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High-power ultrasound was used to study fatigue behavior of three alloys (Udimet 500, 17-4PH, and Ti-6A1-4V) since the loading frequency is very high (f = 20 kHz) and a great number of cycles can be obtained in a short time. The specimens were excited in resonance at the first longitudinal mode in such a way that maximum strain occurred in the middle and maximum displacement amplitude was obtained at the end of the specimen. Fatigue life behavior of the three alloys were investigated at ultrasonic (/ = 20 kHz) and conventional loading frequency (f = 20 to 50 Hz). Miniature dynamic strain gages were employed to measure maximum strain values during ultrasonic fatigue tests. The effect of high strain rate on fatigue crack initiation and propagation failure mechanism was examined. Fracture surface of specimens was observed by means of a scanning electron microscope. Fatigue resistance was found to depend upon ultrasonic and mechanical characteristics and microstructure of the material, loading regime, and surface condition of the specimen.
fatigue limit, ultrasonic frequency, resonance vibration, strain rate effect, failure mechanism, miniature dynamic strain gages
Professor and research engineer, CNAM, Paris,
Doctor of Mechanics, Beijing University of Aeronautics and Astronautics, Beijing,