3.5.1 Fracture behavior of SZs
According to the fracture of the four SZ samples, four steps were concluded. The first was the initiation of fatigue crack based on the lamellar structure. The lamellar structure was not disappeared in the SZs after rolling as shown in Fig. 13 (a). During fatigue test, the crack generated in the lamellar based on the debonding of lamellar structure as shown in Fig. 13 (b). Therefore, the fracture step was observed on the fracture surface as shown in Fig. 11 (e), (f), Fig. 12 (f), and (g). After crack initiation, the crack propagated. If the strength of SZ was low, such as the SZs with a thickness of 6 mm and 4 mm, the crack instantaneously propagated. If the SZ had a high strength, the crack would gradually propagate and form the fatigue striation on the fracture surface as shown in Fig. 11 (g), Fig. 12 (h), and Fig. 13 (c). When the fatigue crack propagated to a certain degree, the SZs would rapidly fractured and formed the complete fracture region as shown in Fig. 11 (h), (i), Fig. 12 (i), (j), and Fig. 13 (d).