This figure shows a significant fatigue strength decrease of all
welded specimens in comparison to the base material, which can
be explained by both the decrease of mechanical strength and
the notch effect due to the stress concentration at the weld toe.
However, an important fatigue resistance increase with the friction
stir processing of the MIG welds can also be clearly observed.
The favourable effect of friction stir post-processing increases
50% the fatigue strength at 106 cycles relatively to unprocessed
MIG butt welded specimens. Therefore, a favourable effect of friction
stir post-processing on the fatigue strength was obtained,
although fatigue strength of MIG + FSP series remains significantly
lower in comparison with the base material.
The fatigue strength increase induced by the friction stir processing
is partially related to the geometry modification, mainly
due to the toe radius increase. To evaluate the geometry influence
on the fatigue life, the Lawrence equation (Eq. (1)) for butt welds
[24] and Peterson equation (Eq. (2)) [25], can be used to determine
the theoretical, kt, and fatigue, kf, concentration factors of the
welded specimens, respectively.