Issue34

M. Goto et alii, Frattura ed Integrità Strutturale, 34 (2015) 427-436; DOI: 10.3221/IGF-ESIS.34.48 429 crack length was measured using a plastic replication technique. The stress value referred to is that of the nominal stress amplitude,  a , at the minimum cross-section (5-mm diameter). Figure 1 : Illustration showing the location of the drilled hole as a crack starter, and the shear plane of the final ECAP pressing. The distribution of the grain sizes and grain boundary misorientation were determined by electron backscatter diffraction (EBSD) analysis. The specimens were ground using silicon carbide papers and polished with polycrystalline 3- and 1-  m diamond suspension. Final polishing was performed using a 0.04-  m colloidal silica suspension for 30–60 min. EBSD mappings were executed using a Tescan Mira II SEM incorporating an EDAX-TSL Hikari EBSD detector. Each hexagonally shaped pixel was 40 nm for the UFG copper samples and 1.0  m for the conventional grain-sized copper samples. Orientation imaging microscopy analysis software version 5.3 was used to analyze the orientation characteristics and misorientation distributions. E XPERIMENTAL RESULTS ig. 2 show the crack growth paths in the zx - and xy -planes under high and low stress amplitudes (  a = 240 and 90 MPa). The surface-cracks within the zx - and xy -planes are referred to hereafter as the “ zx -plane crack” and “ xy - plane crack”, respectively. At  a = 240 MPa, the zx -plane crack that initiated from the hole created a 45° incline to the loading axis and this crack path direction was parallel to the shear direction of the final ECAP processing (Fig. 1). The 45° inclined crack growth direction has been commonly observed in the zx -plane of LCF UFG metals [20, 21, 23]. However, the xy -plane crack grew nearly perpendicular to the loading axis like crack paths in conventional grain-sized materials. At  a = 90 MPa, on the other hand, the macroscale growth direction was perpendicular to the loading axis regardless of the plane where the crack initiated; nevertheless, the crack propagated in a zigzag manner at the microscale. Figure 2 : Effect of stress amplitudes on crack growth paths in the zx - and xy planes: ( a , and b ) the zx -plane crack at  a = 240 and 90 MPa; ( c and d ) the xy -plane crack at  a = 240 and 90 MPa. F