Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 66 hardened metals and alloys generally results in cyclic softening; thus, the intensity of this effect depends on the stability of the hardened structure and on the cyclic conditions, i.e. on the level of the stress or strain amplitude [22]. The hardness of Cu, both annealed and cold worked, tends to reach nearly the same value during cycling, provided the fatigue life is of the order of 10 7 cycles. The fatigue strength, irrespective of the fatigue hardening followed by softening, has been shown to increase nearly linearly with the  UTS reached by cold working. This is fulfilled at least up to 40% of cold work. However, at higher strengths and more severe cold work, a number of anomalous results have been found [1]. Very intensive cold work results often in the fatigue strength, which is even so low as in the case of annealed Cu. The detailed explanation of these results is missing. This is why for engineering applications it is generally recommended that the  UTS of unalloyed Cu is restricted to less than 325 MPa, corresponding to ~ 30 % cold work. As a final note, it can be concluded that the fatigue behaviour of CG Cu is determined by its dislocation structure developing during cyclic loading and which is a function of external loading parameters. The dislocation activity results in a cyclic slip localisation, which manifests itself by development of a surface relief, consisting of extrusions and intrusions. Just deep intrusions are the critical sites for fatigue crack initiation. In fact, there is a clear relation between the surface relief and the underlying dislocation persistent slip band (PSB) structure. The regions of PSBs are characteristic with higher plastic strain amplitudes than the surrounding interior structure. The structural dimensions, i.e. the characteristic dimensions of the vein structure and PSBs, are generally large compared with the characteristic structural dimensions of UFG Cu. This indicates that the basic knowledge on the cyclic strain localisation and on mechanisms of crack initiation obtained on CG Cu cannot be straightforwardly applied to the grain-refined structures having characteristic dimensions of hundreds of nanometres. 10 3 10 5 10 7 10 9 10 11 N f 80 120 160 200 240 280 320 Stress amplitude [MPa] UFG, 1 Hz UFG, 5 Hz UFG, 10 Hz UFG, 124 Hz UFG, 213 Hz UFG, 20 kHz cold worked, 2.1, 32.3 Hz, [1] 50 100 150 Fatigue strength [MPa] 200 300 400 500 Tensile strength [MPa] Cu cold worked, [1] Cu ECAP Figure 6 : Comparison of S-N data for UFG and cold worked Cu. Figure 7 : Relation of tensile and fatigue strength for fatigue limit based on 10 8 cycles for cold worked Cu [1] and UFG Cu. Let’s consider now the UFG structure. The improvement of fatigue performance of Cu by ECAP processing was experimentally documented in, for example, [7, 11, 23-25]. The S-N curve of Cu prepared by eight ECAP passes by the route Bc, having the tensile properties given in Tab.1 and the structure shown in Figs. 2 and 3 is shown in Fig. 6. The fatigue loading was performed in load symmetrical cycle in tension-compression. The number of cycles to failure increases continuously with decreasing stress amplitude in very broad interval ranging from low-cycle fatigue (LCF) region up to the gigacycle region. Arrows indicate the run-out specimens. The reasonable description by power law is possible in the first approximation only in the shorter interval, namely from 10 4 to 10 8 cycles by the Eq. (2) with constants k = 584 MPa and b = -0.078. The fatigue life of UFG Cu is substantially higher than that of annealed CG Cu and also than that of cold worked copper reported by [1]. The S-N curve for UFG material is shifted by a factor of 1.7 towards higher stress amplitudes for a given number of cycles to failure when compared to the cold worked material. From Fig. 6, it follows that the fatigue limit of UFG Cu based on 10 8 cycles is 150 MPa. The  UTS of this copper is 387 MPa. In Fig. 7, which is redrawn from [1], it should be it is shown the relation between the fatigue limit (on the basis of 10 8 cycles) and  UTS for oxygen free cold worked Cu of purity higher than 99.99 %. Increasing tensile strength by cold work increases the fatigue limit. This reasonably holds for  UTS up to ~350 MPa. The experimental point corresponding to severely deformed UFG Cu, which is shown in Fig. 7 by the full symbol, lies on the right-hand side of the very broad scatter-band of data.