Abstract
Fatigue in the very long life regime has assumed great significance in recent years particularly after it has been established that a fatigue limit doesn’t exist in most cases. The paper deals with investigations of gigacycle fatigue and near threshold fatigue crack growth behaviors of AA7075-T6, AA6061-T6 and AA2024-T3 Al alloys using piezoelectric accelerated fatigue at 20KHz. Experiments were conducted in air at room temperature on hourglass-shaped specimens under completely reversed loading conditions (R=−1). It was noticed that failure continues to take place right up to the gigacycle (109 cycles) range, at nominal cyclic stresses. The high cycle fatigue fractures are generally rough in appearance and seen to initiate predominantly from the surface. Furthermore, they contain a significant number of voids, exhibit crystallographic faceting, and discernible fatigue striations. Gigacycle fatigue fracture of AA2024-T3, however, shows features of ductile tearing instead of striations. The studies also show that fatigue crack growth rates of small cracks are greater than those of large cracks for almost the same stress intensity factor range ΔK, and that some small cracks may grow at ΔK values below the large crack threshold in the near-threshold crack growth regime.
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