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Structurally Optimized Beams from Digital Composite Materials

XIAO LIU, RONEN YUDZINSKY, ANDREW BURKE and CHRISTOPHER HANSEN

Abstract


Digital materials are an emerging materials concept, in which a material is fabricated from a (1) discrete set of components, with a (2) discrete set of relative positions and orientations, under (3) explicit control of these components and orientations [1]. Cheung and Gershenfeld created a reversibly assembled cuboctahedral composite structure that exhibited a 12.3 MPa elastic modulus from a 7.2 mg/cm3 density structure [3]. The structure takes advantage of the axial transmission of tensile and compressive forces, thereby simplifying the design to orient the fibers along the strut axis. Recent efforts at structural optimization via cellular materials has created extremely low density structures, typically composed of non-composite isotropic materials. Here, we investigate the design and fabrication of digital cellular composite materials in which the failure strength and buckling resistance of 1-D struts are optimized with respect to mass for NASA-oriented space applications. Struts containing microporous interiors are numerically investigated for specific mechanical performance. A genetic algorithm optimization of buckling performance, coupled with finite element studies, has been applied to investigate the axial buckling resistance of resulting beam designs. The resulting buckling strengths demonstrate that multi-cavity microporous beams have good buckling resistances, which are only marginally less per unit mass than for the unidirectional carbon fiber tubes. In order to manufacture these components, we have developed a circumferential rolling technique to produce carbon fiber composite beams composed of carbon fiber pre-preg sheets and sacrificial elements (e.g., poly(lactic acid) (PLA) filaments). Subsequent to the matrix cure, the sacrificial elements are depolymerized, leaving continuous channels that dramatically reduce the element mass and serve as the foundation for node joining processes. Via this approach, it is possible to produce composite beams with desired cross section (e.g., multi-cavity cellular cross sections). The fabricated struts are tested in compression and the performance compared to previous digital composite materials and predictions from numerical simulations.

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