Fig. 8 Crack in as-welded SZ
The SZ sample with a thickness of 4 mm under a maximum load of 315MPa is show in Fig. 9 (a). It also had an instantaneous elongation during fatigue test. The cross section of the SZ sample with a thickness of 4 mm is shown in Fig. 9 (b). It was observed that the crack initiated and propagated in the SZ region. The crack initiation region is marked in Fig. 9 (c). The crack initiation relied on the debonding of lamellar structure as shown in Fig. 9 (d) and (e). The instantaneous crack propagation is indicated in Fig. 9 (c), (d), and (e) which contributed to the instantaneous elongation during fatigue test.