Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 73 Cu to internal and external parameters, it is difficult to draw reliable conclusions from the comparison of literature data, which cover differently produced materials, different purity and different testing conditions. This is why on UFG Cu, on which the S-N curve in Fig. 6 was determined, the plastic strain controlled tests were conducted. An example of dislocation structures of material from a failed specimen loaded with  ap = 0.1 % is shown in Fig. 18. A well-developed bi- modal microstructure consisting of areas with original fine-grained structure and large recrystallized grains with dislocation structure in their interior can be seen. This observation is in full agreement with results published in [26,49]. For comparison, the characteristic dislocation structure of a specimen loaded with the constant stress amplitude of 340 MPa is shown in Fig. 19. This structure does not exhibit any traces of bimodal structure, though the stress amplitude used is equal to the maximum value of the stress amplitude in the plastic strain amplitude-controlled test with  ap = 1 x 10 -3 . This means that the absolute value of the stress amplitude cannot be the reason for the substantially different stability of UFG structure under both types of tests. Also, the details of ECAP procedure are excluded. The tests were run on the same material. Also the frequency of loading in both tests was similar. The differences in the cumulative plastic strain amplitude in both tests were also not substantially different. The only difference between the two test modes, which can cause the different microstructure, seems to be the stress-strain response at the very beginning of the tests. There is relatively low plastic strain amplitude at the beginning of the stress-controlled test when compared to the strain-controlled test. It can be supposed that just the cycling with low strain amplitude at the beginning of the stress-controlled test can prevent the substantial changes of microstructure due to subsequent loading with increasing  ap . However, this idea is based on a small number of tests; further experimental study is necessary to support this opinion. Figure 18 : Bi-modal dislocation structure after constant plastic strain amplitude loading. Figure 19 : Dislocation structure after constant stress amplitude loading,  a = 340 MPa. The up to now knowledge on the stability of UFG structure of Cu under cyclic loading is not sufficient to draw definite conclusions. On the other hand, it seems to be proven that the enhanced ductility and stable microstructure are major facts that enhance the fatigue properties [32]. If the structural stability is low (due to internal material parameters or type of loading), the fatigue properties of UFG Cu are substantially reduced. C ONCLUSIONS atigue performance of Cu can be substantially improved by severe plastic deformation. The fatigue strength corresponding to 10 8 cycles to failure can reach up to 150 MPa and 120 MPa for 10 10 cycles, provided that the loading is performed in stress-control and the UFG microstructure remains stable. The stability depends on material, i.e. on the details of microstructure produced by SPD. On the other hand, if the fatigue loading is performed in the plastic strain-controlled regime, the UFG structure is more prone to the grain coarsening, and the fatigue life for the same plastic strain amplitude is substantially shorter than that of CG material. F