Titanium-Silicon Carbide Composite Lattice Structures

Sandwich panel structures with stiff, strong face sheets and lightweight cellular cores are widely used for weight sensitive, bending dominated loading applications. The flexural stiffness and strength of a sandwich panel is determined by the stiffness, strength, thickness, and separation of the face sheets, and by the compressive and shear stiffness and strength of the cellular core. Panel performance can be therefore optimized using cores with high specific stiffness and strength. The specific stiffness and strength of all cellular materials depends upon the specific elastic modulus and strength of the material used to make the structure. The stiffest and strongest cores for ambient temperature applications utilize carbon fiber reinforced polymer (CFRP) honeycombs and lattice structures. Few options exist for lightweight sandwich panels intended for high temperature uses. High temperature alloys such as Ti-6Al-4V can be applied to SiC monofilaments to create very high specific modulus and strength fibers. These are interesting candidates for the cores of elevated temperature sandwich structures such as the skins of hypersonic vehicles. This dissertation explores the potential of sandwich panel concepts that utilize millimeter scale titanium matrix composite (TMC) lattice structures.

A method has been developed for fabricating millimeter cell size cellular lattice structures with the square or diamond collinear truss topologies from 240 f.!m diameter Ti-6Al-4V coated SiC monofilaments (TMC monofilaments). Lattices with relative densities in the range 1 0% to 20% were manufactured and tested in compression and shear. Given .the very high compressive strength of the TMC monofilaments, the compressive strengths of both the square and diamond lattices were dominated by elastic buckling of the constituent struts. However, under shear loading, some of the constituent struts of the lattices are subjected to tensile stresses and failure is then set by tensile failure of the TMC monofilaments. Analytical expressions are derived for the elastic moduli and strength of the square and diamond TMC lattices and the predictions compared with measurements over the range of relative densities investigated in this study. Excellent agreement between the measurements and predictions is observed. In terms of specific shear strength, the TMC lattices outperform all other cellular materials investigated to-date including CFRP honeycombs while their compressive properties are comparable to CFRP honeycombs. However, the TMC lattices have a brittle response with catastrophic failure at their peak load. Thus, the TMC lattices appear promising candidate as cores in sandwich structures for elevated temperature and multifunctional applications, provided their limited ductility is not a significant constraint.

Digitization of this thesis was made possible by a generous grant from the Jefferson Trust, 2015.

Thesis originally deposited on 2015-09-28 in version 1.28 of Libra. This thesis was migrated to Libra2 on 2017-03-23 16:38:01.