Issue 30

G. Zucca et alii, Frattura ed Integrità Strutturale, 30 (2014) 409-416; DOI: 10.3221/IGF-ESIS.30.49 411 observed. Therefore, Zone A was the region of the final, unstable fracture. Zone B was characterized by a fracture surface flat, smooth and bright with beach marks typical of fatigue crack growth. (Fig. 4). On the external surface’s edge there were ratchet marks, showed by black arrows in Fig. 4. However, also in the 45° orientation areas, there was a small flat area running all around on the external side (black arrow in Fig. 5) showing ratchet marks. Zone A Zone B Figure 2 : Cap entangled inside the fracture surface. Figure 3 : Fracture surface. Figure 4 : Beach marks and ratchet marks on the surface’s edge. Figure 5 : Fracture detail. Electronic Microscopy FESEM observations of the Zone B of fracture surface confirmed the presence of the fatigue crack mechanism Fig. 6 and Fig. 7. Multiple initiation sites of the fatigue crack are all around the external edge of the fracture surface. Zone A showed dimples and microvoid, these are evidences of unstable fatigue crack propagation (Fig. 8) [1].