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

DSTO-RR-0208
50
Total Fatigue Life, Programs
40
30
20
10
0
0 20 40 60 80 100
Percentage <strong>of</strong> Fatigue Life Expended before <strong>Peening</strong>, %
Figure 19: <strong>The</strong> fatigue life <strong>of</strong> specimens peened after various periods <strong>of</strong> service. <strong>The</strong> specimens were
fatigue tested to a percentage <strong>of</strong> total life then peened before continuing the test until
failure. (Note: there was no surface removal between fatigue testing and peening).
Figure 19 clearly shows the benefit in removing the damage layer to restore maximum life.
<strong>The</strong>re is a significant drop in LIF if the damaged layer is not removed after the component
had reached 40-50% <strong>of</strong> its full life. In fact, the peening has almost no LIF effect, because the
damage is now deeper than the beneficial compressive residual stresses. Section 3.3 shows
that even greater care must be taken to remove a peened damage layer, otherwise not only
can no LIF can be associated with the repair process but in fact a reduced life may occur at
some stresses.
3.3.1 Different rework process comparisons
In many localised regions, the F/A-18 has been peened or re-peened, usually by contractors.
AMRL therefore examined the peening rework processes performed on the F/A-18 by these
contractors. Due to the repair requirements for the full-scale fatigue test articles in the
International Follow On Structural Test Program (IFOSTP) run by the RAAF and Canadian
Forces (CF) several <strong>of</strong> rework processes had to be examined. AMRL recommendation was
that the OEM original peening be removed by polishing before any re-peen, and that a total
loss bead system be used; in contrast the CF had adopted the OEM rework procedure <strong>of</strong>
peening directly over the original peening without polishing. In view <strong>of</strong> the potential for this
approach to bury damage in the surface, and cause excessive peening damage, this process
clearly had to be one <strong>of</strong> those examined. Sharp, Clayton and Clark (1994), compared the
effects <strong>of</strong> the different rework processes on fatigue life (Figure 20, summarised in Table 5).
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