A three-dimensionally tunable mechanical metamaterial enabled by a coupled concave–chiral architecture

Abstract This paper introduces a three-dimensionally tunable mechanical metamaterial with a coupled concave–chiral architecture, addressing the limited multidirectional performance of existing concave honeycombs. Current designs and evaluations are largely two-dimensional, and limited initial stiffness, load-bearing capacity, and post-buckling energy absorption restrict their engineering use. Based on the original concave hexahedral unit cell, a concave–chiral re-entrant auxetic lattice (CCRAL) is designed. Under lateral and longitudinal compression, this structure exhibits a stable negative Poisson's ratio (NPR) in all three orthogonal directions. Under transverse loading, the anisotropic mechanical response arises from the synergy between concavity and chiral torsion of the internal ligaments. Under longitudinal loading, chiral-driven torsional folding governs transverse expansion. Stiffness and load-bearing capacity are tunable by changing the concavity angle, link length, and cell width. Experiments and numerical analyses validate the finite-element predictions. Using the validated model, the effects of structural parameters on initial stiffness, load-bearing capacity, and specific energy absorption (SEA) are evaluated. The results show that parameter variations modulate the mechanical response, enabling tunable performance. These findings establish a directly fabricable 3D concave. chiral lattice platform with multidirectional auxetic response and mechanical programmability, and they offer design guidance for lightweight protective and load-bearing components in civil and structural applications.