Micromechanical Design Tool for Innovative Lightweight Composite Sandwich Structures

Existing Navy aircraft create significant design challenges because aerodynamics require complex shaped contours, while structural efficiency leads to lightweight, strong composite sandwiches for load bearing components. The structures employ high strength graphite/epoxy face sheets over honeycomb cores. Honeycomb cores, however, are difficult to form into doubly-curved shells and are prone to water ingress. In order to tailor their properties it is necessary to splice sections from different billets, adding costs. Honeycomb cores offer limited bond area and are prone to delaminations that are difficult to inspect and repair. An alternative core employs aerospace-grade polymer foam that is reinforced with pultruded unidirectional graphite/epoxy pins. Since the pins can be rapidly inserted via robotic fabrication it is possible to machine unreinforced core to complex contours and insert pins for tailored stiffness and strength. Reinforced foam cores provide more bond area and are less susceptible to water infiltration. In order for aircraft structures to capitalize on benefits of pin reinforced foam cores, it is necessary that accurate design tools are available. Recently Materials Research & Design (MR & D) and the University of Maryland (UM) have been assessing the structural behavior of pin reinforced cores under separately funded programs. MR & D, under Army funding provided by AEC, is pursuing a micromechanics-based design tool. UM, under a Navy grant, is using specialized experimental methods to measure deformations, and developing detailed finite element models of pin reinforced foam cores. This proposed STTR effort seeks to combine the two approaches to develop a reinforced foam core design tool that can be applied to existing and future aircraft structures. When developed and verified, the design tool will enable structural engineers to specify and fabricate complex shaped sandwich structures with tailored properties. This will lead to more structurally efficient, lower cost, and durable composite airframes.