Mechanical Performance of Tubular Microtruss Materials Reinforced With Nanocrystalline Sleeves
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Mechanical Performance of Tubular Microtruss Materials Reinforced With Nanocrystalline Sleeves Eral Bele, Mishaal Azhar, and Glenn D. Hibbard University of Toronto, Department of Materials Science and Engineering 184 College Street, Toronto, Ontario, Canada, M5S3E4 ABSTRACT Microtruss cellular materials are assemblies of struts with characteristic features in the μm to mm scale, arranged in a periodic, three-dimensional architecture. Compared to conventional cellular architectures (e.g. stochastic foams and honeycombs), they can possess improved structural efficiency, because externally applied loads are resolved axially along the constituent struts. We have recently fabricated composite microtruss materials by electrodepositing reinforcing nanocrystalline sleeves on tubular polymeric scaffolds. These materials can offer enhanced structural performance by exploiting advantageous properties along three length scales: the inherent strength of the electrodeposited material (grain size reduction to the nm scale), its location away from the bending axis of the struts (cross-sectional efficiency in the μm scale), and the spatial arrangement of the struts (architectural efficiency in the mm scale). This study uses finite element analysis and experimental methods to characterize the mechanical properties of these composite materials. INTRODUCTION Materials with low densities and high strength to density ratios are desired for many weight-sensitive applications [1]. This desirable region in material property space can be occupied by cellular metals, which are composites of air and metal [1]. In this class of materials, microtruss cellular metals are particularly attractive due to the high structural efficiency provided by the architectural design in the macroscopic scale [1, 2]. New regions of material property space can be accessed by combining this level of architectural design with microstructural design at the nm-scale. Using electrodeposition, new types of structural nanomaterials have been developed: they incorporate the low density benefits of cellular architectures with the large strength increase associated with grain size reduction to below 50 nm [3]. Examples include reinforcement of conventional metallic and polymeric foams [4, 5] and microtruss assemblies [6, 7]. Further improvements in structural properties can be achieved by considering the crosssectional shape of the microtruss strut members. Architectures with exceptionally high strengths at low densities have been achieved by aligning hollow cylindrical struts in a three dimensional architecture [8]. In this case, the buckling strength was improved by increasing the crosssectional efficiency (i.e. second moment of area) of the struts. To exploit the inherent strength of the nanocrystalline metal and the advantageous cross-sectional efficiency of hollow cylinders, we have deposited the nanocrystalline material on a precursor microtruss core constructed from polymeric tubular strut members. Finite element results and analytical models show that these structur
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