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).