METAL FATIGUE AT ULTRASONIC FREQUENCY.
GEORGE WASHINGTON UNIV WASHINGTON D C INST FOR THE STUDY OF FATIGUE FRACTURE AND STRUCTURAL RELIABILITY
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Studies by scanning electron microscope, extending previous preliminary work, show that the mechanism of plastic fatigue at ultrasonic frequencies c. 20,000 Hertz differs from that at the relatively low frequencies of testing normally used for engineering design c. 2000 cpm. At the low frequency a small strain amplitude starts plastic deformation in slip zones. The zones multiply, covering the metal grain, and also degenerate slowly with progressive cycling until they form slip zone microcracks. Thus the low frequency fatigue is marked by a multiplicity of such slip zone cracks- But at the high frequency a comparable small amplitude starts only isolated slip zones in occasional grains. It turns such a slip zone into a crack quickly and causes it to propagate at a much higher velocity than at low frequency. Thus the high frequency fracture in comparison is not only localized it is also virtually catastrophic in the localized areas. Author
- Metallurgy and Metallography