Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 68 effect is more pronounced at higher plastic strain amplitudes. This result seems to be obvious, because the UFG Cu is harder but less ductile than CG Cu. F ATIGUE CRACK RESISTANCE Fatigue crack initiation mechanism yclic strain localisation resulting in fatigue crack initiation is an important stage of the fatigue process. In homogeneous materials without defects it represents a substantial part of the fatigue life. As in CG copper, cyclic slip localisation in UFG Cu results in a development of surface relief. Population of cracks associated with extrusions develops during cycling. Because the dimension of slip bands is substantially larger than the grain size of UFG structure, and because they have been often observed to be oriented approximately 45° from the tension-compression axis [7], we can denote them “shear bands” (SB). Since the early studies of the surface relief development, there are open questions concerning the nature and the mechanism of this phenomenon. The original belief that persistent slip bands (PSBs) with the ladder like dislocation structure might be active in UFG Cu is dubious, since the width of PSBs known from CG Cu is larger than the grain size of UFG structure. On the other hand, grain coarsening and development of bimodal structure with large recrystallized grains was also observed, particularly under plastic strain-controlled fatigue loading [26]. Moreover, the dislocation patterns typical for fatigued CG Cu had developed in coarser grains. Ultimately, the relation of formation of SB and grain coarsening is not fully clarified up to now. There is an open question as to whether the process of shear banding is initiated by the local grain coarsening, which leads to the strain localisation destroying the original UFG structure, or the shear localisation takes place abruptly at first and the coarse structure is formed subsequently [32]. Investigation of acoustic emission during cyclic deformation indicates that large-scale shear banding might be an important period of fatigue damage [33]. A characteristic surface relief on fatigued UFG Cu is shown in Fig. 10. The set of parallel slip bands is the result of fatigue loading in the HCF region. The copper is the same as the material on which the S-N curve, Fig. 6, was determined; i.e., material that did not exhibit any grain coarsening under stress-controlled loading. Hence, the local coarsening of UFG structure is not the necessary prerequisite for the formation of these surface reliefs. The length of the cyclic slip bands, see Fig. 10, substantially exceeds the grain size. The cyclic slip bands are formed by extrusions rising over the surface and deep intrusions. The main fatigue crack develops by connection of suitably located intrusions. Cyclic slip bands are also observed in gigacycle fatigue region. An example of a surface relief developed after application of 10 10 cycles is shown in Fig. 11. The density of cyclic slip bands in gigacycle region is very low. The cyclic slip bands produced in gigacycle fatigue region are broad and make an impression of highly deformed local areas incorporating some neighbouring near-by oriented grains. The related intrusions are not deep at all. Figure 10 : Cyclic slip band on the surface of UFG Cu loaded in HCF region. Figure 11 : Cyclic slip band on the surface of UFG Cu loaded in gigacycle region. Based on the present-day state of knowledge it can be concluded that the slip markings in UFG Cu, their shape and main features resemble the slip markings formed in CG Cu. The local grain coarsening is not a necessary condition for formation of cyclic slip markings and initiation of fatigue cracks. However, the grain coarsening, observed mainly under plastic strain controlled tests, is definitely an important effect that takes place in UFG Cu. The role of the coarsened C