## 4 Results and Discussion
### Fractography  
First, the fracture morphology was examined using a magnifying glass, and an obvious crack initiation zone, crack propagation zone, and instantaneous fracture zone [14] were observed. In Figure 15a,b, I is the crack initiation zone, II is the crack propagation zone, and the size of the  
Figure 14: The stress and Vickers hardness in the failure zone. Figure 13: Vickers hardness profiles at \(-60\,\mathrm{\SIUnitSymbolCelsius}\). crack propagation zone is related to the material toughness and the load level. III is the instantaneous fracture zone, which is the fresh section formed by the instantaneous fracture after the crack expands to a certain size [45]. Most cracks are generated in zones with defects or high stress [46]. The crack propagation zone was smooth and flat and appeared as a \"beach mark\" in the zone. The instantaneous fracture zone was rough and white. Figure 15: The fractography of two welded joints: (**a**) the fractography of T-welded joints and (**b**) the fractography of cruciform welded joints. For the fracture of the T-welded joints, stress concentration was present in one side, and there was no stress concentration in the other side; therefore, the crack was generated in the side (I) with stress concentration and expanded downward in the form of an ellipse until it was broken, as shown in Figure 15a. For the fracture of the cruciform welded joints, since there was a strong stress concentration in both sides, the crack was preferentially generated on the side with defects. As shown in Figure 15b, cracks were generated in one side (I) and then expanded in the form of an ellipse until failure. The fractography of T-welded and cruciform welded joints at \(-60\)\({}^{\circ}\)C was also analyzed using SEM images. The morphology of the crack initiation zone, the crack propagation zone, and the instantaneous fracture zone clearly differed [47]. Some radiation stripes were observed in the crack initiation zone, as shown in zone I of Figure 15a,b. These stripes are steps formed by many tear planes that were parallel to each other but at different heights. In addition, the crack propagation zone contained obvious transgranular fatigue cracks (zone II of Figure 15a,b). The instantaneous fracture zone shows signs of ductile fracture (zone III of Figure 15a,b) [48]. (I) Crack initiation zone  
The crack initiation zone consisted of a relatively large planar defect, and a number of microcracks were observed in the crack initiation zone [49]. These cracks were generated at the surface with stress concentration. There were more fatigue sources at the crack initiation zones of cruciform welded joints than those of T-welded joints, and the stress concentration factors of cruciform welded joints were higher than those of T-welded joints, as shown in zone I of Figure 15a,b. This phenomenon is consistent with the conclusion of Section 4.1. (II) Crack propagation zone  
At least 10 fatigue striation spacings were taken as the population from the crack propagation zone at \(\Delta K=30\) MPa \(\sqrt{\text{m}}\) and the mean of the striation spacing was obtained by averaging the population. The results showed that the striation spacings of cruciform weld joints were 0.84 \(\upmu\)m, while the striation spacings of T-welded joints were 0.79 \(\upmu\)m.  
In the SEM images of the crack propagation zone, clear fatigue striations were observed in the crack propagation zone of the two welded joints, as shown in zone II of Figure 15a,b. According to the \"plastic passivation model\" proposed by [PERSON] [46], the spacing of the fatigue striations represents the crack propagation of the stress cycle, which can reflect the fatigue crack growth rate \(da/dN\)[50]. From the zone II of Figure 15a,b, the spacing of fatigue striations of T- welded joints was less than that of cruciform welded joints. This means that the crack propagation rate of the T-welded joints was lower than that of the cruciform welded joints.