Effects of residual stresses on fatigue crack propagation in friction stir welded 2198-T8 and 2195-T8 Al-Li alloy joints

Effects of weld residual stresses on fatigue crack growth (FCG), both parallel and perpendicular to aluminium friction stir welds, were investigated by experiments. A range of sample geometries were machined from identical welds; hence the hardness and microstructures were identical and the residual stress fields were different. Tests were conducted on M(T), C(T) and ESE(T) (eccentrically-loaded single edge crack tension) specimens with either longitudinal or transverse welds. Cracks growing into or growing away from the weld centre, as well as in the nugget zone were investigated. It is shown that residual stresses have significant effect on FCG rates and in the cases of M(T) with longer cracks and ESE(T) with crack growing into the weld compressive residual stresses caused extra crack closure and FCG rate reduction. For M(T) and C(T) samples with cracks parallel to the weld, the FCG rate was lower in the nugget than in parent material. Facture mechanics analysis is conducted to understand and explain the experimental findings.