The double-peening technique is consistent as an improvement
on fatigue life of leaf springs when using smaller second-peening
media (0.4 mm and 0.3 mm diameters). These components operate
at high stress amplitudes and are processed at low tempering temperatures
for higher hardness. In these cases crack nucleation controls
the durability. This is supported by the residual stress profile
results, which shows that the fatigue life is strongly affected only
by surface compressive residual stresses (up to 0.02 mm depth).
Even with a stress relief mechanism working sub-superficially,
the fatigue life suffered no negative impact. So the crack propagation,
for these components, is considerably less important compared
to crack nucleation, as supported by the lack of discernible
propagation features in the fracture surfaces of some of the
samples.