The use of safety cuts in fatigue damaged fastener hole repair

Safety cuts (or confidence cuts) are used in the repair of fatigue damaged fastener holes in aircraft structure to ensure that any residual crack is removed in the repair process. The necessity of using safety cuts was investigated by growing cracks in laboratory specimens and then “repairing” the specimens in the manner that would be done on in-service aircraft, both with and without safety cuts. The post-repair fatigue life for the safety-cut specimens showed a bimodal log normal distribution with the lower mode arising from machining flaws in the repair process. The results for the no safety-cut material, while having a similar mean life to the lower mode of the safety-cut coupons, showed nearly four times as much scatter in life. The results strongly suggest that residual cracks in no safety-cut coupons served as initiation sites for further crack growth. Consequently, despite the fact that safety cuts result in lower edge margins, they are a necessary part of the repair process.