13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Crack growth rate in friction stir welded nugget under different R ratio fatigue load Yu E Ma1,*, BaoQi Liu1, ZhenQiang Zhao1 1School of Aeronautics, Northwestern Polytechnical University, 710072, P.R. China * Email: ma.yu.e@nwpu.edu.cn Abstract 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 (Kres) 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 Kres and then Kres was used to calculate effective R values. Predicted crack growth rates were compared with experimental results. Keywords Friction stir weld, Welded nugget, Fatigue crack growth rate, different R ratio. 1. Introduction Friction stir welding (FSW), invented at The Welding Institute (TWI) in 1991, was widely used in modern aerospace industry. This new welding technique have successfully taken the pilot study stage and developed into a useful alternative in the manufacture of industrialized aluminum structures solid phase connection technology. Compared to traditional welding techniques, FSW strongly reduces the presence of distortions and residual stresses [1-4]. Also, the FSW performed well in the strength, fatigue and fracture resistant of aerospace aluminum welds, which has long inhibited the widely use of those alloys such as 2XXX and 7XXX series [5, 6]. These aluminum alloys are generally classified as non-weldable because of the poor solidification microstructure and porosity in the fusion zone [7]. Previous studies show that the grain size of the weld nugget has a strong effect on the mechanical properties [8]. In 2007, the air bus published a report which claimed that the FSW application in fuselage longitudinal seam connection of A340-500s and A340-600s has reduced 0.9 kg weight per meter relative to traditional riveting technology. A similar instance occurred in Boeing that the production efficiency in the manufacture of C-17 Global/MasterⅢ has improved tenfold with the FSW application. 2. Experimental procedure 2.1. Fatigue test procedure 2198-T8 Al-Li alloy plates were friction stir welded. The welding directions are perpendicular to the rolling direction. Cracks are the weld centre and grow in the weld. Center cracked tension (CC(T)) specimens ware chosen for the fatigue experiment. All samples have been cut into 220mmX70mm. The specimens have different rotation speed and welding speed: 800(rpm)/400(mm/min), 600(rpm)/200(mm/min) and 400(rpm)/100(mm/min). Fig.1 shows the sample geometries.
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