3.1 Fatigue performance of SZ
The fatigue life of the BM and SZs is shown in Fig. 2 (a). It was concluded that the life of as-welded SZ (6 mm) was greatly short than that of BM. After rolled, the fatigue life of the rolled SZs was gradually extended. According to Fig. 2 (a), the fatigue life of BM and SZs under a stress of 315 MPa was derived as shown in Fig. 2 (b). The fatigue life of original BM was approximately 15000 cycles. The fatigue life of SZ with a thickness of 6 mm was 0 cycles since the low UTS of as-welded SZ (lower than 315 MPa). After rolled to 4 mm, the fatigue life of the SZ increased to 4300 cycles. After rolled to 3 mm and 2 mm, the fatigue life of SZ was increased to 19000 cycles and 27000 cycles, respectively, and was longer than that of BM as shown in Fig. 2 (b).
Based on the fatigue life of the BM and SZs, the fatigue life of as-welded SZ was short than the original BM. After rolling process, the fatigue life was gradually increased. When rolled to 4 mm and 3 mm, the fatigue life became longer than that of original BM.