Very-High Cycle Fatigue Testing Under Spectrum Loading for Endurance Limit Structural Design

A program has recently concluded that generated fatigue test data for the influence of a rotorcraft main rotor blade root bending spectrum (Helix) on the crack nucleation mechanisms in 7075-T651 aluminum. High frequency tests were performed that generated spectrum fatigue failures out to nearly 10⁹ cycles. Fractographic examination showed a distinct change in crack nucleation from slip initiated to inclusion initiated cracking as the spectrum peak stress level was increased. Spectrum life predictions were made using three different baseline constant-amplitude S-N curves, one using a traditional rotorcraft OEM fitting methodology, one using the HCF portion of a strain-life curve, and one that was fit to S-N data with test lives out to 3x10⁸ cycles. The spectrum life prediction using the S-N curve that properly modeled material behavior in the Very High-Cycle Fatigue (VHCF) regime provided a good correlation to the spectrum fatigue test data. Predictions using the other S-N curves were highly conservative.