In Search of Rotorcraft Airframe Fatigue Assessment Methodology

Rotorcraft hub loads are functions of main and tail rotor rotational frequencies and introduce n per revolution stresses into an airframe structure. Airframe fatigue life is currently computed using the stress-life method. This approach uses an S-N curve with an endurance limit where low amplitude cycles below the endurance limit do not contribute to damage accumulation. This method does not consider the low-cycle (high stress) and high-cycle (low stress) interaction effects. To overcome these deficiencies, an ε-N (strain-life isoprobability) approach is proposed. The recorded strain history was analyzed using Fourier analysis to identify n per revolution vibratory stress components. It was further analyzed with a peak-valley algorithm to extract once per maneuver and GAG cycle values. The recorded strain history was transferred to a critical location using the strain transfer function developed by finite element analysis (FEA). The stress-life, strain-life, and proposed approach (ε-N) were utilized to compute stub wing fatigue lives with various probabilities of failure. The prediction was verified using 21 cracked and 293 uncracked stub wings in the fleet at various flight hours. The probability of fleet stub cracking was determined using a Weibull distribution fit. The analytical predicted fatigue lives correlated well with observed stub fatigue cracking in the fleet at 1.86% probability of failure. The ε-N approach can predict fatigue cracking observed in fleet rotorcraft in comparison to significantly higher fatigue life prediction by stress-life and strain-life methods.