Issue 40

V. Mazánová et alii, Frattura ed Integrità Strutturale, 40 (2017) 162-169; DOI: 10.3221/IGF-ESIS.40.14 167 (a) (b) Figure 8 : Surface of the specimen cycled in-phase cyclic straining to fracture: (a) central part of the macroscopic crack, (b) parallel PSMs in surface grains. We were interested in the early stages of the damage i.e. the initiation of the cracks. Fig. 8b shows the surface with grains covered by a system of PSMs. No clear crack is apparent from this picture. Also high resolution image did not allow to distinguish extrusions-intrusions and cracks. FIB cut performed on the area covered by a platinum layer reveals the shapes of extrusions and intrusions and the presence of a deep crack starting from one of the intrusions (Fig. 9a). Fig 9b shows the details of the surface relief in the location of the cut revealing several cracks starting from intrusions. (a) (b) Figure 9 : Surface profile of PSMs produced by in-phase cycling: (a) FIB cut, (b) details of the surface profile. Similar surface cracking and initiation of fatigue cracks was found in out-of-phase cyclic straining. Fig. 10a shows typical secondary crack and Fig. 10b the area with the PSMs. The image of the surface profile extracted from the FIB cut perpendicular to the direction of the PSMs is inserted in the picture. PSMs consist mostly of extrusions but PSMs B, E and F have also an intrusion which runs parallel to the extrusion. D ISCUSSION yclic plastic response of 316L steel in in-phase biaxial loading is close to that in uniaxial tension-compression loading [1, 6-8]. Cyclic hardening/softening curves which show long-term softening followed by cyclic hardening at higher strain amplitudes are nearly identical with those measured by Hong et al. [8] in tension-compression. Cyclic hardening/softening curves in 90° out-of-phase cycling are similar but are shifted to higher stress amplitudes.