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

DSTO-RR-0208
showed a significant improvement in fatigue life using ceramic beads compared to glass
beads. <strong>The</strong> reason for the different results is not clear, but may be due the different testing
conditions or process details. Figure 25 indicates that an operator can rework a local area a
number <strong>of</strong> times and each time the original peened fatigue life is restored to that area. On
an aircraft <strong>of</strong> course, the reduction in thickness (increased stress) with each polish would
have to be taken into account.
3.3.3 Summary <strong>of</strong> AMRL Rework <strong>Peening</strong> Specification
AMRL recommendation was that any future rework peening be done to PS14023 with the
following changes;
1) Remove original peening surface damage (or other damage) before peening,
2) Polish surface prior to peening,
3) ALMEN intensity 6-8A,
4) 200% coverage in main area,
5) Nozzle perpendicular to the surface,
6) Total loss glass bead system or recycled (filtered) ceramic beads,
7) Start peening in corners and work to flat region to reduce laps and folds,
8) Ensure peening runout region ie 100% coverage.
<strong>The</strong> improvement over the specification set out in section 3.2.3 is the verification that the
removal <strong>of</strong> “damage layer” is critical to the process and the life improvement factor derived
from any peening process.
This complete process was published and distributed in late 1996 (Sharp and Byrnes, 1996)
and has been used on both fleet aircraft and structural test articles (Section 3.5). CF adopted
this process for any peening repairs to the FT55 test article. To enhance control <strong>of</strong> the
amount <strong>of</strong> material to be removed AMRL developed a spring punch process for controlled
surface removal (Barter and Houston, 1997). <strong>The</strong> process essentially places a large dent into
the surface <strong>of</strong> a controlled depth. Once the dents have been polished away a controlled
depth <strong>of</strong> material has been removed.
3.4 <strong>Peening</strong> Life Improvement Factor (LIF)
<strong>The</strong> basis <strong>of</strong> any repair is the LIF that can be derived from that repair. For the F/A-18 while
many components are peened no LIF is taken account <strong>of</strong> in the lifing. <strong>The</strong> main reason for
this has been the unreliability with the peening process. Figure 26 shows how the coupon
fatigue life varies with the peening parameters ALMEN intensity and peen coverage. <strong>The</strong>
variability in life is an indication <strong>of</strong> why robotic or automated peening is far better than
human operator peening.
Figure 26 also shows that using 6-8A ALMEN intensity at 200% coverage provides
maximum LIF for 7050-T7451 aluminium alloy coupons.
35