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

DSTO-RR-0208
Table 5:
Results from local peening rework processes. <strong>The</strong> OEM rework – peen over OEM original
peen, AMRL rework – remove OEM original peen by polishing then peen. Specimens
(Sharp, Clayton and Clark (1994)) were tested with a RAAF post-LEX spectrum Spec 16 -
21010 turning points 302.9 hrs per block at a peak stress 410 MPa. Material was 5 inch
plate,
Rework Process Log Av. Fatigue Surface Roughness Surface Roughness Residual Stress
Life (SFH)
R max (µm)
R z (µm)
MPa
Hand polish (#800) 9287 3.4 2.9 -35±20
Simulated original 19079 63.9 46.5 -270±40
aircraft peen
“Overpeen” rework 15073 51.7 38.0 -230±40
AMRL rework
specification
22895 28.2 21.4 -260±40
Table 5 shows that the residual compressive stress is very similar no matter which treatment
is used - in other words they all have reached saturation. Interestingly, in all the x-ray
residual stress analyses completed, the residual compressive stress range is between -
230MPa and -270MPa. <strong>The</strong> ranking order is very similar to the order presented in Table 4,
which were fatigue tests, conducted using a different spectrum ST-16.
Figure 21 shows typical surface finishes achieved with each <strong>of</strong> the peening rework processes
trialled. <strong>The</strong> AMRL rework process (polish/peen) provides the best surface finish and also
the longest fatigue life.
Figure 21a: A typical OEM peen simulation surface. Note that because glass beads were used in the
simulation, the damage to the surface could not be made to fully match the damage seen on
some aircraft where a flapper wheel (Figure 50 was used. <strong>The</strong> arrow indicates lifting grains
typical <strong>of</strong> over-pressure or excessive peening time.
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