Crack growth rate in friction stir welded nugget under different R ratio fatigue load

2.0 mm thick 2198-T8 Al-Li alloy sheets with the parallel friction stir welds in the middle were used in this work. The center cracked tension (CC(T)) specimens with transverse welds were designed and welded by different welding parameters. Fatigue tests were performed and fatigue crack growths in the nuggets were measured and compared. Effects of rotation speed and welding speed on fatigue crack growth rates parallel to the friction stir welds were studied. It was shown crack growth rate in the nuggets can relate to the welding parameter. Residual K (K_res) approaches was used to predict fatigue crack growth rates in residual stress fields. Finite element models of the samples were built and the measured residual stress data put into the model. The virtual crack closure technique was used to calculate K_res and then K_res was used to calculate effective R values. Predicted crack growth rates were compared with experimental results.