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

DSTO-RR-0208
Figure 13: Typical fatigue crack initiating defects from different peening processes. <strong>The</strong> majority <strong>of</strong>
fatigue cracks initiate at embedded fractured glass beads, surface laps and folds and
fractured sub-surface intermetallics.
Experiments conducted by a number <strong>of</strong> researchers (Butz and Lyst 1961, Fuchs 1956 and
Almen 1947) have suggested the use <strong>of</strong> “dimple diameters” to estimate depth <strong>of</strong> peening
effect. <strong>The</strong> results show essentially a 1-to-1 relationship between dimple diameter and depth
<strong>of</strong> compressed layer, with the residual stresses dropping away rapidly after this point. In
more recent research (Al-Hassani 1984) it has been shown that in high strength steels this 1-
to-1 ratio applies but in aluminium alloys ratios <strong>of</strong> 1-to-2 have been obtained.
A research program was also conducted at AMRL to obtain a better understanding <strong>of</strong>
peening parameters and their effect on fatigue life. In particular gas pressure, irradiation
time, bead flux and bead type were examined. <strong>The</strong> tests were conducted on small 4-point
bend specimens (100mm long, 5mm wide and 3mm thick). Each specimen was polished to
#800 grade finish before treating. All testing was done at 10Hz and R=0.05 and a test stress
<strong>of</strong> 186MPa. Seven treatments, Table 2, were compared to a polished finish.
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