Fatigue failure of aluminum alloy friction stir welded joints under two-stage variable amplitude loading

The fatigue failure rule of 7075-T651 aluminum alloy stir welded joint is studied under two-stage variable amplitude loading by fatigue experiments and finite element simulation. The results show that the friction stir welding (FSW) joints have different weak areas under different loading conditions. The fracture position of the FSW joint is related to the loading sequence of variable amplitude load and cycle number. The cycle number and crack length under the first-stage loading influence the fatigue life and fracture location of the FSW joints. When the cycle ratio of the first-stage loading is 70 ~ 80%, the fracture position of the corresponding specimen does not change. The simulation results show that the corresponding cycle ratios of critical damage under low–high load and high-low load are 77.8% and 74.4%, respectively, which are consistent with the experimental results. When the crack length is greater than 400 μm under low–high load, or when the crack length is greater than 500 μm under high-low load, the crack location does not change.