Abstract
Pariss law of fatigue crack propagation rate is well applied in the defect-tolerance fatigue approach. When carry out same approach in the very high cycle fatigue domain, the understanding of mechanism about fatigue crack propagation threshold which is obviously important, is helped. In the present work here, the fatigue crack propagation threshold of a surface crack for an Armco iron loaded in the VHCF regime was investigated by a new approach which combines the fracture surface analysis and the temperature recording on the surface during the test by an infra-red camera. The experiments were carried out on a sheet specimen under a 20 kHz ultrasonic frequency loading with IR images registration. Three stages of fatigue crack were identified with different mechanisms. It is found that the transition between initiation and crack propagation corresponds to the intrinsic fatigue threshold. It takes more than 99% of the gigacycle fatigue life to achieve this transition size.
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