Mechanical metamaterials
   HOME

TheInfoList



OR:

Mechanical metamaterials are artificial structures with mechanical properties defined by their structure rather than their composition. They can be seen as a counterpart to the rather well-known family of
optical metamaterials A photonic metamaterial (PM), also known as an optical metamaterial, is a type of electromagnetic metamaterial, that interacts with light, covering terahertz (Terahertz radiation, THz), infrared (IR) or visible wavelengths. The materials employ a ...
. They are often also termed ''elastodynamic metamaterials'' and include
acoustic metamaterials An acoustic metamaterial, sonic crystal, or phononic crystal, is a material designed to control, direct, and manipulate sound waves or phonons in gases, liquids, and solids (crystal lattices). Sound wave control is accomplished through manipulating ...
as a special case of vanishing shear. Their mechanical properties can be designed to have values which cannot be found in nature.


Examples of mechanical metamaterials


Acoustic / phononic metamaterials

Acoustic or phononic metamaterials can exhibit acoustic properties not found in nature, such as negative effective
bulk modulus The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume. Other moduli describe ...
, negative effective mass density, or double negativity. They find use in (mostly still purely scientific) applications like acoustic subwavelength imaging, superlensing, negative refraction or transformation acoustics.


Materials with negative Poisson's ratio (auxetics)

Poisson's ratio In materials science and solid mechanics, Poisson's ratio \nu ( nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Pois ...
defines how a material expands (or contracts) transversely when being compressed longitudinally. While most natural materials have a positive Poisson's ratio (coinciding with our intuitive idea that by compressing a material it must expand in the orthogonal direction), a family of extreme materials known as auxetic materials can exhibit Poisson's ratios below zero. Examples of these can be found in nature, or fabricated, and often consist of a low-volume microstructure that grants the extreme properties to the bulk material. Simple designs of composites possessing negative Poisson's ratio (inverted hexagonal periodicity cell) were published in 1985. In addition, certain origami folds such as the
Miura fold The is a method of folding a flat surface such as a sheet of paper into a smaller area. The fold is named for its inventor, Japanese astrophysicist Kōryō Miura. The crease patterns of the Miura fold form a tessellation of the surface by par ...
and, in general, zigzag-based folds are also known to exhibit negative Poisson's ratio.


Metamaterials with negative longitudinal and volumetric compressibility transitions

In a closed thermodynamic system in equilibrium, both the longitudinal and volumetric
compressibility In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a fl ...
are necessarily non-negative because of stability constraints. For this reason, when tensioned, ordinary materials expand along the direction of the applied force. It has been shown, however, that metamaterials can be designed to exhibit negative compressibility transitions, during which the material undergoes contraction when tensioned (or expansion when pressured). When subjected to isotropic stresses, these metamaterials also exhibit negative volumetric compressibility transitions. In this class of metamaterials, the negative response is along the direction of the applied force, which distinguishes these materials from those that exhibit negative transversal response (such as in the study of negative Poisson's ratio).


Pentamode metamaterials or meta-fluids

A pentamode metamaterial is an artificial three-dimensional structure which, despite being a solid, ideally behaves like a fluid. Thus, it has a finite
bulk Bulk can refer to: Industry * Bulk cargo * Bulk liquids * Bulk mail * Bulk material handling * Bulk pack, packaged bulk materials/products * Bulk purchasing * Baking * Bulk fermentation, the period after mixing when dough is left alone to ferm ...
but vanishing
shear modulus In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain: :G \ \stackrel ...
, or in other words it is hard to compress yet easy to deform. Speaking in a more mathematical way, pentamode metamaterials have an
elasticity tensor In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of th ...
with only one non-zero eigenvalue and five (penta) vanishing eigenvalues. Pentamode structures have been proposed theoretically by Graeme Milton and Andrej Cherkaev in 1995 but have not been fabricated until early 2012. According to theory, pentamode metamaterials can be used as the building blocks for materials with completely arbitrary elastic properties. Anisotropic versions of pentamode structures are a candidate for transformation elastodynamics and elastodynamic cloaking.


Cosserat and Micropolar Metamaterials

Very often Cauchy elasticity is sufficient to describe the effective behavior of mechanical metamaterials. When the unit cells of typical metamaterials are not centrosymmetric it has been shown that an effective description using chiral micropolar elasticity (or Cosserat ) was required. Micropolar elasticity combines the coupling of translational and rotational degrees of freedom in the static case and shows an equivalent behavior to the
optical activity Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...
.


Willis materials

In 2006 Milton, Briane and Willis showed that the correct invariant form of linear elastodynamics is the local set of equations originally proposed by Willis in the late 1970s and early 1980s, to describe the elastodynamics of inhomogeneous materials. This includes the apparently unusual (in elastic materials) coupling between stress, strain and velocity and also between momentum, strain and velocity. Invariance of Navier's equations can occur under the transformation theory, but would require materials with non-symmetric stress, hence the interest in Cosserat materials noted above. An elastostatic cloak with polar material with a distribution of body torque that breaks the stress symmetry was fabricated and successfully tested. The theory was given further foundations in the paper by Norris and Shuvalov. A mathematical theory of near cloaking for linear elasticity has been developed based on these papers.


Meta-tribomaterials

Meta-tribomaterials proposed in 2021 are a new class of multifunctional mechanical metamaterials with intrinsic sensing and energy harvesting functionalities. These material systems are composed of finely tailored and topologically different triboelectric microstructures. Meta-tribomaterials, a.k.a. self-aware composite mechanical metamaterials, can serve as nanogenerators and sensing media to directly collect information about its operating environment. They naturally inherit the enhanced mechanical properties offered by classical mechanical metamaterials. Under mechanical excitations, meta-tribomaterials generate electrical signals which can be used for active sensing and empowering sensors and embedded electronics.


Hyperelastic cloaking and invariance

Another mechanism to achieve non-symmetric stress is to employ pre-stressed hyperelastic materials and the theory of "small on large", i.e. elastic wave propagation through pre-stressed nonlinear media. Two papers written in the Proceedings of the Royal Society A in 2012 established this principal of so-called ''hyperelastic cloaking and invariance'' and have been employed in numerous ways since then in association with elastic wave cloaking and phononic media.


References

{{Reflist Metamaterials