Frequency-dependent fatigue damage in polycrystalline copper analyzed by FIB tomography

Abstract Pure polycrystalline copper has been subjected to cyclic loading with frequencies from 4 Hz to 20 kHz. With increasing frequency, the S-N curve was shifted to a higher number of cycles to fracture. Quasi-static and dynamic tensile tests were performed to identify the strain rate sensitivity corresponding to the increase of the cyclic frequency in order to explain the shift of the S-N curve with the frequency. Surface relief evolution has been studied in specimens cycled with various stress amplitudes and frequencies. Scanning electron microscopy inspection of the surface and the perpendicular cuts produced by focused ion beam sectioning allowed characterizing the surface relief and internal structure of the individual specimens subjected to cycling. Characteristic types of slip bands were found. In all cases, cavities formed in or around the primary slip plane below the slip markings. Focused ion beam slicing and image recording with subsequent 3D reconstruction allowed obtaining the distribution of cavities below the slip markings. A close relation was established between the distribution of cavities and resulting surface relief. Formation of cavities in the slip band led to the development of extrusions and in some cases also to tiny intrusions. Both rough surface relief and the cavity distribution below the surface along the slip plane contributed to the initiation of the fatigue cracks. Localized cyclic straining with a high strain rate has been analyzed in terms of vacancy generation within the slip band leading to cavity formation.