Fatigue damage in polycrystalline copper below the fatigue limit

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

“…To our knowledge, the key role of twin boundaries on microplasticity initiation was never highlighted for ultrasonic fatigue tests. Comparisons between our results obtained at 20 kHz and literature results obtained at low frequencies [20,[24][25][26]31,32] show that, in both cases, (i) the length, the height, the width and the location of slip markings of type I and II are similar and (ii) the relative amount of the type II slip marking with regard to type I slip band increases with decreasing stress or strain amplitudes. These comparisons suggest that similar mechanisms are responsible for formation of PSBs despite the plastic strain amplitude is much lower and the number of cycles much higher in the VHCF characterized by 20 kHz fatigue tests than in the HCF characterized by low frequency tests.…”

“…In LCF and HCF, the numerous type I persistent slip markings or equivalently PSBs hide the type II persistent slip markings explaining why few literature results mentioned slip bands along grain and twin boundaries. Besides, the literature results quoted above [20,[24][25][26]31,32] were obtained after symmetrical strain-controlled fatigue tests operated at frequencies below 100 Hz, i.e. much lower than the present frequency (20 kHz).…”

“…They are commonly observed in low and high cycle fatigue regime (see e.g. [18,19,26] for polycrystalline copper). PSBs are usually associated with the ladder-like dislocation structure under the slip marking [27,28].…”

“…More precisely, the high majority of the early type II persistent slip markings is located at twin boundaries while triple junctions are preferred sites for type III persistent slip markings. Polak and Vasek [26] observed PSBs similar to type II persistent slip markings at strain amplitudes lower than the ''conventional fatigue limit'' (De/2 = 5 Â 10 À5 [29]) in polycrystalline copper. In the same way, Neumann and Tönnessen [24] and Peralta et al [30] have observed PSBs along twin boundaries oriented at around 45°from the loading direction but not inside the grains at low stress amplitudes.…”

Gigacycle fatigue initiation mechanism in Armco iron

“…Microstructural processes of fatigue crack initiation are different in these two types of metal. Very high cycle fatigue crack of Type I material always related to the formation of persistent slip bands on the surface [17][18][19][20][21]. On contrary, fractographic of Type II materials has ''fisheye'' as typical character after very high cycle fatigue testing [1][2][3].…”