Very high cycle fatigue for single phase ductile materials: Comparison between α-iron, copper and α-brass polycrystals

In this paper, the main results obtained in the framework of a National French Agency project called DISFAT, standing for “Dissipation in Fatigue”, are presented. The project was dedicated to the microplastic mechanisms leading to crack initiation in the case of ductile metals loaded in very high cycle fatigue. Fatigue tests were carried out at 20 kHz using an ultrasonic facility. In order to investigate the microplastic mechanisms, slip markings at the surface of the specimens were observed and the self-heating of the specimen during the tests was measured by thermography to deduce the dissipated energy. Polycrystalline copper, α-brass and α-iron were investigated. A good correlation was found between persistent slip bands and dissipated energy. The dissipated energy for the three materials was of the same order of magnitude but while α-iron reached a stable dissipative state, the dissipated energy in the case of copper and α-brass was found to continue to increase gradually with increasing numbers of cycles. That change in dissipated energy during cycling was consistent with the development of persistent slip bands. Both were discussed with regard to the materials.