Mechanisms of strain localization, crack initiation and fracture of polycrystalline copper in the VHCF regime

Abstract

Abstract. Development of areas with strain localization is regarded as first step of fatigue damage and failure. At small load amplitudes only very few such areas are generated, but their number increases with number of cycles, until all grains are covered by them. Slip bands become persistent (PSBs) during high enough numbers of cycles. Formation of numerous persistent slip bands together with deep intrusions or small stage I (mode II) shear cracks, however, is not sufficient to form a long and propagating crack and thus specimen failure below a specified strain/stress amplitude. Specimen fracture requires an approximately 100% higher cyclic load than generating PSBs. The threshold value for PSB formation is lower than traditionally reported. Quantitative results on cyclic plastic strain measurements, TEM observations of the dislocation structure in the VHCF regime and determination of the "short" crack length by use of a Kitagawa diagram are reported. Its relevance for fracturing and endurance limit is discussed.

Very high cycle fatigue of copper: Evolution, morphology and locations of surface slip markings

“…The latter is promoted by the presence of large-angle grain boundary and the decrease of the stacking fault energy [22]. In the VHCF regime, Stanzl-Tschegg and Schönbauer [12] mentioned that the grain boundaries are the preferred crack nucleation and propagation sites for pure copper loaded at 19 kHz in agreement with the present results.…”

“…Most current studies have focused on the VHCF behavior of type II materials and revealed that, in the transition from high cycle fatigue (HCF) to VHCF, the origins of fatigue failure changed from surface to interior ''fish-eye'' fracture for instance at nonmetallic inclusions [1,7]. For type I materials, only few investigations mainly on copper [9][10][11][12] are available regarding the shape of the fatigue life curve and the damage evolution [9][10][11][12][13][14]. Hessler et al [9] and Weidner et al [11] showed that cyclic strain localization in VHCF below the PSB threshold occurs in some form of PSBs.…”

Through thickness property variations in friction stir welded AA6061 joint fatigued in very high cycle fatigue regime

“…8b, Persistent slip bands could be observed locally in an area as plotted with dash line, from where the fatigue crack were initiated. The same fatigue crack initiation mechanisms in the VHCF regime, the development of persistent slip bands leading to the surface fatigue cracks, can also be observed in some single phase metals [11,[29][30][31][32]. Generally, the plastic deformation features for these materials were extensive on the specimen surface, but in this work the persistent slip bands seem to be limited in a very local small region distributed sporadically at the boundary between the TMAZ and HAZ due to the heterogeneities of microstructural and mechanical properties.…”

Development of Cyclic Slip Bands in Ufg Copper in Gigacycle Fatigue

“…Nevertheless, failure does not occur even if slip markings cover the whole surface. The very high cycle fatigue threshold for formation of persistent slip bands (VHCF-PSB threshold) [19] obtained for CG copper is ∆σ/2 ≈ 45 MPa for 2.7×10 8 cycles. It is expected that persistent slip bands and small cracks can be formed at even lower stress amplitudes.…”

“…By contrast, fatigue properties in gigacycle region (alternatively called ultrahigh-cycle or very high cycle region) have been studied to substantially lesser extent, though fatigue failure of engineering components may appear after a number of cycles of 10 10 or even higher [15]. Recent studies [16][17][18][19][20] concerning gigacycle fatigue behaviour of CG metals and alloys distinguish two kinds of materials. The first one, called Type I, includes pure fcc materials like copper.…”