Ultralight, ultrastiff mechanical metamaterials

Abstract

Abstract:The mechanical properties of ordinary materials degrade substantially with reduced density, due to the bending of their structural elements under applied load. We report a class of micro-architected materials that maintain a nearly constant stiffness per unit mass density, even at ultra-low density. This performance derives from a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. Production of these microlattices, with constituent materials ranging from polymers to metals and ceramics, is made possible by using projection microstereolithography, an additive micromanufacturing technique, combined with nanoscale coating and postprocessing. We found that these materials exhibit ultra-stiff properties across more than three orders of magnitude in density, regardless of the constituent material. One Sentence Summary:We report a class of micro-architected materials that change their stiffness linearly with reduced density.Main Text: Nature has found a way to achieve mechanically efficient materials by evolving cellular structures. Natural cellular materials, including honeycomb (1) (wood, cork) and foamlike structures, such as trabecular bone (2), plant parenchyma (3), and sponge (4), combine low weight with superior mechanical properties. For example, lightweight balsa has a stiffness-toweight ratio comparable to that of steel along the axial loading direction (5). Inspired by these naturally occurring cellular structures, manmade lightweight cellular materials fabricated from a wide array of solid constituents are desirable for a broad range of applications including structural components (6, 7), energy absorption (8, 9), heat exchange (10, 11), catalyst supports (12), filtration (13,14), and biomaterials (15,16). However, the degradation in mechanical properties can be drastic as density decreases (17,18). A number of examples among recently reported low-density materials include graphene elastomers (19), metallic micro-lattices (20), carbon nanotube foams (21), and silica aerogels (22,23). For instance, the Young's modulus of low-density silica aerogels (22, 23) decreases to 10 kPa (10 -5 % of bulk ) at a density of less than 10 mg/cm 3 (< 0.5% of bulk). This loss of mechanical performance is because most natural and engineered cellular solids with random porosity, particularly at relative densities less than 0.1%, exhibit a quadratic or stronger scaling relationship between Young's modulus and density as well as between strength and density. Namely, E/E s ~ (/ s ) n and  y  ys ~ (/ s ) n , where E is Young's modulus,  is density,  y is yield strength, and s denotes the respective bulk value of the solid constituent material property. The power n of the scaling relationship between relative material density and the relative mechanical property depends on the material's microarchitecture. Conventional cellular foam materials with stochastic porosity are known to...

Reexamining the mechanical property space of three-dimensional lattice architectures

“…Experimental and theoretical work on lattices have shown mixed results on the exact role of topology in governing strength and modulus scaling; a wide range of reported strength and stiffness power law scaling relationships exists, even for topologically identical systems, and no experimentally realized M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 3 lightweight lattice matches the performance predicted by the Gurtner-Durand bound [1,2,18,[21][22][23]37,38].…”

“…These materials are 3D assemblies of beams with micro-and nanoscale constituent dimensions, and it is the confluence of nanometer-sized dimensions and architecture that gives rise to their unique properties [1][2][3]15,[18][19][20][21][22][23][24][25][26]. The theoretical maximum Young's modulus ‫)ܧ(‬ and yield strength (ߪ ௬ ) of a lightweight porous material are set by the Voigt bound, which are functions of the relative density (ߩ̅ ) as ‫ܧ‬ = ‫ܧ‬ ௦ ߩ̅ and ߪ ௬ = ߪ ௬௦ ߩ̅ .…”

“…Incorporating three-dimensional architecture into materials design across multiple length scales has led to the creation of advanced materials with novel mechanical properties, such as ultralight weight [1][2][3], negative Poisson's ratios [4,5], and near infinite bulk-to-shear modulus ratios [6,7]. The versatility of current fabrication methods and processing techniques engenders a virtually unbounded potential design space by which new materials can be created [8][9][10][11][12][13][14][15][16][17].…”

High‐Entropy Alloy (HEA)‐Coated Nanolattice Structures and Their Mechanical Properties

“…[1][2][3][4][5][6][7][8][9] The strength of lattices is determined not only by the order and periodicity of its structure, but also by the constituent materials. [10,11] Recently, numerous engineering materials such as Al 2 O 3, [12][13][14] Ni-P alloy, [4,15] glassy carbon, [16] copper, [17] gold, [18] and metallic glass [19,20] have been employed as constituent materials to significantly enhance the mechanical properties of pristine polymer scaffolds with respect to its strength and stiffness. Nevertheless, exploring a kind of lightweight, low-cost, and easily fabricated material with promising mechanical features that are easily to be coupled with pristine lattice structure is still a challenge.…”

On the Use of Mechanical Metamaterials for Innovative Seismic Isolations Systems

“…These may include: exceptional strength-and stiffness-to-weight ratios; excellent strain recoverability; very soft and/or very stiff deformation modes; auxetic behavior; phononic bandgaps; sound control ability; negative effective mass density; negative effective stiffness; negative effective refraction index; superlens behavior; and/or localized confined waves, to name some examples (cf. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and references therein). The category of "extremal materials" has been introduced in [3] to define materials that simultaneously show very soft and very stiff deformation modes (unimode, bimode, trimode, quadramode and pentamode materials, depending on the number of soft modes).…”