Friction stir welding (FSW) is now widely used in industrial manufacturing fields to join metal materials. Although it avoids solidification cracking, the combination of thermal excursion and plastic deformation introduces significant residual stresses. These residual stresses will combine with the applied stresses in service to cause unexpected failure. Therefore, a detailed knowledge of the residual stress effects on fatigue characteristics of FSWs is required. In this review, the experimental, theoretical and numerical studies of residual stress effects on fatigue characteristics of FSWs are summarized, including fatigue crack initiation, fatigue crack growth, stress-life and residual stress relaxation. The applications of residual stress modification techniques for fatigue improvement in FSWs are also presented.

Residual stress (RS) in laser powder bed fusion (LPBF) additive manufactured structures can significantly affect mechanical performance, potentially leading to premature failure. The complex distribution of residual stresses, combined with the limitations of full-field measurement techniques, presents a substantial challenge in conducting damage tolerance analyses of aircraft structures. To address these challenges, this study developed a comprehensive simulation framework to analyze the 3D distribution of residual stresses and fatigue crack growth in LPBF parts. The 3D residual stress profiles of as-built samples in 15° and 75° build directions were computed and compared to experimental data. The fatigue crack propagation behavior of the 75° sample, considering 3D residual stress, was predicted, and the effects of residual stress redistribution under cyclic loading were discussed. It shows that the anisotropy of residual stress, influenced by the build direction, can lead to mixed-mode fracture and subsequent crack deflection. Tensile residual stress in the near-surface region and compressive stress in the inner region can cause an inverted elliptical crack front and accelerate fatigue crack growth.