Correlation of the low and high frequency fatigue responses of pure polycrystalline copper with mechanisms of slip band formation

Ultrasonic fatigue testing at 20 kHz was developed to accelerate fatigue tests and explore the very high cycle fatigue range. However, the use of ultrasonic fatigue systems raises the open question of the impact of frequency on the material fatigue response. In this paper, the impact of the loading frequency on the fatigue response of face-centered cubic polycrystalline copper is investigated. Firstly, experiments with conventional fatigue devices inducing loading frequencies lower than 100 Hz were carried out and compared with previously published results obtained with an ultrasonic fatigue machine at 20 kHz. At a stress amplitude of 100 MPa, the number of cycles required for failure was found to be more than 200 times greater for tests at 20 kHz than for tests at frequencies below 100 Hz. Secondly, for both types of fatigue tests, the conditions in stress amplitude and number of cycles needed for the emergence of the early slip markings were investigated. The early slip marking S-N curve was found to have the same sensitivity as the failure S-N curve to the frequency effect. Thirdly, the different possible reasons responsible for this frequency effect are discussed. It is concluded that the physical mechanisms of slip marking formation is preserved while their kinetics is sensitive to the loading frequency. Finally, two simplified models were considered and for the first time, the frequency effect on the fatigue response of copper is quantitatively correlated with the occurrence of time-dependent dislocations cross slip and vacancies production/diffusion involved in the persistent slip band formation for face-centered cubic structure. The obtained results could be extended to a full class of materials where slip band formation plays a key role in the crack initiation process while suggesting no frequency effect for materials with a lower degree of plasticity development at crack initiation (High Strength Steels for example).