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.