Influence of the metallurgical instability on the gigacycle fatigue regime

In this paper several cases of metallurgical instability at high frequency are shown in order to explain what is the limitation of ultrasonic fatigue compared to regular frequency fatigue, for engineering applications. The response of the micro structure around a flaw, that is to say a stress concentration, after a great number of cycles applied in the case of ultrasonic fatigue with a high strain rate is taken into account in gigacycle fatigue initiation. The instability of the microstructure is often related to a heating dissipation with an increase of the temperature. The increase of temperature associated with a microstructural transformation seems to explain a frequency effect which does not exist for many stable metals.