Fatigue crack growth in 2024-T351 friction stir welded joints: Longitudinal residual stress and microstructural effects

The role of longitudinal residual stress on propagation of fatigue cracks was examined in friction stir welds produced in 2024-T351 aluminum alloy. Fatigue crack growth rate was obtained through constant Δ K_Iapp tests for notches at different distances from the weld centerline. Subsequently, crack growth was correlated to weld residual stress measured by the cut-compliance method. It was found that residual stresses correspond to low crack growth rates outside the weld zone during fatigue loading. Once in the weld zone, the crack growth was affected by microstructural and hardness changes. Furthermore, weld residual stresses were mechanically relieved and effects on crack propagation behavior were observed. A comparative analysis between relieved and unrelieved joints indicated that fatigue crack growth behavior is dominated by residual stress outside the weld zone.