VALIDATED DESIGN MODEL OF LIGHTWEIGHT CONTOURED SANDWICH STRUCTURE FOR AIRCRAFT COMPONENTS

Navy aircraft employ components that are contoured for aerodynamic performance and made of composite sandwich structures for high strength-to-weight ratio. Many of these components presently use honeycomb cores surrounded by thin high strength face sheets. While honeycomb is used extensively, it can be difficult to form to contoured shapes, must be spliced to reinforce high stress locations, provides limited bond area, and is prone to moisture absorption. Many of the limitations of standard honeycomb can be addressed with cores made from aerospace-grade foam reinforced with unidirectional graphite composite pins. These cores can be thermoformed to aerodynamic shapes, provide larger face sheet bond area, absorb less water, and can employ tailored pin distributions in high stress locations. Under separate Army funding MR&D had developed Technicore, a micromechanical design tool that allows a designer to specify core stiffnesses and strengths and define resulting foam and pin fabrication parameters. In the Phase I STTR, the team of Materials Research & Design (MR&D) and the University of Maryland (UMD) validated Technicore for flat cores by comparison with results provided by UMD including ASTM test data, detailed digital image correlation (DIC) measurements, and 3D finite element analysis. The goal of the Phase II STTR will be to extend and validate Technicore for contoured aircraft sandwich structure. The Phase II Base technical effort will consist of four tasks including: 1) extension of Technicore micromechanical theory to curved structures with variable pin orientations; 2) theoretical validation of Technicore by comparison to detailed 3D finite element models; 3) experimental validation of the design method by testing a contoured subcomponent, and; 4) transfer of the technology by software distribution and training for end users such as AEC, Boeing, Sikorsky, Bell, etc. The Phase I Base program will focus on structures with single curvature such as a cylindrical shell. The proposed Option program includes four additional tasks that will extend the theory and experiments to doubly curved structures like a spherical shell. The proposed Phase II will be coordinated with an ongoing program at Albany Engineered Composites (AEC) that is developing an X-cor tunnel cover for a CH47 helicopter.