STP677

    Variations in Crack Growth Rate Behavior

    Published: Jan 1979


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    Abstract

    Three variable amplitude stress histories based on a single repeating flight of a bomber aircraft are applied to two 610 mm wide, 4.6 mm thick, center-crack panels of 7075-T6 aluminum. The stresses in each stress history are controlled so that the stress intensity factor coefficient (K/σ) is maintained constant as the crack grows. The resulting fatigue crack growth (FCG) data are utilized to show that the derived variable amplitude fatigue crack growth rate (FCGR) behavior is controlled by a stress intensity factor parameter. These FCG data are also used to evaluate the influence that differentiation method, crack length measurement interval, and stress history have on the FCGR behavior and its variability. Within the limits spelled out in the text, the value of the (statistical) mean FCGR is shown to be independent of differentiation method and of crack length measurement interval. The choice of stress history is shown to affect the mean FCGR value as would be expected.

    The variability in FCGR behavior, as characterized by the coefficient of variation, is affected by all the parameters studied. As the crack length measurement interval increases, the FCGR variability associated with the secant method of differentiation is shown to decrease toward the almost constant level of FCGR variability exhibited by the seven-point incremental polynomial method. Also, the coefficient of variation, while significantly affected by crack length measurement interval for a given stress history, is shown to be independent of stress history when the FCGR data for the three stress histories are derived using similar crack length measurement intervals.

    Keywords:

    fatigue (materials), crack growth, statistical methods, stress intensity factor, variable amplitude loading, crack propagation


    Author Information:

    Artley, ME
    Aerospace engineers, Air Force Flight Laboratory, Wright-Patterson Air Force Base, Ohio

    Gallagher, JP
    Group leader, Service Life Management, University of Dayton Research Institute, Dayton, Ohio

    Stalnaker, HD
    Aerospace engineers, Air Force Flight Laboratory, Wright-Patterson Air Force Base, Ohio


    Paper ID: STP34906S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP34906S


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