Fatigue damage in copper polycrystals subjected to ultrahigh-cycle fatigue below the PSB threshold

Anja Weidner,Dorothea Amberger,Florian Pyczak,Bernd Schönbauer,Stefanie Stanzl-Tschegg,Hael Mughrabi

doi:10.1016/j.ijfatigue.2009.04.004

Anja Weidner, Dorothea Amberger + Show 4 more

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https://doi.org/10.1016/j.ijfatigue.2009.04.004

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Abstract

In High-Cycle Fatigue (HCF) of materials like copper, the most common failure modes originate from cyclic strain localization in persistent slip bands (PSBs). The latter form only when the loading amplitudes exceed the PSB thresholds. In contrast to this well-known HCF behaviour, it is shown in the present study that fatigue damage (cyclic strain localization, surface roughening, stage I crack initiation) develops even at loading amplitudes well below the PSB threshold in copper subjected to UltraHigh-Cycle Fatigue (UHCF). These findings are attributed to the accumulation of very small cyclic slip irreversibilities over very large numbers of cycles (>10 10).

Highlights

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 108 to 109 cycles
Observations and discussion 3.1 focused ion beam (FIB) observations of surface roughness and stage I cracks Figures 2 and 3 show examples of scanning electron microscopy (SEM) micrographs obtained on the ion-machined surface that was approximately perpendicular to the surface traces of the persistent slip bands (PSBs)
The lamellae of localized cyclic slip differ noticeably in appearance from the dislocation patterns in the form of PSBs with the ladder structure embedded in a matrix structure of veins which are observed at conventional frequencies in cyclic saturation in HCF

Summary

Introduction

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 108 to 109 cycles. 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 amplitudes [5] must be exceeded It had been postulated in earlier work that, even at very low amplitudes, cyclic slip still retains a small but nonnegligible 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, perhaps to PSB formation at the sites of local stress concentration and fatigue crack initiation. Fatigue damage can be expected to develop in the UHCF regime even below the PSB threshold [5], as illustrated in Fig. 1 (σ: stress, specimen axis vertical)

Discussion

Conclusion