STP982: The Effect of Closure on the Near-Threshold Fatigue Crack Propagation Rates of a Nickel Base Superalloy

    Zawada, LP
    Materials research engineer and senior scientist, Materials Laboratory, Air Force Wright Aeronautical Laboratories, AFWAL/MLLN, Wright-Patterson Air Force Base, OH

    Nicholas, T
    Materials research engineer and senior scientist, Materials Laboratory, Air Force Wright Aeronautical Laboratories, AFWAL/MLLN, Wright-Patterson Air Force Base, OH

    Pages: 20    Published: Jan 1988


    Abstract

    An experimental study of the effects of closure on the near-threshold fatigue crack growth rates in a nickel-base superalloy is presented. Fatigue crack propagation (FCP) tests were conducted on Rene' 95 at 650°C under either constant load amplitude (increasing ΔK) or under threshold load shedding (decreasing ΔK) test conditions. Particular emphasis was placed on the level of closure in the near threshold regime as a function of load history, specimen size and thickness, and environment. Test results indicated that for decreasing load threshold tests the recorded threshold levels were independent of load shedding rates. Specimen thickness was found to have no influence on FCP rates, threshold values, or closure loads, thereby eliminating plasticity induced closure as a dominant mechanism. Tests performed in vacuum produced closure values identical to those obtained in an oxidizing environment. It was concluded that the primary operating closure mechanism was roughness induced closure.

    Keywords:

    crack closure, crack growth mechanisms, elevated temperature, nickel base superalloy, fatigue crack propagation, fracture mechanics, oxidation, threshold


    Paper ID: STP27232S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP27232S


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