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

DSTO-RR-0208
Like all the structural aluminium parts the Y470.5 bulkhead has been IVD coated for
corrosion protection. As part <strong>of</strong> this IVD production process the fresh aluminium surface is
etched. This etching has a detrimental effect on the fatigue life <strong>of</strong> coupons made from a
similar material 7050-T7451 (Barter 1999). A comparison <strong>of</strong> peened fatigue life versus etch
fatigue life, Figure 32, shows the benefit <strong>of</strong> peening compared to an etched surface over a
large range <strong>of</strong> stresses.
Etched Fatigue Life
Peened Fatigue Life
500
450
Max Stress (MPa)
400
350
300
250
0 50 100 150 200 250 300 350 400
IARP03a programs
Figure 32: Fatigue test results using IARP03a+marker spectrum comparing an etched surface finish
with a peened surface finish. <strong>The</strong> peening was done as per AMRL standards.
All testing was done under spectrum loading using a fighter aircraft representative spectrum
(IARPO3a), which equates to approximately one year <strong>of</strong> flying. This is the spectrum applied
to the FT55 fatigue test and FT488/2 bulkhead fatigue test.
<strong>The</strong> crack growth rate and initial discontinuity size were measured for each specimen. This
provided a distribution <strong>of</strong> fatigue initiation starter sizes (“y” intercept in Figure 33) and crack
growth rates (slopes in Figure 33). For a particular stress the fatigue crack growth rate
showed very little variation, therefore only the mean slope was used for all subsequent
analysis. By using the distribution <strong>of</strong> fatigue starters and the mean crack growth rate, Figure
33, it is possible to determine the status <strong>of</strong> each aircraft and the repair procedure needed.
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