Trending News
Home / Technology / Manufacturing / Mechanical Metamaterials enable new materials for Aerospace, energy storage, biomedical, acoustics, photonics, and thermal management

Mechanical Metamaterials enable new materials for Aerospace, energy storage, biomedical, acoustics, photonics, and thermal management

For centuries, the development of materials has solely relied on the modifications of its composition to alter mechanical properties. Despite being effective, it usually takes more than a decade for a newly discovered material to be in the market. Researchers have found new way to develop new materials via Mechanical metamaterials. They are artificial structures with mechanical properties defined by their structure rather than their composition. Their mechanical properties can be designed to have values which cannot be found in nature

 

They can be seen as a subset to the rather well-known family of  metamaterials, which are artificially structured materials designed to control and manipulate physical phenomena such as light and other electromagnetic waves, sound waves and seismic waves in unconventional ways, resulting in exotic behavior that’s not found in nature.  They are often also termed elastodynamic metamaterials and include acoustic metamaterials as a special case of vanishing shear.

 

On the other hand, numerous materials found in nature often exhibit intriguing properties unachievable with conventional materials. These natural materials, particularly cellular materials, have evolved over the course of several million years such that they developed an optimized architecture which could span over multiple hierarchies across different length scales. For instance, the highly complex porous architecture of a bone core consisting of intricately-shaped ligaments and density gradients allows it to achieve a much higher structural efficiency compared to most of the synthetic cellular materials developed by humans, which possess significantly less sophisticated architectures and
are far from ideal.

 

Although the concept of incorporating architecture into materials may not seem new, the recent developments in advanced manufacturing technologies such as additive manufacturing (AM), also known as 3D printing, has enabled the fabrication of cellular materials with complex architectures across several length scales, with feature sizes down to the nanometer scale for a wide range of materials.

 

This brings about another benefit to the mechanical properties of materials, typically known as the “size-effect”, exhibited in certain nanomaterials where decreasing the size of a material down to the nanoscale allows them to exhibit unusual properties, such as increase in strength compared to its bulk counterpart, also known as the “smaller is stronger” trend in materials. Altering the topology of mechanical metamaterials such that it exhibits unique and tailorable properties (e.g., tunable stiffness, negative Poisson’s ratio, negative compressibility, vanishing shear modulus) arising from rationally engineered deformation mechanisms is possible as well.

 

Along with the improvement in computational methods developed over the years, mechanical metamaterials, which utilize the synergy between topology, material composition and in some cases, “size-effect” in nanomaterials, have the potential to produce novel materials
with unprecedented properties that approaches the theoretical limits of materials and could be optimized for specific industrial applications across multiple disciplines.

 

In the past decade, mechanical metamaterials have garnered increasing attention owing to its novel design principles which combine the concept of hierarchical architecture with material size effects at micro/nanoscale. This strategy is demonstrated to exhibit superior mechanical performance that allows us to colonize unexplored regions in the material property space, including ultrahigh strength‐to‐density ratios, extraordinary resilience, and energy absorption capabilities with brittle constituents.

 

In the recent years, metamaterials with unprecedented mechanical behaviors such as negative Poisson’s ratio, twisting under uniaxial forces, and negative thermal expansion are also realized. This paves a new pathway for a wide variety of multifunctional applications, for example, in energy storage, biomedical, acoustics, photonics, and thermal management. They are predicted to be able to protect the building from earthquakes by bending seismic waves around it, Similarly, tsunami waves could be bent around towns, and sound waves could be bent around a room to make it soundproof.

 

Mechanical metamaterials

The mechanical response of natural materials can generally be investigated with respect to four elastic constants, i.e. the Young’s modulus E for strength, the bulk modulus B and shear modulus G for rigidity, the deformation resistance K for stiffness, and the Poisson’s ratio v for deformability between strains.

 

The moduli of natural materials generally denote the mechanical properties while the stiffness is about the relationship between force (load) and displacement (deformation). For the recoverable deformations in the elastic regime, the Hooke’s law is used to describe the linearly continuous relationship between stress and strain under the small deformation assumptions, which treats stress as force per area and strain as displacement per length. Due to fact that the deformation resistance tends to push the deformed body back to its original configuration, natural materials have positive material properties such as positive stiffness that displacement is in the same direction as the applied force, and positive Poisson’s ratio that the elongation (or shortening) in the loading direction results in the shortening (or elongation) in the transverse direction.

 

Recent studies have demonstrated that certain natural materials (e.g. 2D materials graphene) can exhibit negative Poisson’s ratio. In particular, negative Poisson’s ratio was obtained due to the normal-auxeticity mechanical phase transition of the graphene films subjected to the uniaxial tension

 

A. Metamaterial types based on Structural characteristics

Since their promising mechanical response is primarily due to the microstructures, many researchers categorises mechanical metamaterials with respect to the structural characteristics of the microstructures including the lattice metamaterials, cellular metamaterials, chiral metamaterials, and origami metamaterials.

 

1. Lattice or cellular metamaterials

Analogous to the lattice structure of a crystal, lattice materials typically refer to cellular materials with periodic arrangements of unit cells in two or three dimensions. Cellular solids are widely found in nature: cork, wood, coral, sponge, and honeycomb.

Among different material architectures, lattice structures can achieve the highest structure efficiency per unit weight; therefore, they are broadly used in weight-critical applications like aerospace engineering, automobiles, armors, and rotor blades. Recently, the research interest in lattice metamaterials has expanded from purely mechanical to general physical, chemical, and biological properties.

 

Lattice metamaterials are commonly generated by assembling the unit cells consisted of lattice elements and therefore, the geometry and assembly of the unit cells are crucial.  Account for the two major factors of unit cells, the mechanical response of micro/nanoscale lattice metamaterials is affected by the cellular architectures and their density. Recently, computer-aided techniques have been applied to design and optimise the lattice structures of the metamaterials, predict the mechanical response or develop for novel applications.

 

Currently, 3D printing offers precise control of the lattice geometry from the sub-μm to cm scale technology and it is evolving more rapidly than ever. Great advances have been made to improve print speed, increase print precision, and expand printable structures and materials.  Aided by these cutting-edge printing techniques, the study of lattice metamaterials has achieved fruitful results. For instance, ultralight, ultra-stiff, and ultra-strong lattices have been successfully fabricated.

 

2.Chiral metamaterials

Chiral metamaterials, designed by the left- or right-handed substructures such as chiral hexagons, are characterised as non-superimposable mirrored configurations. Comparing with the symmetric metastructures, chiral metamaterials are generally found with regular polygons and chiral ligaments. As a consequence, the chirality of the metamaterials is resulted in the chiral or anti-chiral connections between two-dimensional unit cells. The unique mechanical behaviour of the chiral metamaterials is reported as negative stiffness or negative Poisson’s ratio. Functionally materials are used to fabricate chiral metamaterials to obtain advanced, controllable mechanical performance, such as composites materials  or shape memory polymers. Numerical approaches have been used in chiral metamaterials for response-oriented inverse design  and optimisation.

 

3. Origami metamaterials

Origami metamaterials are reported as the two-dimensional structures generated by emerging elegant patterns and shapes through folding planar films. Origami metamaterials are inspired by the art of folding paper originated from Japan. Architecting regular sheets into well-designed decorative shapes, origami has been applied to obtain ultra-stiff, ultra-light engineered structures, or extraordinary mechanical characteristics such as negative Poisson’s ratio.

 

Three types of origami strategies are found in the existing studies, including the Miura-ori pattern, non-periodic Ron Resch pattern, and square twist pattern. Origami has been used as basic blocks (e.g. cellular origami) to form metastructures with advanced functionalities such as enhanced flexibility, deformability and compactness, etc.  Combining with the dynamically folding patterns, origami leads to particular metamaterials such as bio-origami hydrogel scaffolds or certain buckled surfaces. As a special type of origami, Kirigami mainly focuses on cutting regular sheets to form two-dimensional structures.

 

4. Highly porous materials

Highly porous materials, typically made of metals, ceramics or polymers, form an important subcategory of functional materials due to their promising mechanical properties. For example, highly porous metals have been reported with well flexibility (up to 70% axial strain) and low weight under nearly constant stress, negative Poisson’s ratio. Since highly porous materials are assembled by cellular microstructures, the morphology and arrangement of the cellular units (e.g. orientation, size or shape) dominate the mechanical performance of functional materials. Highly porous materials have been used in different industrial applications due to their high internal surface and thermal connectivity, such as heat dissipation, thermal insulation, packaging, or comfortability design. Computer-aided techniques have been used to model, topologically optimise  and predict  highly porous materials.

 

Mechanical Metamaterial types based on Function

Acoustic / phononic metamaterials

More recently, the metamaterial concept has been extended to acoustic waves in a variety of scenarios of interest such as acoustic clocking, super-lensing and sound focusing and confinement. Acoustic metamaterial is defined as an artificial composite material displaying novel acoustic properties unavailable in naturally occurring materials and radically different from those of any of the constitutive components. The novel acoustic properties derive from closely spaced constituent elements with specifically chosen geometrical and mechanical characteristics.

 

Acoustic or phononic metamaterials can exhibit acoustic properties not found in nature, such as negative effective bulk modulus, 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.

 

Acoustic waves are a type of longitudinal waves that propagate by means of adiabatic compression and decompression. Longitudinal waves are waves that have the same direction of vibration as their direction of travel. Important quantities for describing acoustic waves are sound pressure, particle velocity, particle displacement and sound intensity. Acoustic waves travel with the speed of sound which depends on the medium they’re passing through. According to the oscillation frequency, acoustic waves have been classified to different fields that cover the audio, ultrasonic and infrasonic frequency range, or seismic waves at much larger scale which are waves of energy travelling through the Earth’s layer.

 

The material properties of interest for acoustic metamaterials are the effective mass density ρ and the effective bulk modulus κ, which is analogous to a spring’s stiffness. Of particular interest for acoustical applications is the ability to manipulate an acoustic wave’s speed. Because acoustic metamaterials have a broad range of effective properties, they can produce propagating waves with extremely high, zero, or even negative speeds, not to mention the purely imaginary values that correspond to nonpropagating evanescent waves.

 

Materials with either negative mass density and positive bulk modulus or positive mass density and negative bulk modulus are called single negative. Single-negative materials cannot support propagating waves, so any acoustic wave in those materials will exponentially decay. That makes them superior sound absorbers.

 

 

Materials with negative Poisson’s ratio (auxetics)

Poisson’s ratio 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. Poisson’s ratio which is approximately 0.5 for rubber and 0.3 for glass and steel.

 

Negative Poisson’s Ratio (NPR) materials, also known as auxetic materials, have attracted attention due to their unique behavior. When materials are compressed (stretched) along a particular axis they are most commonly observed to expand (contract) in directions orthogonal to the applied load. The property that characterizes this behavior is the Poisson’s ratio which is defined as the ratio between the negative transverse and longitudinal strains.

 

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 and, in general, zigzag-based folds are also known to exhibit negative Poisson’s ratio.

 

This unusual behavior results in a unique feature that the material can concentrate itself under the compressive load to better resist the load. Furthermore, NPR materials also become stiffer and stronger when the amplitude of the load increases. Studies and experiments have demonstrated that auxetic materials can improve mechanical properties, including shear resistance, indentation resistance and fracture toughness, compared to conventional materials from which they are made. These auxetic materials also offer very good sound and vibration absorption and could have many potential applications to aerospace and defense areas.

 

A significant challenge in the fabrication of materials with auxetic properties is that it usually involves embedding structures with intricate geometries within a host matrix. As such, the manufacturing process has been a bottleneck in the practical development towards applications. K. Bertoldi, and others from Harvard wrote, “However, recently we showed that instability induced pattern switches in porous elastomeric structures characterized by an initial simple microstructures may lead to auxetic behavior. Furthermore, we showed that the hole shape provides a convenient parameter to control the compaction (quantified as change of structure planar area divided by original area) and negative Poisson’s ratio of the periodic structures.

 

Our results demonstrate that by simply changing the shape of the holes the response of porous structure can be easily tuned and soft structures with optimal compaction can be designed. Surprisingly, we show that circular holes do not lead to optimal response and that the compaction of the system can be significantly improved through a careful design of the pore shape. Furthermore, the insights gained by performing a numerical parametric exploration serve as an important design guideline in fabricating practical materials towards applications.”

 

Metamaterials with negative longitudinal and volume compressibility transitions

In a closed thermodynamic system in equilibrium, both the longitudinal and volumetric compressibility 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 but vanishing shear modulus, 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 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 [22] 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.

 

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 required materials with non-symmetric stress, hence the interest in Cosserat materials noted above. The theory was given further foundations in the paper by Norris and Shuvalov.

 

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.

 

 

University of California, Irvine Team creates new ultralightweight, crush-resistant tensegrity metamaterials, in March 2021

In a study published this week in Advanced Materials, engineers at the University of California, Irvine and the Georgia Institute of Technology describe the creation of a new class of mechanical metamaterials that delocalize deformations to prevent failure. They did so by turning to tensegrity, a century-old design principle in which isolated rigid bars are integrated into a flexible mesh of tethers to produce very lightweight, self-tensioning truss structures.

 

Starting with 950 nanometer-diameter members, the team used a sophisticated direct laser writing technique to generate elementary cells sized between 10 and 20 microns. These were built up into eight-unit supercells that could be assembled with others to make a continuous structure. The researchers then conducted computational modeling and laboratory experiments and observed that the constructs exhibited uniquely homogenous deformation behavior free from localized overstress or underuse. The team showed that the new metamaterials feature a 25-fold enhancement in deformability and an orders-of-magnitude increase in energy absorption over state-of-the-art lattice arrangements.

 

“Tensegrity structures have been studied for decades, particularly in the context of architectural design, and they have recently been found in a number of biological systems,” said senior co-author Lorenzo Valdevit, a UCI professor of materials science and engineering who directs the Architected Materials Group. “Proper periodic tensegrity lattices were theoretically conceptualized only a few years ago by our co-author Julian Rimoli at Georgia Tech, but through this project we have achieved the first physical implementation and performance demonstration of these metamaterials.

 

“While developing structural configurations for planetary landers, the Georgia Tech team discovered that tensegrity-based vehicles could withstand severe deformation, or buckling, of its individual components without collapsing, something never observed in other structures. “This gave us the idea of creating metamaterials that exploit the same principle, which led us to the discovery of the first-ever 3D tensegrity metamaterial,” explained Rimoli, aerospace engineering professor at Georgia Tech.

 

“In familiar nano-architected materials, failure usually starts with a highly localized deformation,” said first author Jens Bauer, a UCI research scientist in mechanical and aerospace engineering. “Shear bands, surface cracks, and buckling of walls and struts in one area can cause a chain reaction leading to the collapse of an entire structure.” He explained that truss lattices begin to collapse when compressive members buckle, since those in tension cannot. Typically, these parts are interconnected at common nodes, meaning that once one fails, damage can quickly spread throughout the entire structure.

 

In contrast, the compressive members of tensegrity architectures form closed loops, isolated from one another and only connected by tensile members. Therefore, instability of compressive members can only propagate through tensile load paths, which—provided they do not rupture—cannot experience instability. Push down on a tensegrity system and the whole structure compresses uniformly, preventing localized damage that would otherwise cause catastrophic failure.

 

According to Valdevit, who’s also a professor of mechanical and aerospace engineering at UCI, tensegrity metamaterials demonstrate an unprecedented combination of failure resistance, extreme energy absorption, deformability and strength, outperforming all other types of state-of-the-art lightweight architectures. “This study provides important groundwork for design of superior engineering systems, from reusable impact protection systems to adaptive load-bearing structures,” he said

 

University of Missouri researchers have developed a new Lattice  metamaterial to help buildings withstand the ground shockwaves from earthquakes

The longitudinal and sheer energy waves produced by an earthquake travel through the ground and can destroy buildings miles from the epicentre. Preventing that damage requires a solution that can withstand these multidirectional waves travelling through a solid material but that is also flexible.

 

Enter metamaterials, a term for artificially constructed material, usually a composite, engineered in patterns that give them unique properties often to do with the way they manipulate waves. Dr Guoliang Huang, a James C. Dowell Professor in the mechanical and aerospace engineering department at the University of Missouri’s College of Engineering leads a team that has developed a lattice-type material that protects against both types of wave and is flexible enough to wrap around the objects it is protecting – a building or vehicles, for example.

 

The polar metamaterial is an ideal material for elastic wave cloaking. It is constructed by a lattice structure that can bend waves or vibrations so that objects inside the polar metamaterial coating are untouched by these waves or vibrations. Therefore, it is particularly useful for protecting against vibrations that might damage a structure. The polar metamaterial was fabricated by 3D printing. We performed static tests with tension and shear loadings. We are planning to do dynamic testing in the near future, said Dr Guoliang Huang.

 

The US Army Research Office funds the research, which has clear defence applications, including protection against vibration in mechanical parts, such as aircraft or submarine engines, and flexible protection for soldiers and equipment against blast energy. Two papers on the research, ‘Polar metamaterials: a new outlook on resonance for cloaking applications’ and ‘Physical realization of elastic cloaking with a polar material’, were published in Physical Review Letters, a journal of the American Physical Society.

About Rajesh Uppal

Check Also

Edge Computing technology

Over the past decades, cloud computing has been greatly developed and applied owing to its …

Leave a Reply

Your email address will not be published. Required fields are marked *

error: Content is protected !!