Structural Integrity Assessment Procedure (SINTAP) was applied to thin aluminium alloy 6013 T6 base material and friction stir welded (FSW) wide plates containing flaws to predict their residual strength. The weld joints exhibit strength undermatching and, hence, plastic strain localization occurs within the lower strength FSW weld region which requires special considerations when applying the SINTAP-based failure assessment route. Two different (J-integral and CTOD) R-curves were used as material input parameters in the analysis. The SINTAP estimations of the experimental residual strength value for the base material panel were remarkably accurate due to the use of the highest analysis level of the SINTAP procedure. The weld strength mismatch analysis level also successfully provided a conservative estimate of the maximum load level of the FSW panel.