The structural symmetry of a material can be manifested at a multitude of length scales such as spatial arrangement of atoms in a crystal structure, preferred orientation of grains in a polycrystalline material, alignment of reinforcing particles/fibers in composites or the micro-architecture of members in cellular solids. This paper proofs, in a simple yet rigorous manner, that six axes of fivefold structural symmetry is necessary and sufficient for isotropy of the elastic moduli tensor in the three-dimensional (3D) context.

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