Narrow Results

The objective of this study was to investigate 3D woodpile metamaterials for mitigating impact-induced vibrations by leveraging their local resonant and nonlinear contact characteristics. For experimental demonstrations, we designed, fabricated and tested prototypes of sandwich-structured woodpile metamaterials consisting of two plates, slender cylindrical rods and fasteners. We experimentally and numerically obtained impact responses of sandwich-structured woodpile metamaterials under various geometries and boundary conditions. We found that sandwich-structured woodpile metamaterials could efficiently manipulate and attenuate the impact vibrations due to their local bending motions and nonlinear contact between members. In addition, sandwich-structured woodpile metamaterials could have high damping as well as high stiffness by controlling the rod spacing. The findings from this study suggest sandwich-structured woodpile metamaterials can be used as structural components for impact-induced vibration mitigation.

As one of the most popular 3D printed metamaterials, the auxetic structure with its tunable Poisson’s ratio has attracted huge amount of attention recently. In this study, we designed an auxetic shape-memory metamaterial, which showed designable buckling responses by using the thermomechanically coupled in-plane instability. The influence of viscoelasticity on in-plane moduli and Poisson’s ratios of shape-memory auxetic metamaterial was experimentally investigated. Based on the generalized Maxwell model and finite-element method, the buckling behaviors and their main influence factors were studied. The analytical results and experimental ones showed a good agreement. Thermomechanical properties of the printed metamaterials govern the temperature and strain rate-dependent buckling, and a controllable transition from the negative to positive Poisson’s ratio in the metamaterials can be achieved. Based on the shape memory effect, the buckled state and the Poisson’s ratio of the metamaterials can be tuned by programmed thermomechanical processes. This study provides a simple and efficient way to generate morphing structures using the designable buckling effect.

The development of engineering technology puts forward more demanding requirements for the performance of materials, many of which cannot be met by the existing natural materials. In addition to exploring and refining new natural materials, it is possible to arrange designed microstructures according to periodicity or other rules to produce man-made materials that do not exist in nature itself, mostly due to the development of additive manufacturing (AM) technology. These artificial materials are known as metamaterials. In the past few years, the design and application of metamaterials have made great progress. This paper reviews the research of elastic metamaterials in recent years, which includes both traditional metamaterials with fixed structural properties and active metamaterials with tunable properties. Among many application scenarios, this paper mainly focuses on elastic metamaterials’ applications in vibration control, impact mitigation and wave attenuation. In addition, this paper also discusses the development of metamaterial inverse design or optimization design, with special attention to the application of topology optimization (TO) technology in this area.

In the past two decades, auxetic metamaterials have shown their potential in many fields such as energy absorption and vibration mitigation. Many mechanisms have been proposed to guide the design of their microstructures. More recently, structural optimization methods, especially topology optimization, have been employed for the design and optimization of metamaterials. In this paper, topology optimization was employed with the SIMP method and MMA optimizer for the design of auxetic metamaterials. The negative Poisson’s ratio was verified by numerical simulation. Bandgap study was also conducted on the optimized layout and it showed that the optimization also achieved auxetic metamaterials with two narrow locally resonant bandgaps in addition to broadening and lowering the bandgap of the initial configuration.

With the development of metamaterials, programmable and assembled auxetic structures have attracted extensive attention due to their unusual mechanical behaviors. In this study, we design a 3D printed metamaterial structure with significantly improved stress and programmable auxetic behavior by means of the cooperativity of viscoelastic and elastic materials. The effects of porosity, temperature, the shape of pore and Young’s modulus of the elastic material on the mechanical behavior of 3D printed metamaterial have been characterized using finite element method (FEM) analysis and experimental measurements. The constitutive relationships between stress, strain, porosity and the shape of pore have been formulated to explore the working principles of these parameters in the mechanical performances.

3D printing metamaterial structures have attracted extensive attentions, due to their multifunctional, programmable and tailorable mechanical behaviors. Currently, the buckling behaviors of irregular and non-uniform metamaterial structures have become a prominent challenge due to their unstable deformations. In this study, we designed a 3D printed metamaterial structure with tailorable buckling behaviors by means of viscoelastic materials and holey column structure. Effects of pore shapes, porosity, rotation angles, and temperature on the buckling modes and mechanical properties of metamaterial structures have been investigated using finite element analysis and experimental tests. Furthermore, the constitutive relationships among critical buckling stress, strain, pore shape, porosity and rotation angle have been formulated to explore the design principle of local instability in holey-column metamaterial structure towards tailorable buckling modes.

With the development of structural metamaterials and 3D printing technology, the polymorphic shape-memory metamaterials have attracted extensive attention. This study aims to design a structural 3D printed shape-memory metamaterial, of which the epigenetic bi-stability of shape-fixity and recovery behaviors have been achieved by means of the dual matching nominal moduli and geometrical size optimization. Epigenetic bi-stability and dual matching refer to the combination of bi-materials in a specific structure, in response to external stimuli to show adjustable bi-stability. Elastic thermoplastic polyurethane (TPU) and viscoelastic polylactic acid (PLA) both are thermally responsive shape-memory polymers (SMPs) and have been employed to fabricate the structural metamaterial, of which the nominal modulus is tailorable owing to the dual matching shape-memory effects (SMEs) of two SMP components. Furthermore, the effects of structural parameters, i.e., width of framework and width of internal support, on the nominal modulus have been investigated for the structural metamaterials, of which the shape-fixity and recovery ratios have been characterized using finite element method (FEM) analyses and experimental measurements. Finally, a constitutive relationship among structural parameters, dual matching SME and nominal modulus has been identified to explore the working principle of epigenetic bi-stability in shape-memory metamaterials. This study provides a design strategy for a shape-memory metamaterial with a post-switchable bi-stability, through dual matching SME and geometrical size optimization.

This paper explores the use of carbon fiber-reinforced elastomeric skins for advancing flexible morphing wing technologies, highlighting exceptional 1D morphing capabilities. The study focuses on synthesizing a specially engineered elastomer-based skin with a zero Poisson’s ratio, achieved through precise formulation and fabrication onto unidirectional carbon fiber. Poisson’s ratio of the 1D skin is confirmed to be zero via image analysis. Micro-CT tomography evaluates carbon fiber quality and voids under varied prestretch conditions, revealing proportional increases in void sizes. X-ray tomography also provides insights into void distribution and morphology during pre- and post-cyclic stretching. The innovative approach enables seamless 1D morphing, achieving an impressive 200% stretch with a slight increase in actuation force and hysteresis loss percentage. Inspired by continuum mechanics, the proposed “Exp-ln-ln” framework accurately models the loading–unloading stretching of the skin. An extensive parametric investigation assesses key parameters, advancing fiber-reinforced elastomeric materials for aerospace morphing skins.

This study investigates the compressive characteristics of rigidly folded interleaved origami tube metamaterials, following the “Flip-Flop” pattern, across principal directions. The geometry of the metamaterial’s fundamental unit is outlined, establishing theoretical models for compressive stresses in the $X$/$Y$-directions and mean compressive stress in the $Z$-direction, which show strong agreement with numerical simulations. These simulations assess the effects of thickness-to-length ratio and angular parameters on compressive performance. When compared to hexagonal honeycomb structures, the findings indicate that, with carefully selected structural parameters, origami tube metamaterials exhibit superior energy absorption efficiency and compressive energy absorption characteristics, surpassing the constraints of relative density.