Abstract
Ultralight sandwich panel structures which utilize light, stiff, and strong face sheets, and stiff, strong, compressible cellular cores have attracted significant research interest for use in aerospace and energy absorption applications. The cores of these sandwich structures are typically fabricated using high strength cellular materials, such as aluminum and titanium alloys, or polymer foams and honeycombs. However, for weight sensitive, ambient temperature applications, carbon fiber composites have emerged as a promising material due to its high specific strength and low density. Carbon fiber reinforced polymer (CFRP) composite cellular materials, when combined with structurally efficient sandwich panel designs, offer new opportunities for fabricating ultralight structures. This dissertation explores carbon fiber and carbon fiber hybrid sandwich panel design concepts, details the novel fabrication methods which have been developed for these structures, investigates the mechanical performance of the structures made using these approaches, and develops micromechanical models which predict the relationships between the mechanical properties of these structures and parent material properties, unit cell topology, and core density.
The dissertation develops four fabrication approaches for making carbon fiber and carbon fiber hybrid sandwich structures. These include a mechanical snap fitting and adhesive bonding method, a braided carbon fiber net method, a simplified linear braid approach, and a pultruded rod/linear tow technique. CFRP lattice structures made from carbon fiber laminates using a mechanical snap-fitting and adhesive bonding technique have been found to have high strength in through-thickness compression and in-plane shear. However, under compression or shear loading, these structures typically fail abruptly, exhibiting little ability to support load after initial strut failure. The trusses are also brittle, and absorb little energy during impact. The braided net fabrication approach utilizes a braided carbon fiber net within polymer and syntactic foams to form the core, woven carbon fiber face sheets, polymer and syntactic foam molds, and Kevlar stitching of core to the face sheets, followed by vacuum assisted resin transfer molding. This approach has facilitated the fabrication of empty lattice (truss and face sheet only) and hybrid core specimens ranging in density from 35 – 478 kg/m3, and the mechanical performance of these structures characterized. The cellular structures made using this approach have been shown to have high compressive and shear strengths, as well as excellent energy absorption capacity. The linear braid method represents a simplified fabrication approach which utilizes linear braids to form the trusses within a hybrid cellular material. This method has facilitated the manufacture of hybrid cellular materials with a truss volume fraction ranging from 1.5 – 17.5% of the core. The fraction of the truss contained in the nodes is found to lead to saturation in core strength at higher densities. The pultruded rod/linear tow method enabled the fabrication of hybrid octet lattice structures which have a cell size independent of face sheet separation, and near isotropic properties. The micromechanical models presented in this dissertation offer a basis for materials design and analysis for composite structures for a wide variety of uses and applications.
