Hybrid repair of primary metallic structures by combining a stress optimised cut-out and bonded patches

Abstract

A hybrid repair solution combining a stress optimised cut-out and a bonded patch is an effective approach for primary structure repairs, in which the optimised cut-out enhances the residual strength and helps meet certification requirement while the bonded patch provides further stress reduction in the repaired structure and promotes significant additional fatigue life enhancement. In this study, a cracked metallic specimen was repaired with an optimised cut-out at the crack tip and two boron/epoxy patches bonded back-to-back. The patches were positioned adjacent and some distance away from the cut-out and thus potential crack re-occurrence/growth can be visually monitored without the patch concealing the area. The effectiveness of the hybrid repair was assessed experimentally by comparing against three other specimen configurations namely specimens with a crack, a standard stop drill at the crack tip, and an optimum cut-out hole at the crack tip.

Finite element modelling results indicated that compared with the standard stop-hole specimen, the stress concentration factors of the specimens with optimum cut-out hole and hybrid repairs were reduced by 63 % and 73 %, respectively. Based on a simple cubic law, these stress reductions would result in the fatigue life before crack re-initiation to be 20 and 52 times longer. Fatigue tests under variable amplitude loads confirmed the significant extension of fatigue initiation life, where 28 and 53 times of enhancement were achieved for the optimum cut-out hole and hybrid repairs, respectively. The fatigue test results further showed that for the stop drill and optimum cut-out configurations, once the crack was reinitiated, its growth was relatively fast, whilst in the case of the specimen with the hybrid repair the crack growth rate was significantly slower.