Figure 8 Stress distributions on the damaged structure and patch
of a stop-drill repair.
Finally, for damage-removal repair, upon the initial crack appears, the
entire damage cracked area is removed with a hole and a patch is placed.
The fatigue loading continues to be applied. For damage-removal repair
technique, the new crack occurs after 69,146 service cycles, a
significant improvement compared to stop-drill repair (at 15,134 service
cycles). The simulation continues to run until structural failure.
Fatigue crack propagation analysis for further extended service life
evaluation is then performed. The crack finally reaches 64.1mmm with
82645 service cycles. Damage-removal repair extends the wing structural
life by 20-fold compared to no-repair, 10 times more effective compared
to live crack repair, and 3-times more effective compared to stop-drill
repair. Figure 9 shows the von-Mises stress distributions of the damaged
structure and the patch at different loading cycles. In Figure 9a, the
crack starts to re-initiate at 70146 loading cycle and the stress
concentrates begins to appear. The titanium patch has a relatively even
stress distribution. At the end of the structural life (Figure 9b), the
crack has propagated to the end of the patch region. Stress is
concentrated at the end of the crack tip as well as the patch where the
crack occurs.