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Additively manufactured auxetics (structures exhibiting a negative Poisson’s ratio) offer a unique combination of enhanced mechanical strength and energy absorption. These properties can be further improved through strategic material placement and architectural design. This study investigates the feasibility of fabricating bi-material rotating-square auxetic structures composed of flexible and rigid constituents in their squares and hinges. Rotating-square auxetic structures are manufactured via material extrusion using rigid polylactic acid (PLA) and flexible thermoplastic polyurethane (TPU) to explore the effects of material distribution on mechanical performance and failure characteristics at the macro (i.e., component) and meso (i.e., cell) scales. Baseline tests are conducted to quantify single- and bi-material interfacial strength and failure modes under normal, shear, and combined loading conditions. Upon validation of interface integrity, single- and bi-material auxetic structures are fabricated and tested in uniaxial compression. Relative to the TPU single-material structure, the PLA square-TPU hinge structure provides a 33% increase in structural stiffness, increases energy absorption, delays the global densification strain by 10%, yields a structural Poisson’s ratio at least 0.3 lower than its single-material counterpart through global axial strains of 20%, and demonstrates partial shape recovery. Multiscale experimental analyses supplemented by a kinematic model reveal the rotation-dependent stiffening mechanisms of these structures, highlighting the benefits of flexible hinge materials. Bi-material structures with flexible hinges are shown to have bilinear trends in structural stiffness and energy absorption, not intrinsic to their single-material counterparts. These findings highlight the potential of bi-material design strategies in advancing the functionality and tunability of auxetic structures for the next generation of mechanical metamaterials.
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This content will become publicly available on March 1, 2026
Multiscale experimental characterization of nonlinear mechanics and auxeticity in mechanical metamaterials with rotating squares
Auxetic (negative Poisson’s ratio) structures made from rotating squares have attracted considerable attention due to their tunable shape control, strength, and strain energy absorption capacity. The present study aims to explore the interrelations between mesoscale kinematics and the macroscopic mechanical behavior of additively manufactured rotating-square auxetics under compressive loads. Specifically, correlations between the rotational degree of freedom of the squares, mechanical deformation of the cell hinges, and the macroscopic nonlinear mechanical and Poisson’s behaviors are investigated using experimental measurements supplemented by mathematical models. Structures with variable cell hinge thicknesses are fabricated by stereolithography additive manufacturing technique and then subjected to compressive loads applied at quasi-static and dynamic conditions with several orders of magnitude difference in strain rate. Multiscale mechanical deformation of the structure in each case is analyzed using digital image correlation (DIC). Experimental characterizations indicate strongly nonlinear and rate-sensitive auxetic behaviors in the examined structures. The role of cell hinge thickness is discussed in terms of the mechanical constraint that these components impose on the rotational degree of freedom of the solid squares in the structure, concurrently causing a nonlinear strain hardening behavior.
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- Award ID(s):
- 2035660
- PAR ID:
- 10613836
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Composite Structures
- Volume:
- 357
- ISSN:
- 0263-8223
- Page Range / eLocation ID:
- 118931
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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