To illustrate the effectiveness of different repair techniques, the existing crack is treated using three ways using simulations: a) live crack repair (patch without crack treatment), b) stop-drill crack repair, and c) damage-removal repair (crack cutout), as shown in Figure 4. For each of the repair techniques, on the fatigue damaged fuselage skin structure with pre-crack, a bonded patch is placed.
For live crack repair, a patch is applied without any crack treatment, the fatigue loading continues to be applied until structural failure. The crack grows to 130.1mm from its initial 53.4mm initial crack. It is noted the two crack fronts grow additional 44.6mm and 32.1mm. The fatigue life for live crack repair is extended from 4064 service cycles if no repair at all to 7598 service cycles, extending the wing structural life by nearly 2-fold. As crack propagates, repair patch carries more and more loads leading to higher stresses as shown in Figure 7. Figure 7a shows the stress contours of the damaged structure with the crack and the titanium patch at 4249 loading cycles. Figure 7b shows the stress of contours at 7598 cycles (at the end of the structural life). It is evident that the stress concentrates at the end of the crack tips and the crack continues to grow as more loading is applied. The stress on the titanium patch also continues to increase as more loading is applied and the crack grows.