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

DSTO-RR-0208
5. Conclusion
1. <strong>The</strong> benefits <strong>of</strong> peening in terms <strong>of</strong> fatigue life extension are dependent on the applied
stress - the higher the applied stress the lower the benefit. For the test stresses described
in this report, the fatigue life improvement factor for peening ranges from 1.2 to 6 when
using the AMRL proposed rework specification. For a peak stress between 400-420MPa,
common for highly stressed regions on F/A-18 bulkheads, a LIF=1.5 is applicable, which
is slightly higher than the 1.39 LIF declared by the OEM for peening.
2. <strong>The</strong> research undertaken has provided a good understanding <strong>of</strong> the competing effects <strong>of</strong>
residual stress and surface damage in controlling fatigue life in peened components.
3. <strong>The</strong> AMRL research program has developed a rework process which <strong>of</strong>fers the potential
<strong>of</strong> repeatedly restoring the fatigue life <strong>of</strong> critical locations on the F/A-18 aircraft.
4. <strong>The</strong> progressive change in peening media (steel shot to glass bead to ceramic bead) has
caused significant improvements in fatigue life. <strong>The</strong>se changes, combined with a total
loss system or better filtering, has reduced the number <strong>of</strong> broken beads (glass and
ceramic) and odd-shaped beads (steel) that impact the surface. Broadly, this has reduced
the number <strong>of</strong> defects embedded into the surface.
5. Surface roughness (related to peening quality) can be correlated to fatigue life only if the
peening is performed on a clean polished surface - the smoother the peened surface finish
the better the fatigue life. Where peening has occurred directly over earlier peening,
surface roughness cannot be correlated with fatigue life.
6. <strong>The</strong> magnitude <strong>of</strong> the residual compressive stress appears to vary little under the different
conditions examined in this report, although in all cases peening saturation <strong>of</strong> the surface
was ensured. In the case <strong>of</strong> aluminium alloys, the plastically deformed region is many
times larger than the bead size used - this is not the case when peening steel, in which the
deformed region is comparable to the bead size.
7. <strong>The</strong> OEM peening specification does not give the optimal peening conditions for
obtaining the maximum life improvement factor for aluminium alloys. <strong>The</strong> main problem
arises because the peening operation is not mechanised, but relies on human operators,
and the environment is far from ideal in terms <strong>of</strong> making detailed quality control
assessments “on the fly”.
8. Further research is underway and planned to establish a more through statistical basis for
implementing peening methods.
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