EVALUATION OF ADVANCED SANDWICH STRUCTURE DESIGNED FOR IMPROVED DURABILITY AND DAMAGE TOLERANCE

This report presents information regarding research conducted at Boeing Helicopters to evaluate advanced material systems in sandwich structure designed for rotorcraft primary airframe structure. The purpose of this work has been to improve the durability and damage tolerance of thin-gage sandwich panels suitable for rotorcraft airframes. Four design concepts were evaluated for their improvements over baseline configurations. These four concepts are: 1) toughened thermoset facesheets, 2) thermoplastic facesheets, 3) interleaves, and 4) a denser core. Sandwich specimens utilizing either the toughened thermoset with high strain-to-failure fibers or thermoplastic facesheets showed significant improvements in impact resistance over specimens fabricated using baseline facesheets. These two configurations also showed improvements in residual strength over the baseline graphite/epoxy systems. Facesheet toughness and fiber strain-to-failure were shown to be key parameters in determining overall sandwich durability. Increased core density increased undamaged and residual compression strength but did not significantly improve impact resistance. Finally, the interleaves also did not improve impact resistance but may be able to prevent moisture ingression through undamaged and damaged facesheets.