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.