Figure 7 Stress distributions on the damaged structure and patch of a live repair.
For stop-drill crack repair, upon the initial crack appears, two holes of 3mm in radius are drilled at each end of the crack to stop them from further growth. A patch is then placed. The fatigue loading continues to be applied. A re-initiation of a new crack of 1mm occurs at 15,134 service cycles with stop-drill repair [23]. The fatigue crack propagation analysis is further performed. The crack ultimately reaches 76.9mm with 27574 service cycles with stop-drill repair. The stop-drill repair technique extends the wing structural life by 6-fold compared to no repair, and 3 times more effective compared to live crack repair. Figures 8a and 8b show the stress contours of fatigue crack propagation following its re-initiation at different load cycles. Figure 8a shows the stress concentration near the crack tips after re-initiation of the new crack upon stop-drill repair. The titanium patch exerts stress mostly concentrated in the general vicinity of the crack. Figure 8b shows the stress concentration of the crack tip at the end of its structural life, where the crack has grown to the end of the patched area. The patch stress increases correspondingly as more loading is applied and yields a more concentrated stress where the crack is located.