If the specimen failed at the TMAZ, the typical example of the fatigue crack initiation site was shown in Fig. 6. Fig. 6a shows that the critical fatigue crack was initiated from the surface. The detailed view of the crack initiation site is shown in Fig. 6b, where a particle with a maximum length of 15 μm can be observed at the surface. The crack path on the side surface of this specimen is also presented in Fig. 6c. It can be noted that the crack length at the TMAZ is much shorter than that at the NZ after the fatigue test, which indicates that the TMAZ has a better fatigue crack growth resistance in comparison with the NZ. This is consistent with the results of fatigue crack propagation test that the crack propagation rate across the FSW welds increases once in the weld nugget [20]. Moreover, the enlarged view of the crack initiation site with OM indicated that the critical particle that inducing the crack is located at the border of the TMAZ-AS, which is adjacent to the NZ with a distance of about 90 μm.