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

DSTO-RR-0208
4. Discussion
<strong>Peening</strong> research around the world and at AMRL has shown that a higher level <strong>of</strong> quality
control is needed for peening aluminium alloys than for high strength steels. This arises
because in both materials, fatigue life is controlled by the residual compressive stress, but for
aluminium alloys the surface damage caused by peening is a major competing effect, tending
to reduce fatigue life. In steels, the residual compressive stress controls fatigue life because
<strong>of</strong> the limited surface damage which can be imparted by the glass beads, and the higher
compressive stresses generated At low fatigue stresses, the growth <strong>of</strong> small surface flaws is
a dominant factor in fatigue life, so large life extension can be achieved by appropriate
peening methods, which will reduce the propagation <strong>of</strong> these small flaws.
This effect is highlighted in early AMRL experiments, Figure 11, which show that a crossover
point exists above which peening can reduce fatigue life depending on the type <strong>of</strong>
peening treatment (ie surface finish). Considering this information, AMRL postulated a
correlation between surface finish (ie roughness) and fatigue life. Further experiments on a
reworked surface supported this correlation except where extensive re-peening <strong>of</strong> damaged
surfaces had occurred. In such cases the surface roughness was low but the fatigue life was
short. Visual examination <strong>of</strong> cross-sections revealed that any debris that was present in the
surface after the original peen, had been hammered deeper into the surface along with the
flattening <strong>of</strong> many laps and folds. This result means that surface roughness does not
necessarily correlate with fatigue life for specimens which have peening directly over
previous peening. High-saturation peening will suffer the same limitations in terms <strong>of</strong>
surface finish/ fatigue life correlation when subjected to these high stresses.
AMRL experiments have clearly demonstrated the competing processes involved in
aluminium alloy fatigue life extension due to peening and have been instrumental in
developing an AMRL rework procedure which can be used to extend the fatigue life <strong>of</strong>
critical areas by repeated polishing and re-peening.
<strong>The</strong>re appears to be very little variation in the residual compressive stresses achieved by
different peening processes. At this stage, only x-ray residual stress measurements have
been made, although a number <strong>of</strong> other residual stress methods are currently being
examined. <strong>The</strong>se include “hole drilling”, neutron radiography and a method developed in
Canada by Marchand. <strong>The</strong> residual stress (240±40MPa) appears to be approximately half the
yield stress (480MPa) <strong>of</strong> 7050 aluminium alloy.
At the stresses tested (410MPa) using load spectrum Spec 16 (RAAF-22010 turning points),
peening is always a benefit to fatigue life compared to a polished finish. However, at higher
stresses poor peening can be detrimental to fatigue life. Early AMRL test results show a
mean LIF <strong>of</strong> between 1.6 to 2.1 due to AMRL rework peening, though this mean life increase
did not fully address the scatter in the peened fatigue life results. If the worst scatter case
were to be used then the LIF would be 1.2. This is significantly lower than the LIF <strong>of</strong> 1.39
suggested by MDA for peening.
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