Issue34

M. Goto et alii, Frattura ed Integrità Strutturale, 34 (2015) 427-436; DOI: 10.3221/IGF-ESIS.34.48 430 The SEM micrograph in Fig. 3a shows the post-fatigued surface at repeated stressing of  a = 240 MPa. A high population of SBs was observed over the whole surface of the specimen. The SBs initiated along the shear plane of the final ECAP processing, and their lengths were less than ten micrometers. On the other hand, the post-fatigued surface formed under  a = 90 MPa (Fig. 3b) had slip-band like traces greater than a few tens of micrometers. The difference in formation mechanism of surface damage between high and low stress amplitudes should affect the formation mechanisms of crack paths under high and low stresses. Figure 3 : SEM micrographs of damaged traces in the post-fatigued specimens: (a) SBs at  a = 240 MPa ( N f =1.95×10 5 ); (b) slip band like traces at  a = 90 MPa ( N f =1.33×10 7 ) . To study the crack growth path inside the specimen fatigued at high and low stress amplitudes, the specimens were sectioned in the longitudinal direction (parallel to the x -axis), at specific crack lengths (Figs. 4 and 5), followed by etching of the sectioned planes. Figs. 4b and 4d show the crack growth path formed at  a = 240 MPa in the sectioned plane for the zx - and xy -plane crack, respectively. The inner growth direction of the zx -plane crack was perpendicular to the specimen surface ( zx -plane). For the xy -plane crack, although the inner growth direction just after the initiation was nearly perpendicular to the loading axis, the crack gradually changed its direction to the plane inclined 45° to the specimen surface ( xy -plane). Although the inclination direction of the crack path to the loading axis was different between the zx - and xy -plane cracks, the paths for both cracks were parallel to the shear plane of the final ECAP pressing (Fig. 1), except for the initial growth path in the xy -plane crack. Figs. 5b and 5d show the crack growth path formed at  a = 90 MPa in the sectioned plane for the zx - and xy -plane crack, respectively. The crack growth paths for both planes were vertical to the specimen surface. The vertical growth path was commonly observed in the conventional grain-sized materials. Figure 4 : Crack paths at  a = 240 MPa: ( a and c ) sectioning position described in the crack paths of the zx - and xy -planes; ( b and d ) crack paths in the sectioned plane for the zx - and xy -plane cracks.