Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 65 for number of passes between 3 and 10, and [14] 443 MPa for 12 passes by the processing route Bc. In [15] the value ~520 MPa for Cu processed by 12 Bc passes is reported. Generally, during the first passes a rapid increase of strength is observed. However, later on, the strength saturates or even decreases. From the comparison of tensile diagrams of CG and UFG Cu, Fig. 5, it can be seen that there is a substantial difference in (a) the initial part of the diagram, indicating very high yield strength of UFG Cu and very low one for CG material, and (b) in the tensile strength, being two times higher for the UFG state. 0 0.05 0.1 0.15 0.2 Strain 0 100 200 300 400 Stress [MPa] 1mm/min 100mm/min 0 0.2 0.4 0.6 Strain 0 100 200 300 400 Stress [MPa] UFG Cu CG Cu Figure 4 : Tensile diagrams of UFG Cu prepared by ECAP, route Bc. Figure 5 : Comparison of tensile diagrams of UFG and CG Cu. F ATIGUE STRENGTH urphy [1] summarised in a comprehensive review the basic knowledge on copper acquired until the eighties of the last century. The minimum fatigue strength,  c , of annealed Cu at 10 9 cycles to failure is 50 MPa; most of data published in literature falls into the interval of 50 to 60 MPa. The ultimate tensile strength is in the range of 200 – 250 MPa for CG Cu, reflecting the wide range of annealing times, temperatures and the source material. The S-N curve of CG Cu of commercial purity 99.98 % with the average grain size of 70  m can be well described in the interval form 10 4 to 10 7 cycles by Eq. (2): b a f kN    (2) where  a is the stress amplitude, N f number of cycles to failure, k = 388 MPa and b = 0.107 [16]. The copper was annealed for 1 hr. in vacuum; its  UTS was 220 MPa and the yield stress,  0.2 , was 37 MPa. The dependence of fatigue strength on the grain size was found to be quite weak. In practice, there is no effect of grain size ranging in the interval 3.4 to 150 μm on the fatigue life in the high-cycle fatigue (HCF) region [17]. This behaviour was attributed to easy cross-slip. Later on, the grain size effect was not substantiated even though the grain size was varied from 50 μm to 0.5 mm. Generally, it can be summarised that the fatigue life curves expressed both as S-N curves or dependences of number of cycles to failure on the total strain amplitude depend on the grain size of CG Cu insignificantly. This holds especially for fatigue limits based on 10 7 cycles [16]. On the other hand, the Coffin-Manson plot depends strongly on the grain size. The plot is shifted to lower values of plastic strain amplitude,  ap , for a given number of cycles to failure with increasing grain size D . Experimental data on the number of cycles to failure for plastic strain amplitude of 1 x 10 -4 indicate a roughly linear increase of N f with 1/2 d  [18- 20]. The plastic strain amplitude fatigue limit based on 10 7 cycles was found to be grain size dependent, being 4 x 10 -5 for fine-grained copper and 2.3 x 10 -5 for coarse-grained Cu. The explanation of this effect is based on the different conditions for propagation of short cracks, which physically determine the fatigue limit and whose dynamics is a function of obstacles, like grain boundaries. Copper exhibits strong work hardening, which is a typical effect for most single-phase f.c.c. structures. In fact, the tensile strength of annealed material can be increased by 100 % due to 80 % cold working. Cyclic loading of annealed Cu results in rapid cyclic hardening followed by a long period of cyclic softening, in a broad temperature interval [21]. Fatigue of M