Influence of material properties and testing frequency on VHCF and HCF lives of polycrystalline copper

•Structural features on all scales give insight into different ways of fatigue response.•Ultrasonic-fatigue is most appropriate for studying structural changes in the VHCF regime.•Polycrystalline copper is used to study basic mechanisms of fatigue behavior.•Different SEM techniques are applied and correlated with S-N results.•Material properties play a more important role than loading frequency for its VHCF response. Predicting life times of structures, machines and their parts as well as very small electronic devices for medical purposes is especially difficult for very low stress amplitudes and very high numbers of cycles. The development of the ultrasonic-fatigue method resulted in a reliable testing technique for VHCF loading. Some of its properties are described in this paper. To offer an explanation for the somewhat controversial ultrasonic-fatigue properties reported in the literature, S-N tests were performed at ∼19kHz and 20Hz on polycrystalline copper – a nominally homogenous and ductile material. Moreover, plastic-strain measurements were performed. In addition, micro-structural features and their changes in the high and very-high cycle regimes are reported.