Extreme Specific Stiffness Through Interactive Cellular Networks in Bi-Level Micro-Topology Architected Metamaterials

Architected lattice materials, realized through artificial micro-structuring, have drawn tremendous attention lately due to their enhanced mechanical performances in multifunctional applications. However, the research area on the design of artificial microstructures for the modulation of mechanical properties is increasingly becoming saturated due to extensive investigations considering different possibilities of lattice geometry and beam-like network design. Thus, there exists a strong rationale for innovative design at a more elementary level. It can enhance and grow the microstructural space laterally for exploiting the potential of geometries and patterns in multiple length scales, and the mutual interactions thereof. A bi-level design is proposed, where besides having the architected cellular networks at an upper scale, the constituting beam-like members at a lower scale are further topology-engineered for most optimum material utilization. The coupled interaction of beam-level and lattice-level architectures can enhance the specific elastic properties to an extreme extent (up to ≈25 and 20 times, depending on normal and shear modes, respectively), leading to ultra-lightweight multifunctional materials for critical applications under static and dynamic environments.     

Active Metamaterials with Negative Static Electric Susceptibility

Although well-established textbook arguments suggest that static electric susceptibility χ⁽⁰⁾ must be positive in “all bodies,” it has been pointed out that materials that are not in thermodynamic equilibrium are not necessarily subject to this restriction. Media with inverted populations of atomic and molecular energy levels have been predicted theoretically to exhibit a χ⁽⁰⁾ < 0 state, however the systems envisioned require reduced temperature, reduced pressure, and an external pump laser to maintain the population inversion. Further, the existence of χ⁽⁰⁾ < 0 has never been confirmed experimentally. Here, a completely different approach is taken to the question of χ⁽⁰⁾ < 0 and a design concept to achieve “true” χ⁽⁰⁾ < 0 is proposed based on active metamaterials with internal power sources. Two active metamaterial structures are fabricated that, despite still having their power sources implemented externally for reasons of practical convenience, provide evidence in support of the general concept. Effective values are readily achieved at room temperature and pressure and are tunable throughout the range of stability −1 < χ⁽⁰⁾ < 0, resulting in experimentally-determined magnitudes that are over one thousand times greater than those predicted previously. Since χ⁽⁰⁾ < 0 is the missing electric analog of diamagnetism, this work opens the door to new technological capabilities such as stable electrostatic levitation.     

Folding at the Microscale: Enabling Multifunctional 3D Origami‐Architected Metamaterials

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura‐Ori tubes. The origami‐architected metamaterials, achieved by means of microfabrication, display remarkable mechanical properties: stiffness and Poisson’s ratio tunable anisotropy, large degree of shape recoverability, multistability, and even reversible auxeticity whereby the metamaterial switches Poisson’s ratio sign during deformation. The findings here reported underscore the scalable and multifunctional nature of origami designs, and pave the way toward harnessing the power of origami engineering at small scales.     

Microarchitected Stretching‐Dominated Mechanical Metamaterials with Minimal Surface Topologies

Historically, the creation of lightweight, yet mechanically robust, materials have been the most sought‐after engineering pursuit. For that purpose, research efforts are dedicated to finding pathways to emulate and mimic the resilience offered by natural biological systems (i.e., bone and wood). These natural systems evolved over time to provide the most attainable structural efficiency through their architectural characteristics that can span over multiple length scales. Nature‐inspired man‐made cellular metamaterials have effective properties that depend largely on their topology rather than composition and are hence remarkable candidates for a wide range of application. Despite their geometrical complexity, the fabrication of such metamaterials is made possible by the emergence of advanced fabrication techniques that permit the fabrication of complex architectures down to the nanometer scale. In this work, we report the fabrication and mechanical testing of nature‐inspired, mathematically created, micro‐architected, cellular metamaterials with topologies based on triply periodic minimal surfaces (TPMS) with cubic symmetries fabricated through direct laser writing two‐photon lithography. These TPMS‐based microlattices are sheet/shell‐ and strut‐based metamaterials with high geometrical complexity. Interestingly, results show that TPMS sheet‐based microlattices follow a stretching‐dominated mode of deformation, and further illustrate their mechanical superiority over the traditional and well‐known strut‐based microlattices and microlattice composites. The TPMS sheet‐based polymeric microlattices exhibited mechanical properties superior to other micrloattices comprising metal‐ and ceramic‐coated polymeric substrates and, interestingly, are less affected by the change in density, which opens the door for fabricating ultralightweight materials without much sacrificing mechanical properties.

     

Nonlinear deformation behaviour of auxetic cellular materials with re‐entrant lattice structure

Recent frontiers in material development are represented by a class of so‐called auxetic metamaterials that, thanks to their structure rather than composition, are characterized by a negative Poisson's ratio. In the present paper a two‐dimensional auxetic plate, made by structural straight elements forming a lattice periodic structure with re‐entrant cells, is considered. A thorough discussion on the linear and geometrically nonlinear deformability of the auxetic plate is presented. The key geometric parameters governing the deformability of the plate are identified, and some analytical expressions for calculating the Poisson's ratio, as a function of the applied strain, are given. Numerical (finite element) analyses and experimental tests on 3D printed specimens are carried out to verify the theoretical findings. For the latter ones, full field strain maps are obtained by means of a suitable interpolation of the sampled displacement field measured by digital image techniques.     

A CDM Approach of Ductile Damage with Plastic Compressibility

The paper proposes a modified formalism of continuum damage mechanics in order to describe plastic compressibility in the context of ductile damage. The model uses two damage state variables, one of them playing role of porosity in micromechanics based approaches like Gurson’s model. Various versions of the model are determined and compared with Gurson’s model, in terms of the constitutive responses for various loading conditions, as well as for simple structural examples like a free and a clamped plate under plane strain, and an axisymmetric notched bar under tension. The classical CDM is also applied and some advantages of the proposed approach are underlined.     

A Decomposition Method for the Analysis of Viscoelastic Structural Dynamics with Time‐Dependent Poisson's Ratio

Abstract: This paper presents a decomposition method for the dynamic analysis of elastic–viscoelastic composite (EVC) structures with time‐dependent Poisson's ratio. The analysis splits the viscoelasticity matrix with time‐dependent Poisson's ratio into two matrices in a simple form in which the time‐dependent Poisson's ratio does not appear. The decomposition simplifies the process of dynamical analysis for EVC structures with time‐dependent Poisson's ratio. The approach also makes it possible to apply existing analysis methods for constant Poisson's ratio structures directly to structures with time‐dependent Poisson's ratio. Based on the numerical results of three case studies, it is found that the time‐dependent Poisson's ratio has little influence on the structure's natural frequencies and damping properties. Therefore, it could be concluded that the effect of time‐dependency in Poisson's ratio may be ignored in the EVC structural dynamic analysis without introducing notable errors.     

Multifunctional Cellular Materials Based on 2D Nanomaterials: Prospects and Challenges

Recent advances in emerging 2D nanomaterial‐based cellular materials (2D‐CMs) open up new opportunities for the development of next generation cellular solids with exceptional properties. Herein, an overview of the current research status of 2D‐CMs is provided and their future opportunities are highlighted. First, the unique features of 2D nanomaterials are introduced to illustrate why these nanoscale building blocks are promising for the development of novel cellular materials and what the new features of 2D nanoscale building blocks can offer when compared to their 0D and 1D counterparts. An in‐depth discussion on the structure–property relationships of 2D‐CMs is then provided, and the remarkable functions that can be achieved by engineering their cellular architecture are highlighted. Additionally, the use of 2D‐CMs to tackle key challenges in different practical applications is demonstrated. In conclusion, a personal perspective on the challenges and future research directions of 2D‐CMs is given.