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

DSTO-RR-0208
Figure 11: <strong>The</strong> effect <strong>of</strong> different peening variables on surface finish, Top- over-pressure or excessive
time, Bottom left- nozzle angle and Bottom right- even indentations but numerous surface
cuts from debris.
Aircraft are subject to spectrum loading and there is the potential (a) for a high spectrum
loads to change the beneficial residual stresses in the component and (b) for different fatigue
lives as a result <strong>of</strong> a changed balance between crack initiation and crack growth. Clark and
Clayton (1991) examined the effect <strong>of</strong> constant amplitude and spectrum loading on the
fatigue life <strong>of</strong> peened components, using small numbers <strong>of</strong> specimens, at different overall
stress levels. A more detailed test program examining the effect on fatigue life <strong>of</strong> peening
treatment under spectrum loading was carried out at AMRL (Sharp, Clayton and Clark,
1993). <strong>The</strong> test results, Figure 12, clearly show that at the high stress (low cycle) end <strong>of</strong> the
fatigue curve, shot peening can be detrimental to fatigue life. At the low stress (high cycle)
end <strong>of</strong> the fatigue curve, shot peening is beneficial to fatigue life but the amount <strong>of</strong> benefit is
controlled by the peening process quality.
17