Fatigue endurance experiments have been performed on five kinds of Aluminum alloys, such as 2219, 6061, LY10, LY12 and AS5U3G-Y35,
using the
ultrasonic fatigue testing method at 20?kHz. The S—N curves show that fatigue failure can occur over 10~7 cycles, and that fatigue endurance stress continues to decrease with increasing cycles to failure between 10~7 and 10~(10) cycles. Fatigue strength at 10~(10) cycles is obviously lower than that at 10~7 cycles. Scanning electron microscopy is used to examine the fatigue rupture section of all specimens. It was found that fatigue fracture initiation in duralumin such as 2219, 6061, LY10 and LY12 are mostly at the surface of specimen since the materials are very sensitive to the surface condition and also due to the lack of microstructural defects at the interior. There is no clear fish-eye and optically dark area (ODA) seen in the crack initiation region, different from fatigue initiation characteristics of high strength steel and Titanium alloy tested in high cycle range. High strain rate and dislocation transfer at ultrasonic frequency accelerates the crack of fragile particles and the dissolution of deposit phase in the Aluminum alloys. This results in local failure of the microstructure, and accelerates the fatigue crack initiation and growth.