Unveiling New Mechanical Couplings in 3D Lattices: Axial-Bending and the Role of Symmetry Breaking

Abstract: Mechanical couplings with symmetry breaking open up novel applications such as robotic metamaterials and directional mechanical signal guidance. However, most studies on 3D mechanical couplings have been limited to ad-hoc axial-twist designs due to a lack of comprehensive understanding of 3D non-centrosymmetry and chirality. Few theoretical methods exist to identify and quantify mechanical couplings in non-centrosymmetric and chiral lattices, typically relying on crystal physics (point group symmetry) and generalized constitutive equations. By extending symmetry breaking to mirror and inversion symmetries, we identify a broader range of mechanical couplings beyond axial-twist, such as axial-bending couplings. We develop a generalized 3D micropolar model of curved cubic lattices, encompassing both non-centrosymmetric achiral and chiral geometries, to quantify anisotropic physical properties and mechanical couplings as functions of curvature and handedness. Integrating point group symmetry operations with micropolar homogenized constitutive equations for curved cubic lattices, including mirror and inversion symmetry breaking, provides a clear design framework for identifying and quantifying anisotropic physical properties and mechanical couplings beyond axial-twist. This study uncovers a novel axial-bending coupling in non-centrosymmetric structures and highlights the weak correlation between chirality and both axial-bending and axial-twisting couplings. It also offers design guidelines for achieving multimodal couplings. The relationship between metamaterials' geometry and physical properties aligns with Neumann's principle. This work presents a robust framework for understanding mechanical couplings related to symmetry breaking and spatial anisotropy in metamaterial design, drawing an analogy to crystal physics and crystal chemistry.