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

DSTO-RR-0208
Figure 15b: Scanning electron microscope pictures <strong>of</strong> a replica from a RAAF reworked surface,
Aircraft A21-110 (left) and Aircraft A21-15 (right) (Sharp et al. 1997).
3.2.2 Surface roughness/residual stress measurement
Since a smoother surface appeared to provide substantial improvements in fatigue life, it was
desirable to explore the possibility that a correlation could be found between surface
roughness condition and fatigue life.
AMRL purchased a laser surface pr<strong>of</strong>iler, which allowed very accurate non-contact
measurements <strong>of</strong> the surface roughness. <strong>The</strong> laser surface pr<strong>of</strong>iler is accurate to 0.1µm over
a range <strong>of</strong> 500µm. While this level <strong>of</strong> accuracy is not required in this case, many “stylus” type
<strong>of</strong> surface roughness measuring systems grind through the surface due to the complex
surface roughness. <strong>The</strong> AMRL instrument is now computerised to allow a 3-D surface
pr<strong>of</strong>ile to be built up.
For the RAAF aircraft <strong>of</strong>f-cuts, the wide difference in surface quality is very evident in the
measured surface roughness, Table 3, where R max is the max roughness depth, R z is the mean
roughness depth and R a is the arithmetical mean deviation (See Appendix 1).
Table 3: Some Typical F/A-18 FS488 Bulkhead Off-cut Surface Roughness Measurements (Sharp et al
1997).
Tail No. R max (µm) R z (µm) R a (µm)
A21-13 63.96 30.95 3.66
A21-18 73.12 41.36 5.03
A21-109 103.1 35.43 3.72
A21-112 124.6 55.53 6.23
Table 4 compares coupon surface roughness with the coupon fatigue life at 410MPa (F/A-18
peak applied stress under a USN ST-16 load spectrum, which gives a k t s=432MPa).
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