Fatigue Damage in Copper Polycrystals Subjected to Ultrahigh-cycle Fatigue below the PSB Threshold

There is currently an increased interest in understanding the specific damage and failure mechanisms which occur in the UltraHigh-Cycle Fatigue (UHCF) or Very High Cycle Fatigue (VHCF) range above about 10 to 10 cycles. Most current studies have focused on the UHCF behaviour of high-strength materials containing microstructural heterogeneities, in which internal fatigue failure frequently is caused by fatigue cracks which are initiated at internal heterogeneities such as inclusions, compare [1]. In contrast, the present work is part of an experimental study of the UHCF behaviour of pure ductile single-phase face-centred cubic (fcc) materials such as copper. In these materials, the most common high-cycle fatigue (HCF) failure modes originate from cyclic strain localization in persistent slip bands (PSBs). The general belief is that a necessary prerequisite for cyclic strain localization in PSBs is that the so-called PSB threshold amplitude [2] must be exceeded. However, it had been postulated in earlier work that, even at very low amplitudes, cyclic slip still retains a small but non-negligible irreversible component which, accumulated in a random fashion over a very large number of cycles in the UHCF regime, can lead to surface roughening (and irreversible changes of the dislocation substructure) and, ultimately, perhaps to PSB formation and fatigue crack initiation. In that picture, fatigue damage can be expected to develop in the UHCF regime even below the PSB threshold [3], as illustrated in Fig. 1 ( : stress, axis vertical). In order to test this hypothesis, a project was started in which the UHCF behaviour of commercial purity copper polycrystals that had been ultrasonically fatigued up to more than 10 cycles was investigated in some detail. In this work, a detailed study of the surface features was conducted by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The most important results of this earlier work have been published [4,5,6] and are summarized as follows: The “traditional” axial PSB threshold stress amplitude was found to be ca. 63 MPa [4,5,6]. Well below the PSB threshold, marked cyclic slip localization occurred in intense slip bands after very large numbers of cycles and intensified with increasing numbers of cycles [4,5].