Issue 77

T. Jiao et alii, Fracture and Structural Integrity, 77 (2026) 362-385; DOI: 10.3221/IGF-ESIS.77.21

(c) final fracture zone Figure 7: Fatigue fracture morphology characteristics of a sound joint observed via high-magnification SEM: (a) crack initiation zone; (b) crack propagation zone; (c) final fracture zone. In summary, the fatigue behavior of the sound joint is essentially the result of the interplay between two competing microstructural mechanisms introduced by the FSW process: the strengthening effect due to grain refinement in the stir zone and the weakening effect due to material softening and stress concentration in the HAZ [24].

Figure 8: Microhardness distribution in a sound joint.

For the joint with oxide inclusion defects (Fig. 10), the crack initiation sites are concentrated in the weld root area. Cracks rapidly propagate transversely across the joint width. Multiple cracks merge into a main crack during propagation, extending toward the arc-striated surface. The fracture surface has fewer radial striations, which are gently curved, indicating that crack propagation is dominated by the physical barrier effect of the inclusion defects, resulting in less consistent propagation paths.

373