue crack growth was detected in aqueous 3.5% NaCl solution than in laboratory air. This is presumably attributed to crack closure that has been induced by accumulation of corrosion product in the crack-tip.

    Keywords:

    fatigue, near-threshold fatigue crack growth, stress ratio, vacuum, laboratory air, 3.5% NaCl solution, threshold stress intensity range, maximum stress intensity


    Author Information:

    Lee, EU
    Materials Engineer, Naval Air Warfare Center Aircraft Division, Aerospace Materials Division, Patuxent River, MD

    Sanders, HC
    Materials Engineering Technician, Naval Air Warfare Center Aircraft Division, Aerospace Materials Division, Patuxent River, MD

    George, K
    Materials Engineer, Naval Air Warfare Center Aircraft Division, Aerospace Materials Division, Patuxent River, MD

    Agarwala, VV
    Materials Engineer, Naval Air Warfare Center Aircraft Division, Aerospace Materials Division, Patuxent River, MD


    Paper ID: STP10232S

    Committee/Subcommittee: G01.05

    DOI: 10.1520/STP10232S


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