The Effect of Peening on the Fatigue Life of 7050 Aluminium Alloy

DSTO-RR-0208
Figure 22a: A typical surface pr<strong>of</strong>ile from the OEM peen simulation.
Figure 22b: A typical surface pr<strong>of</strong>ile from re-peening directly over the original OEM peen.
Figure 22c: A typical surface pr<strong>of</strong>ile from the AMRL rework. <strong>The</strong> original OEM peening damage has
been removed and the surface re-peened.
This observation indicates that if the component has been peened directly over the original
peening then surface roughness cannot be correlated with fatigue life. This non-correlation
causes a major problem because, without a very confident knowledge about the peening
history and conditions, the peening quality and fatigue life can no longer be simply
quantified from the surface roughness.
To show more directly the effect <strong>of</strong> peening on fatigue crack growth rate, fractographic
analysis <strong>of</strong> the striation marks left on the fracture surface was undertaken. <strong>The</strong> results are
shown in Figure 23 and Figure 24.
It can be seen clearly that in the two peened specimens the initial crack growth rate is much
lower while the crack is contained in the first 200-300µm. After this depth, the crack growth
rate <strong>of</strong> the peened specimens increases, approaching that <strong>of</strong> the unpeened specimens,
becoming very similar by a crack depth <strong>of</strong> 600µm. Note that the specimens were all tested at
a peak applied stress <strong>of</strong> 410MPa and not at the high stresses (>450MPa) shown earlier where
peening had the potential to reduce specimen fatigue life compared to polished specimens.
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