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M. Goto et alii, Frattura ed Integrità Strutturale, 34 (2015) 427-436; DOI: 10.3221/IGF-ESIS.34.48 435 maximum shear planes which correspond to the shear plane of the final pressing. Therefore, questions regarding why one type of SBs nucleates, as observed, are still unanswered. C ONCLUSIONS he main findings of this study can be summarized as follows: The direction of the crack growth paths strongly depended on both stress amplitudes and crack initiation sites along the circumferential direction of the round-bar specimen. At low stress amplitudes, like conventional grain- sized materials, the cracks grew perpendicular to both the loading direction and specimen surface regardless of the crack initiation sites. At high stress amplitudes, the inclination of crack paths were observed. On the specimen surface, the inclination of the crack paths to the loading direction was 45° in the zx -plane and 90° in the xy -plane, whereas the inner crack path was inclined 90°and 45°to the specimen surface in the zx - and xy -plane crack, respectively. Profile of crack at low stress amplitudes showed the aspect ratio of b / a = 0.82 for both plane cracks. At high stress amplitudes, the values of aspect ratios was b / a = 0.38 and 1.10 for the zx - and xy -plane crack, respectively. The crack at low stress amplitudes propagated via the striation formation mechanism, which is associated with crack tip retardation and blunting. At high stress amplitudes, however, the formation of crack paths was strongly affected by the in-plane shear-mode deformation and the SB decohesion process around the crack tip areas. A CKNOWLEDGMENTS his study was supported by a Grant-in-Aid for Scientific Research (C) (KAKENHI: No. 26420021) from the Ministry of Education, Science and Culture of Japan, as well as a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (No.2011-0030058), and by a grant from the Global Frontier R&D Program (2013M3A6B1078874) on Global Frontier Hybrid Interface Materials R&D Center funded by the Ministry of Science, ICT and Future Planning. The authors are very grateful to the members of the Strength of Materials Laboratory of Oita University for their excellent experimental assistance. Thanks are also extended to the members of the Korea Institute of Materials Science, for performing the ECAP processing of our copper rods. R EFERENCES [1] Valiev, R.Z., Kozlov, E.V., Ivanov, Yu. F., Lian, J., Nazarov, A.A., Baudelet, B., Deformation behaviour of ultra-fine grained copper, Acta Metal.l Mater., 42 (1994) 2467-2475. [2] Valiev, R.Z., Structure and mechanical properties of ultrafine-grained metals, Mater. Sci. Eng., A234-236 (1997) 59- 66. [3] Iwahashi, Y., Horita, Z., Nemoto, M., Langdon, T.G., The process of grain refinement in equal-channel angular pressing, Acta Mater., 46 (1998) 3317-3331. [4] Zhu, Y.T., Lowe, T.C., Observations and issues on mechanisms of grain refinement during ECAP process, Mater. Sci. Eng., A291 (2000) 46-53. [5] Agnew, S.R., Weertman, J.R., Cyclic softening of ultrafine grain copper. Mater. Sci. Eng., A244 (1998) 145-153. [6] Hashimoto, S., Kaneko, Y., Kitagawa, K., Vinogradov, A., Valiev, R.Z., On the cyclic behavior of ultra-fine grained copper produced by equi-channel angular pressing, Mater. Sci. Forum, 312-314 (1999) 593-598. [7] Vinogradov, A., Hashimoto, S., Multiscale phenomena in fatigue of ultra-fine grain materials-an overview, Mater. Trans., 42 (2001) 74-84. [8] Patlan, V., Vinogradov, A., Higashi, K., Kitagawa, K., Overview of fatigue properties of fine grain 5056 Al-Mg alloy processed by equal-channel angular pressing, Mater. Sci. Eng., A300 (2001) 171-182. [9] Höppel, H.W., Zhou, Z.M., Mughrabi, H., Valiev, R.Z., Microstructural study of the parameters governing coarsening and cyclic softening in fatigued ultrafine-grained copper, Philos. Mag. A, 82 (2002) 1781-1794. [10] Wu, S.D., Wang, Z.G., Jiang, C.B., Li, G.Y., Alexandrov, I.V., Valiev, R.Z., The formation of PSB-like shear bands in cyclically deformed ultrafine grained copper processed by ECAP, Scr. Mater., 48 (2003) 1605-1609. [11] Mughrabi, H., Höppel, H.W., Kautz, M., Fatigue and microstructure of ultrafine-grained metals produced by severe plastic deformation, Scr. Mater., 51 (2004) 807-812. T T