Friction stir welds in aluminium: design S-N curves from statistical analysis of literature data

Appropriate S-N fatigue design curves for friction stir (FS) welded joints in aluminium alloys are currently not specified in design codes and standards. The present paper is intended to assist in enabling standardised fatigue design for such joints, through a comprehensive statistical analysis of more than 500 individual sets of data gathered from published literature. These data are used to establish the usual design fatigue curves for welds that give a 97.7% survival probability with 95% confidence. Experimental fatigue data represent defect-free butt joints and include both flat plate and tubular joints between similar aluminium alloys (across the range of 2xxx, 5xxx, 6xxx, and 7xxx series). Weld conditions include as-welded, machined, and post-weld heat treated under constant amplitude cyclic loading at various stress ratios in the range from R = -1 to 0.5. A systematic comparison is presented by categorising the data according to the alloy type, temper condition, postweld heat treatment and stress ratio and the correlation with the S-N design curves from Eurocode 9 is also considered. The fatigue curves presented in this paper will serve as a useful guideline for engineers involved in design of friction stir aluminium joints subjected to in-service fatigue loading.