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    An Engineering Model for Assessing Load Sequencing Effects

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    An engineering model has been developed to assess the impact of load spectrum variables on fatigue crack growth in metal structures. The model is based on the effective stress concept of Willenborg et al and Gallagher et al, and has been extended to accommodate retardation/acceleration phenomena peculiar to transport spectra. Development was accomplished by consideration of basic principles and previously reported results of simple overload/underload spectrum testing. The model does not rely on additional empirical parameters except material constant amplitude crack-growth rates. Formulation is such that flight-by-flight crack growth may be assessed without recourse to cycle-by-cycle integration.

    Predictive accuracy of the model was compared with the results of crack propagation testing conducted on 28 center-cracked 7075 and 2024 aluminum specimens subjected to transport wing and fin load spectra. Mission parameter variables investigated in this testing were: altitude, gross weight, flight duration, speed, and touch-and-go landings. Use of the model produced improved accuracy over that exhibited by other models currently in use in the industry. The capability of the model to accurately predict acceleration effects is considered unique.


    fatigue (materials), acceleration, crack propagation, cyclic loads, damage tolerance, residual stress, retardation

    Author Information:

    Wozumi, JT
    Senior specialist engineers, The Boeing Company, Seattle, Wash

    Spamer, T
    Senior specialist engineers, The Boeing Company, Seattle, Wash

    Lambert, GE
    Senior specialist engineer, The Boeing Company, Wichita, Kan.

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP27486S