Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 454-461; DOI: 10.3221/IGF-ESIS.30.55 459 a) b) c) d) Figure 6 : Fatigue crack path at R=0.1: a) and b) two different values of crack extension, c) main and secondary path corresponding to boundary grains, d) high magnification in a zone in the front of conventional crack tip. For R = 0.1, the crack path is characterized both by the presence of linear paths (Fig. 6a) and by the presence of a sort of “zig-zag” propagation paths (Fig. 6b), with secondary paths that after some hundreds of microns join again the main crack. Presence of “zig-zag” paths are probably due to presence of inhomogeneities boundary grains (brittle precipitates) as shown in Fig. 2b. Secondary crack paths are due to a different interaction of main path to boundary grains, as shown in Fig. 6c. In this case, the main crack is almost to 90° respect the boundary grain and the brittle precipitates deviate the path with an angle that that is analogous to the precipitates angle observed in Fig. 2b. Finally, short microcracks are observed ahead of the crack tip under loading conditions (Fig. 6d), probably due to the stress field around the crack and the consequent microstructure stress-induced transformation. Considering R=0.5, analogously to the results obtained for R = 0.1, crack paths (Fig. 7a and b) are characterized by the presence of the “zig-zag” path propagation and by the presence of secondary cracks that can be observed at boundary grains (Fig. 7c). The main differences can be summarized as follows: - Microcracks are not observed ahead of the crack tip. This is probably due to the absence of stage 4 and the crack tip stress field generate a transformed structure characterized by a homogenous behaviour; - Secondary cracks do not join again the main crack (Fig. 7d).