Alternate Characterizations of Fatigue Load Spectra

This paper improves prior efforts to provide a framework for fair, early, and meaningful assessment of rotorcraft fatigue durability. The authors provide methods to determine fatigue damage accumulation in accordance with piecewise exponential density functions, which represent the load spectra in a manner that is more manageable to the reader than prior efforts. The particular load spectra under consideration are for a conventional rotorcraft performing a scout mission. New formula are derived to calculate exceedances and cumulative damage fraction. Normalized critical endurance limits are determined for the range of expected fatigue curve shape parameters, and charts compare results to allow readers to obtain a sense of the sensitivity to selected curve-fit basis functions. A traditional curve-fit metric (the standard error of the fit) is used to determine and control the quality of the parameterization. Prior results are confirmed by application of the newly derived and independent method.