Bonded repair of minimum-gage composite sandwich structures

The bonded repair of minimum-gage sandwich structures is investigated both experimentally and analytically. The experimental study focuses on the amount of strength recovery and failure mode of repairs with various numbers of plies in the patch. A three-dimensional finite element model was developed to determine the stress and strain states in the patch, adhesive layer, and face sheets. The repair is modeled on a ply-by-ply basis and includes resin-rich areas between the patch plies, and adhesive pockets at the ply drop locations. The model is correlated with experimental data and a phenomenological failure model is developed that predicts the onset of damage and the failure mode.