Crack growth in the gigacycle fatigue regime for helicopter gears

Investigations were performed on helicopters' gears that had failed in service, using quantitative fractographic analyses. It was shown that a combination of LCF and HCF creates damage in gears on a flight-by-flight basis which produces beach marks on the fatigue surface of the gears that correlate one-to-one with flight cycles. The crack increment between beach marks occurs under HCF at 7000–8000  r.p.m. The ratio of the crack growth period to durability is smaller in the gigacycle fatigue area (more than 10⁸ cycles) than in the high cycle fatigue area (10⁶–10⁷ cycles) for those gears investigated that have stress raisers. This relationship for damaged gears depends not only on durability but also on the type of stress raiser and its location.