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    Bithermal Fatigue: A Link Between Isothermal and Thermomechanical Fatigue

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    Many technologically important elevated temperature service cycles are non-isothermal. Nevertheless, major design codes rely on the most severe—usually the highest—temperature of an operational cycle as being the pertinent temperature upon which to base a design. Consequently, most high-temperature fatigue data for design have been generated under isothermal conditions. There is a growing awareness of the potential inadequacy of such a simplistic approach since many thermomechanical fatigue results have been found to exhibit considerably lower fatigue lives than would be expected on the basis of isothermal results at the maximum cycle temperature. Yet, variable-temperature, low-cycle fatigue tests are difficult to conduct and to interpret. The considerable gap between isothermal and thermomechanical fatigue technology can be bridged by an approach which retains the simplicity and ease of interpretation of isothermal fatigue, but captures many of the first order effects of the greater complexities involved in thermomechanical fatigue. We have developed a procedure for conducting what has been designated as bithermal fatigue experiments. In this procedure, the tensile and compressive halves of the cycle are conducted isothermally at two significantly different temperatures. The higher temperature is chosen to be in the time-dependent creep and oxidation prone regime and the lower temperature in the regime wherein time dependencies are minimized due to lack of thermal activation.

    Interestingly, bithermal fatigue tests prior to those performed for this paper have been conducted in conjunction with the evaluation of the isothermal Strainrange Partitioning characteristics of high-temperature alloys, not with thermomechanical behavior per se. Nevertheless, the bithermal fatigue test may well be used as an alternative to thermomechanical cycling. In this paper, we place emphasis on using the bithermal testing concept as a link between isothermal and thermomechanical testing. New bithermal fatigue data for the nickel-base superalloy B1900 + Hf are presented herein.


    fatigue (metal), low-cycle fatigue, thermomechanical fatigue, creep-fatigue, Strainrange Partitioning method, testing techniques, nickel-base alloys

    Author Information:

    Halford, GR
    NASA-Lewis Research Center, Cleveland, OH

    McGaw, MA
    NASA-Lewis Research Center, Cleveland, OH

    Bill, RC
    NASA-Lewis Research Center, Cleveland, OH

    Fanti, PD
    NASA-Lewis Research Center, Cleveland, OH

    European Patent Office, Munich,

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP24510S