Battle damage repair of a helicopter composite frame-to-skin junction - A sole external repair approach

Research was conducted to examine the effectiveness of a rapid repair to a helicopter composite frame-to-skin junction subjected to battlefield damage. The repair design consists of a laminate patch and aluminium angle bracket adhesively bonded and riveted, respectively, to the helicopter external surface. The assessment involved a relative comparison of three models, representing pristine, damaged and repaired configurations. Computational analyses were conducted to examine the stiffness and buckling onset load of the overall structure and the strengths of individual components (laminates, adhesive bondlines and rivets) under three typical load conditions, namely in-plane shear, axial compression and transverse compression. The results showed that the damage would cause significant stiffness and strength reduction. The repair could sufficiently restore the stiffness and static strength for the load cases considered. However, for the specimen without support from its adjacent helicopter structure, it is predicted that the failure mode under the transverse compression loading would be via buckling under a relatively low load. A compression test was conducted to further validate the repair design. The result agreed well with the prediction. It showed that compared with an un-repaired damaged specimen, the external repair increased the strength by 83%. The equivalent far field failure strain exceeded 3300 µe which is considered satisfactory for a rapid field battle damage repair (BDR).