Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 67 A recent overview of the cyclic deformation and fatigue properties of UFG materials evidences that the fatigue behaviour depends strongly on parameters of the ECAP procedure, purity of material and type of fatigue loading [26]. The discussion, interpretation and, in particular, the comparison of results published in literature, requires taking account of all the details of the UFG structures produced in different laboratories and also the external loading parameters and conditions. In the earliest studies it has been experimentally shown that the lamellar-like elongated type B microstructure shows longer durations under the same stress amplitude than the equiaxial type A [11]. Similar observations were made also on other materials like Ti alloys; however, the available knowledge is not enough to declare that the lamellar-like structures of UFG Cu are generally better than equiaxial ones. The experimental data presented in Fig. 6 give evidence that the UFG structure of Cu can exhibit substantially better fatigue strength expressed in terms of S-N curves than the CG Cu. However, there is also data in literature that indicates quite poor or no improvement of fatigue strength in the HCF region. In [27-29] it is observed the strong enhancement of fatigue life in LCF range but a very weak effect in long-life regime. The fatigue strength of 99.99 wt% Cu processed by four passes by route Bc coincided with that of fully annealed copper for 3 x 10 7 cycles. This fatigue strength was only slightly enhanced by an increase in the number of ECAP passes and by a decrease in purity. The  UTS of coppers investigated in these studies was high. Fig. 8 compiles the available majority published experimental results up to now on fatigue life of UFG Cu prepared by ECAP cycled under constant stress amplitudes. S-N data was obtained in different laboratories on different coppers. It is remarkable that the field of the S-N points splits up into two distinct bands. The inspection of the legend in the figure shows that the material purity could be a parameter influencing the HCF strength. The band A covers S-N points for low purity UFG coppers (purity in the range from 99.5 and 99.9 %), while the band B covers S-N points for high purity UFG coppers (purity in the range from 99.96 to 99.9998 %). The details of the ECAP process, particularly the type of paths (Bc or C), seem to have only minor effect on the fatigue performance. The bands merge into one band in the LCF region and obviously diverge in the HCF region. The average stress amplitude corresponding to the 10 7 cycles to failure is around 60 MPa for band A and around 90 MPa for band B. The S-N curves of two coppers of substantially different purity tested in a goal-directed research are shown in Fig. 9. Cu was processed by two ECAP routes, namely Bc and C. The fatigue tests were carried out in one laboratory under the same testing conditions. Thus the effect of variances in testing procedures (except of the different specimen shape) is eliminated. It can be seen that the fatigue strength of high purity copper is lower than that of low purity copper. The figure also shows that the ECAP route affects the fatigue strength of pure material. Both the effects, i.e. purity and route, are more pronounced for low stress amplitudes. 10 3 10 5 10 7 10 9 N f 80 120 160 200 250 300 350 Stress amplitude [MPa] 99.96, 10-16, Bc 99.99, 12, C 99.96, 12, Bc 99.9, 8, Bc 99.99,12, C 99.99, 4, Bc 99.8, 6, C 99.9998, 6, Bc 99.5, 4, C A B purity [%], nr. of passes, route 10 3 10 4 10 5 10 6 10 7 10 8 10 9 N f 60 80 100 120 150 200 300 400 Stress amplitude [MPa] purity passes route [%] 99.5 6 C 99.5 4 C 99.9998 6 Bc 99.9998 6 C Figure 8 : S-N data of UFG Cu of different purity and processed by different routes and number of ECAP passes. Figure 9: Influence of purity on S-N curves of UFG Cu. The explanation of the large differences in the fatigue resistance of UFG Cu in the HCF region shown in Fig. 8 can be sought either in the stability of the microstructure during fatigue loading or in the mechanism of the strain localisation and in the fatigue crack initiation. The stability of UFG structure and crack initiation will be discussed later. The fatigue lives of UFG Cu are generally higher than those of CG Cu when the comparison is made on the basis of S-N plots. However, just the opposite arises from the comparison on the basis of plastic strain amplitudes. Results of strain- controlled fatigue tests expressed as Coffin-Manson plots show shorter lifetime of UFG than CG Cu [7, 11, 30-32]. The