ANALYSIS AND TESTS OF REINFORCED CARBON-EPOXY/FOAM-CORE SANDWICH PANELS WITH CUTOUTS

The results of a study of a low-cost structurally-efficient minimum-gage shear-panel design that can be used in light helicopters are presented. The shear-panel design is based on an integrally stiffened syntactic-foam stabilized-skin concept with an all-bias-ply tape construction for the skins. This sandwich concept is an economical way to increase the panel bending stiffness with a minimum weight penalty. The panels considered in the study were designed to be buckling resistant up to 100 lbs/in. of shear load and to have an ultimate strength of 300 lbs/in. The panel concept uses unidirectional carbon-epoxy tape on a syntactic adhesive as a stiffener that is co-cured with the skin and is an effective concept for improving panel buckling strength. The panel concept also uses pultruded carbon-epoxy rods embedded in a syntactic adhesive and over-wrapped with a bias-ply carbon-epoxy tape to form a reinforcing beam which is an effective method for redistributing load around a rectangular cutout. The buckling strength of the reinforced panels is 83 to 90 percent of the predicted buckling strength based on a linear buckling analysis. The maximum experimental deflection exceeds the maximum deflection predicted by a nonlinear analysis by approximately one panel thickness. The failure strength of the reinforced panels was two and a half to seven times the buckling strength. This efficient shear-panel design concept exceeds the required ultimate strength requirement of 300 lbs/in by more than 100 percent.