The use of safety cuts in fatigue damaged fastener hole repair

• Published in: International journal of fatigue 2016-10, Vol.91, p.242-247
• Main Authors: Underhill, P.R., Juurlink, J., DuQuesnay, D.L.
• Format: Article
• Language: English
• Subjects: Aircraft; Aircraft components; Crack growth; Crack propagation; Damage; Fastener hole repair; Fatigue; Fatigue cracks; Fatigue failure; Fracture mechanics; Probability of detection; Repair; Safety
• ISSN: 0142-1123; 1879-3452
• DOI: 10.1016/j.ijfatigue.2016.06.014

•48 plate coupons with fatigue damaged rivet holes were repaired using standard aircraft techniques.•24 of the coupons had an additional safety cut of 0.030in. (0.75mm).•Safety cut coupons had less fatigue life scatter than non-safety cut coupons.•Residual cracks were serving as initiation sites in non-safety cut coupons.•Safety cuts give lower edge margins but are a necessary part of the repair process. 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.