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Cellular materials' two important properties-structure and mechanism-can be selec tively used fo r materials design; in particular, they are used to determine the modulus and yield strain. The objective o f this study is to gain a better understanding o f these two properties and to explore the synthesis o f compliant cellular materials (CCMs) with com pliant porous structures (CPSs) generated from modified hexagonal honeycombs. An in-plane constitutive CCM model with CPSs o f elliptical holes is constructed using the strain energy method, which uses the deformation o f hinges around holes and the rotation o f links. A finite element (FE) based simulation is conducted to validate the analytical model. The moduli and yield strains o f the CCMs with an aluminum alloy are about 4.42 GPa and 0.57% in one direction and about 2.14 MPa and 20.9% in the other direc tion. CCMs have extremely high positive and negative Poisson's ratios (NPRs) (fA, ~ ±40) due to the large rotation o f the link member in the transverse direction caused by an input displacement in the longitudinal direction. A parametric study o f CCMs with varying flexure hinge geometries using different porous shapes shows that the hinge shape can control the yield strength and strain but does not affect Poisson's ratio which is mainly influenced by rotation o f the link members. The synthesized CPSs can also be used to design a new CCM with a Poisson's ratio o f zero using a puzzle-piece CPS assembly. This paper demonstrates that compliant mesostructures can be used fo r next generation materials design in tailoring mechanical properties such as moduli, strength, strain, and Poisson's ratios.

Adding programmable function to elastic metamaterials makes them versatile and intelligent. The objective of this study is to design and demonstrate thermomechanically tunable metamaterials with a compliant porous structure (CPS) and to analyze their thermomechanical behaviors. CPS, the unit cell of the metamaterial, is composed of rectangular holes, slits, and bimaterial hinges. By decomposing kinematic rotation of a linked arm and elastic deformation of a bimaterial hinge, a thermomechanical constitutive model of CPS is constructed, and the constitutive model is extended to a threedimensional (3D) polyhedron structure for securing isotropic thermal properties. Temperature-dependent properties of base materials are implemented to the analytical model. The analytical model is verified with finite element (FE) based numerical simulations. A controllable range of temperature and strain is identified that is associated with a thermal deformation of the bimaterial hinge and contact on the slit surfaces of CPS. We also investigate the effect of geometry of CPS on the thermal expansion and effective stiffness of the metamaterial. The metamaterial with CPS has multiple transformation modes in response to temperature while keeping the same mechanical properties at room temperature, such as effective moduli and Poisson's ratios. This work will pave the road toward the design of programmable metamaterials with both mechanically and thermally tunable capability, providing unique thermomechanical properties with a programmable function.

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