3.4 Fracture behavior of SZs
The fractured samples of BM (315 MPa) and as-welded SZ (210 MPa) are shown in Fig. 7 (a) and (d). During fatigue test, the load of the two samples had an instantaneous elongation larger than 50 μm. However, the samples did not fracture. The cross section of the BM sample is shown in Fig. 7 (b) and (c). The crack initiated on the surface. The instantaneous crack propagation contributed to the instantaneous elongation. In Fig. 7 (c), the straight crack was the instantaneously propagated crack. The crack in the as-welded SZ sample is shown in Fig. 7 (e). It was observed that the crack initiated in the SZ near the PAZ/BM interface.