Abstract
Fatigue tests were carried out on a 6061 Al alloy under the ultrasonic fatigue test (test frequency 20kHz) and the conventional tension compression fatigue test (50-80Hz). In the ultrasonic fatigue test, the temperature of the specimen rises due to internal friction. In order to suppress the temperature rise in the specimen, the specimens under the ultrasonic fatigue test were cooled by air blow. Small artificial holes were introduced onto the specimen surface to investigate the crack initiation and growth behavior. In the crack initiation period, the crack growth rate at lower stress amplitude under the ultrasonic fatigue test and the conventional tension compression fatigue test decreased with crack propagation. The crack growth rate started to increase at a critical crack length. The critical crack length at lower stress amplitude under the ultrasonic fatigue test was longer than that under the conventional tension compression fatigue test. This might be the reason for that the fatigue lives at lower stress amplitude under the ultrasonic fatigue test were longer than those under the conventional tension compression fatigue test. A few small cracks were observed in the vicinity of the main crack tip of the specimen under the ultrasonic fatigue test. However, such small cracks were not observed on the specimen under the conventional fatigue test. The small cracks shield the tip of the main crack under the ultrasonic fatigue test and influence the crack growth rate.
