Exotic Mechanical Properties Research Articles

Non-linear stress-softening of the bacterial cell wall confers cell shape homeostasis.

The bacillus - or rod - is a pervasive cellular morphology among bacteria. Rod-shaped cells elongate without widening by reinforcing their cell wall anisotropically to prevent turgor pressure from inflating cell width. Here, we demonstrate that a constrictive force is also essential for avoiding pressure-driven widening in Gram-positive bacteria. Specifically, super-resolution measurements of the nonlinear mechanical properties of the cell wall revealed that across a range of turgor pressure cell elongation directly causes width constriction, similar to a "finger trap" toy. As predicted by theory, this property depends on cell-wall anisotropy and is precisely correlated with the cell's ability to maintain a rod shape. Furthermore, the acute non-linearities in the dependence between cell length and width deformation result in a negative-feedback mechanism that confers cell-width homeostasis. That is, the Gram-positive cell wall is a "smart material" whose exotic mechanical properties are exquisitely adapted to execute cellular morphogenesis.

Auxetic Biomedical Metamaterials for Orthopedic Surgery Applications: A Comprehensive Review.

Poisson's ratio in auxetic materials shifts from typically positive to negative, causing lateral expansion during axial tension. This scale-independent characteristic, originating from tailored architectures, exhibits specific physical properties, including energy adsorption, shear resistance, and fracture resistance. These metamaterials demonstrate exotic mechanical properties with potential applications in several engineering fields, but biomedical applications seem to be one of the most relevant, with an increasing number of articles published in recent years, which present opportunities ranging from cellular repair to organ reconstruction with outstanding mechanical performance, mechanical conduction, and biological activity compared with traditional biomedical metamaterials. Therefore, focusing on understanding the potential of these structures and promoting theoretical and experimental investigations into the benefits of their unique mechanical properties is necessary for achieving high-performance biomedical applications. Considering the demand for advanced biomaterial implants in surgical technology and the profound advancement of additive manufacturing technology that are particularly relevant to fabricating complex and customizable auxetic mechanical metamaterials, this review focuses on the fundamental geometric configuration and unique physical properties of negative Poisson's ratio materials, then categorizes and summarizes auxetic material applications across some surgical departments, revealing efficacy in joint surgery, spinal surgery, trauma surgery, and sports medicine contexts. Additionally, it emphasizes the substantial potential of auxetic materials as innovative biomedical solutions in orthopedics and demonstrates the significant potential for comprehensive surgical application in the future.

Kresling origami derived structures and inspired mechanical metamaterial

Origami has attracted more and more attention due to its exotic mechanical properties, and the inspired metamaterials are also popular. However, the main focus of current research is on existing origami patterns and properties, although new origami patterns or results that expand on existing origami patterns are gradually emerging. In this paper, we summarize a series of derived structures of the Kresling origami, demonstrating more stable states and richer structural forms. At the same time, a point-searching method is proposed along the ideas of the truss model, which is effective for irregular stable states of these derived structures. On this basis, we create an origami-inspired mechanical metamaterial with foldable property and high load-bearing capacity, fabricate the prototype, and validate its performance through experiments. These works make important contributions for promoting the Kresling origami and origami-inspired metamaterials.

Integration of additive manufacturing and thermomechanical processing toward a hybrid production route for PH13-8Mo stainless steel

In the current study, PH13–8Mo is fabricated using laser-powder bed fusion and arc-directed energy deposition additive manufacturing (AM) processes. The microstructure and mechanical properties in the as-built and isothermally heat-treated conditions are studied. The isothermal heat treatment includes direct aging at 530 °C to form β-NiAl precipitates. The AM alloys (in the as-built condition) are then thermomechanically processed at the isothermal aging temperature to promote dynamic aging and dislocation multiplication. The resulting mechanical properties are found to be superior to the heat-treated conditions and correlated to the microstructure. This study paves the pathway toward developing a novel hybrid manufacturing route to fabricate alloys with exotic mechanical properties.

Two-dimensional Janus Si2OX (X = S, Se, Te) monolayers as auxetic semiconductors: theoretical prediction.

The auxetic materials have exotic mechanical properties compared to conventional materials, such as higher indentation resistance, more superior sound absorption performance. Although the auxetic behavior has also been observed in two-dimensional (2D) nanomaterials, to date there has not been much research on auxetic materials in the vertical asymmetric Janus 2D layered structures. In this paper, we explore the mechanical, electronic, and transport characteristics of Janus Si2OX (X = S, Se, Te) monolayers by first-principle calculations. Except for the Si2OTe monolayer, both Si2OS and Si2OSe are found to be stable. Most importantly, both Si2OS and Si2OSe monolayers are predicted to be auxetic semiconductors with a large negative Poisson's ratio. The auxetic behavior is clearly observed in the Janus Si2OS monolayer with an extremely large negative Poisson's ratio of -0.234 in the x axis. At the equilibrium state, both Si2OS and Si2OSe materials exhibit indirect semiconducting characteristics and their band gaps can be easily altered by the mechanical strain. More interestingly, the indirect-direct bandgap phase transitions are observed in both Si2OS and Si2OSe monolayers when the biaxial strains are introduced. Further, the studied Janus structures also exhibit remarkably high electron mobility, particularly along the x direction. Our findings demonstrate that Si2OS and Si2OSe monolayers are new auxetic materials with asymmetric structures and show their great promise in electronic and nanomechanical applications.

In-plane elastic property prediction of straight-arc coupled auxetic structures

Auxetic metamaterials with two components exhibit a wide variety of potential engineering applications due to their exotic mechanical properties. In this work, a novel straight-arc coupled structure (SACS) is designed by introducing a circular arc structure to a classical re-entrant structure. This work aims to explore the linear and geometrical nonlinear mechanical of SACS at large strains. According to Castigliano’s second theorem, the in-plane linear theoretical model is established to obtain equivalent Poisson’s ratio and elastic modulus. A geometrical nonlinear model is further established based on large deflection theory and chain algorithm. The finite element method is used to verify the prediction of the theoretical solution, and linear and nonlinear mechanical properties of the SACS are studied by numerical simulation. The influence of geometric parameter re-entrant angle and arc radius on the mechanical properties of the SACS is investigated to compare the linear and nonlinear mechanical properties. The linear numerical simulation of SACS with two transverse ribs (SACS-TR) and classical re-entrant honeycomb structure with two transverse ribs (CRS-TR) is carried out to analyze the in-plane elastic properties. These results demonstrate that considering the geometric nonlinear model can predict the actual structural deformation more accurately, which is verified by the quasi-static compression experiment results at large strains. The SACS design can enhance the auxetic effect and structure Young’s moduli under the same dimension.

Heuristic molecular modelling of quasi-isotropic auxetic metamaterials under large deformations

A two-dimensional molecular model is presented for the elastic non-linear modelling and design of meta-materials. The fundamental unit-cell, based on a heuristic molecule (HM) approach, is composed of atoms that interact through centred and non-centred spring-based bonds. The kinematics formulation allows to consider large displacements and finite strains while the specific topology of the HM can be parametrized to modify the shape of the rigid atoms and the size of the bonds. The HM is frame indifferent and provides a remarkable quasi-isotropic elastic response for both deviatoric and volumetric large deformation modes. At a macro-scale, the relationship with different continuum materials is given through a standard isotropic Cauchy up to an isotropic Cosserat solid. Evidence on the interest of the model as a calculation tool is provided by studying the elastic response of standard and auxetic materials subjected to a non-homogeneous deformation field, as well as the response of auxetic foams under large deformations. Aside the numerical agreement, it is highlighted how the tailoring of the HM topology can be effective to approximate the non-linear geometric effects that occur at finer scales of auxetic foams. In perspective, we address how the exotic mechanical properties provided by the HM, together with the assumed physical-driven framework, can foster the engineering application and the design of new meta-materials.

Programming Multistable Metamaterials to Discover Latent Functionalities.

Using multistable mechanical metamaterials to develop deployable structures, electrical devices, and mechanical memories raises two unanswered questions. First, can mechanical instability be programmed to design sensors and memory devices? Second, how can mechanical properties be tuned at the post-fabrication stage via external stimuli? Answering these questions requires a thorough understanding of the snapping sequences and variations of the elastic energy in multistable metamaterials. The mechanics of deformation sequences and continuous force/energy-displacement curves are comprehensively unveiled here. A 1D array, that is chain, of bistable cells is studied to explore instability-induced energy release and snapping sequences under one external mechanical stimulus. This method offers an insight into the programmability of multistable chains, which is exploited to fabricate a mechanical sensor/memory with sampling (analog to digital-A/D) and data reconstruction (digital to analog-D/A) functionalities operating based on the correlation between the deformation sequence and the mechanical input. The findings offer a new paradigm for developing programmable high-capacity read-write mechanical memories regardless of thei size scale. Furthermore, exotic mechanical properties can be tuned by harnessing the attained programmability of multistable chains. In this respect, a transversely multistable mechanical metamaterial with tensegrity-like bistable cells is designed to showcase the tunability ofchirality.

Active mechanical metamaterial with embedded piezoelectric actuation

Metamaterials are artificially structured materials and exhibit properties that are uncommon or non-existent in nature. Mechanical metamaterials show exotic mechanical properties, such as negative stiffness, vanishing shear modulus, or negative Poisson’s ratio. These properties stem from the geometry and arrangement of the metamaterial unit elements and, therefore, cannot be altered after fabrication. Active mechanical metamaterials aim to overcome this limitation by embedding actuation into the metamaterial unit elements to alter the material properties or mechanical state. This could pave the way for a variety of applications in industries, such as aerospace, robotics, and high-tech engineering. This work proposes and studies an active mechanical metamaterial concept that can actively control the force and deformation distribution within its lattice. Individually controllable actuation units are designed based on piezostack actuators and compliant mechanisms and interconnected into an active metamaterial lattice. Both the actuation units and the metamaterial lattice are modeled, built, and experimentally studied. In experiments, the actuation units attained 240 and 1510 µm extensions, respectively, in quasi-static and resonant operation at 81 Hz, and 0.3 N blocked force at frequencies up to 100 Hz. Quasi-static experiments on the active metamaterial lattice prototype demonstrated morphing into four different configurations: Tilt left, tilt right, convex, and concave profiles. This demonstrated the feasibility of altering the force and deformation distribution within the mechanical metamaterial lattice. Much more research is expected to follow in this field since the actively tuneable mechanical state and properties can enable qualitatively new engineering solutions.

The effect of microstructures and precipitates (γ', γ″, δ) on machinability of Inconel-718 nickel-based superalloy in turning process

Nickel-based superalloys have been extensively used in the aerospace and energy sector due to their excellent mechanical properties at high pressures and temperatures. The exotic mechanical properties of Inconel-718 majorly depend on its microstructure and precipitates, which can be controlled using a heat-treatment process. In this work, a wide range of microstructures with micro-hardness (210–418 HV) and average grain size (12–88 μm) was achieved by solution treatment and aging. Electron microscopy (EBSD, SEM, EDS and TEM), analysis revealed the precipitates and microstructures. A high-volume fraction of γ' and γ″ increases the strength while δ reduces the strength of the sample. The role of microstructure, precipitates and machining parameters (such as feed and depth of cut) on the machinability of Inconel-718 was investigated. There was no direct correlation observed between average grain size and machinability due to the presence of precipitates. The solution treatment of the as received sample enables dissolution of all the precipitates (γ', γ″, δ), leaving only the matrix (γ) phase, thus improving the machinability. The machined surface was characterized using Kernel average misorientation (KAM) and Image quality (IQ) map to quantify the machined affected zone (MAZ). Both the KAM and the IQ, parameters consistently reveal MAZ with large plastic deformation, with no grain fragmentation, micro cracks, and strain localizations in the solution treated samples which are in contrast with the aged samples.

Review: Auxetic Polymer-Based Mechanical Metamaterials for Biomedical Applications.

Over the last three decades but more particularly during the last 5 years, auxetic mechanical metamaterials constructed from precisely architected polymer-based materials have attracted considerable attention due to their fascinating mechanical properties. These materials present a negative Poisson's ratio and therefore unusual mechanical behavior, which has resulted in enhanced static modulus, energy adsorption, and shear resistance, as compared with the bulk properties of polymers. Novel advanced polymer processing and fabrication techniques, and in particular additive manufacturing, allow one to design complex and customizable polymer architectures that are particularly relevant to fabricate auxetic mechanical metamaterials. Although these metamaterials exhibit exotic mechanical properties with potential applications in several engineering fields, biomedical applications seem to be one of the most relevant with a growing number of articles published over recent years. As a result, special focus is needed to understand the potential of these structures and foster theoretical and experimental investigations on the potential benefits of the unusual mechanical properties of these materials on the way to high performance biomedical applications. The present Review provides up to date information on the recent progress of polymer-based auxetic mechanical metamaterials mainly fabricated using additive manufacturing methods with a special focus toward biomedical applications including tissue engineering as well as medical devices including stents and sensors.

Four-penta-graphenes: Novel two-dimensional fenestrane-based auxetic nanocarbon allotropes for nanoelectronics and optoelectronics

The great success of graphene has encouraged tremendous interest in searching for new two-dimensional (2D) carbon allotropes. Based on first-principles calculations, we proposed new 2D carbon allotropes obtained from the assembly of fenestrane molecule unit, named as four-penta-graphenes (fPG). The fPG monolayers are energetically more favorable than pentagraphene and show excellent dynamical, thermal, and mechanical stability. They exhibit exotic mechanical properties including anisotropic in-plane stiffness and auxetic behavior with sign-tunable Poisson's ratio. Their electronic properties are diverse, ranging from narrow band gap semiconductors to metal. The electronic band gap and band edge positions of the fPG semiconductors can be flexibly modulated by strain engineering. Indirect-to-direct band gap and semiconductor-to-metal transitions can be achieved by applying compressive and tensile strains, respectively. High mobility and anisotropic effective mass of carriers make these materials promising for nanoelectronics. Using many-body GW0+BSE approximations, we reveal that the fPG semiconductors exhibit strong excitonic effects with distinct optical absorption peaks in the visible range. These remarkable optical properties make them also promising devices for optoelectronics.

An elastic metal-organic crystal with a densely catenated backbone.

What particular mechanical properties can be expected for materials composed of interlocked backbones has been a long-standing issue in materials science since the first reports on polycatenane and polyrotaxane in the 1970s1-3. Here we report a three-dimensional porous metal-organic crystal, which is exceptional in that its warps and wefts are connected only by catenation. This porous crystal is composed of a tetragonal lattice and dynamically changes its geometry upon guest molecule release, uptake and exchange, and also upon temperature variation even in a low temperature range. We indented4 the crystal along its a/b axes and obtained the Young's moduli of 1.77 ± 0.16 GPa in N,N-dimethylformamide and 1.63 ± 0.13 GPa in tetrahydrofuran, which are the lowest among those reported so far for porous metal-organic crystals5. To our surprise, hydrostatic compression showed that this elastic porous crystal was the most deformable along its c axis, where 5% contraction occurred without structural deterioration upon compression up to 0.88 GPa. The crystal structure obtained at 0.46 GPa showed that the catenated macrocycles move translationally upon contraction. We anticipate our mechanically interlocked molecule-based design to be a starting point for the development of porous materials with exotic mechanical properties. For example, squeezable porous crystals that may address an essential difficulty in realizing both high abilities of guest uptake and release are on the horizon.

Giant negative Poisson's ratio in two-dimensional V-shaped materials.

Two-dimensional (2D) auxetic materials with exceptional negative Poisson's ratios (NPR) are drawing increasing interest due to their potential use in medicine, fasteners, tougher composites and many other applications. Improving the auxetic performance of 2D materials is currently crucial. Here, using first-principles calculations, we demonstrated giant in-plane NPRs in MX monolayers (M = Al, Ga, In, Zn, Cd; X = P, As, Sb, S, Se, Te) with a unique V-shaped configuration. Our calculations showed that GaP, GaAs, GaSb, ZnS and ZnTe monolayers exhibit exceptional all-angle in-plane NPRs. Remarkably, the AlP monolayer possesses a giant NPR of −1.779, by far the largest NPR in 2D materials. The NPRs of these MX monolayers are correlated to the highly anisotropic features of the V-shaped geometry. The exotic mechanical properties of the V-shaped MX monolayers provide a new family of 2D auxetic materials, as well as a useful guidance for tuning the NPR of 2D materials.

A Flexible Carbon Nanotubes-Based Auxetic Sponge Electrode for Strain Sensors.

Soft compliant strain gauges are key devices for wearable applications such as body health sensor systems, exoskeletons, or robotics. Other than traditional piezoresistive materials, such as metals and doped semiconductors placed on strain-sensitive microsystems, a class of soft porous materials with exotic mechanical properties, called auxetics, can be employed in strain gauges in order to boost their performance and add functionalities. For strain electronic read-outs, their polymeric structure needs to be made conductive. Herein, we present the fabrication process of an auxetic electrode based on a polymeric nanocomposite. A multiwalled carbon nanotube/polydimethylsiloxane (MWCNT/PDMS) is fabricated on an open-cell polyurethane (PU) auxetic foam and its effective usability as an electrode for strain-gauge sensors is assessed.

3D printing of chiral carbon fiber reinforced polylactic acid composites with negative Poisson's ratios

Auextic materials possessing negative Poisson's ratios have gained great research interest by virtue of its various exotic mechanical properties while most of these metamaterials rely on the hollow cellular configuration, largely undermining the overall mechanical performance. It therefore restricts their application in load-bearing engineering fields. In order to solve this issue, we first designed a new class of chiral structures by integrating various polynomial curves. Based on the state-of-the-art 3D printing technique, these auxetic geometries were then fabricated into high-performance composites with chopped carbon fiber (CF) as the reinforcement. Guided by experimental characterization and finite element analysis, the influences of the polynomial orders (n) and the incorporation of CF on the mechanical properties were systematically studied. Results showed that the auxetic behaviour can be tailored by varying the n value inside the polynomial equation. Moreover, the tensile modulus, strength and energy absorption at break of printed chiral samples were all enhanced with the addition of CF, especially for the large improvement of modulus and absorbed energy by 2-fold. Hence, these novel chiral composites present great potential in the development of high performance mechanical metamaterials.

Gallium Thiophosphate: An Emerging Bidirectional Auxetic Two-Dimensional Crystal with Wide Direct Band Gap.

Two-dimensional (2D) materials with negative Poisson's ratio (NPR) attract considerable attention because of their exotic mechanical properties. We propose a new 2D material, monolayer GaPS4, which shows NPR for both in-plane (-0.033) and out-of-plane (-0.62) directions. Such coexistence of NPR in two distinctdirections could be explained by its corner- and edge-shared tetrahedra pucker structure. GaPS4 has an ultralow cleavage energy of 0.23 J m-2 according to our calculation, such thatexfoliation of the bulk material is feasible for the preparation of mono- and few-layer GaPS4. Direct wide band gap of 3.55 eV and moderate electron mobility have been revealed in monolayer GaPS4, while the direct gap feature is robust within a strain range of -6% to 6%. These findings render 2D GaPS4 a promising candidate for applications in nanoelectronics and low-dimensional electromechanical devices.

Feasibility Study on 3-D Printing of Metallic Structural Materials with Robotized Laser-Based Metal Additive Manufacturing

Metallic structural materials continue to open new avenues in achieving exotic mechanical properties that are naturally unavailable. They hold great potential in developing novel products in diverse industries such as the automotive, aerospace, biomedical, oil and gas, and defense. Currently, the use of metallic structural materials in industry is still limited because of difficulties in their manufacturing. This article studied the feasibility of printing metallic structural materials with robotized laser-based metal additive manufacturing (RLMAM). In this study, two metallic structural materials characterized by an enlarged positive Poisson’s ratio and a negative Poisson’s ratio were designed and simulated, respectively. An RLMAM system developed at the Research Center for Advanced Manufacturing of Southern Methodist University was used to print them. The results of the tensile tests indicated that the printed samples successfully achieved the corresponding mechanical properties.

Theoretical Investigation of Structural, Electronic, and Mechanical Properties of Two Dimensional C, Si, Ge, Sn

In this article, we investigate the predictions of the first principles on structural stability, electronic and mechanical properties of 2D nanostructures: graphene, silicene, germanene and stenane. The electronic band structure and density of states in all these 2D materials are found to be generic in nature. A small band gap is generated in all the reported materials other than graphene. The linearity at the Dirac cone changes to quadratic, from graphene to stenane and a perfect semimetalicity is exhibited only by graphene. All other 2D structures tend to become semiconductors with an infinitesimal band gap. Bonding characteristics are revealed by density of states histogram, charge density contour, and Mulliken population analysis. Among all 2D materials graphene exhibits exotic mechanical properties. Analysis by born stability criteria and the calculation of formation enthalpies envisages the structural stability of all the structures in the 2D form. The calculated second order elastic stiffness tensor is used to determine the moduli of elasticity in turn to explore the mechanical properties of all these structures for the prolific use in engineering science. Graphene is found to be the strongest material but brittle in nature. Germanene and stenane exhibit ductile nature and hence could be easily incorporated with the existing technology in the semiconductor industry on substrates.

Effect of Al and Mn Content on the Mechanical Properties of Various ECAE Processed Mg-Li-Zn Alloys

The equal channel angular extrusion (ECAE) process is an innovative method to refine grain structure; however, it could be highly technical to perform to result in subsequent exotic mechanical properties. This study will demonstrate how easy or difficult of this operation. Yoshida el at. had applied the ECAE process on Mg-10%Li-1%Zn alloy to obtain a max. superplastic elongation of 421%. This paper tries to reproduce it with Mg-1%Li-1%Zn and Mg-9%Li-1%Zn alloys via the same ECAE process. For the 28 specimens having received 4 passes of ECAE, one of them shows a comparable but lesser elongation of 350% under selective temperature of 523 K and strain rate, 1 x 10 -4 s -1 , and the corresponding strain rate sensitivity exponent is 0.48. Furthermore, the ECAE process is imposed on three other Mg-Li-Zn alloys containing Al and Mn (Mg-9%Li-l%Zn-0.2%Mn, Mg-9%Li-l%Zn-l%Al-0.2%Mn and Mg-9%Li-3%Al-l%Zn-0.2%Mn), and this investigation is unprecedented. The original justification for this exploration is that Al and Mn addition may strengthen the a and β phases which are the sole micro-constituents of Mg-Li alloys when bearing Li content between 5% to 11%. Indeed, it is so at room temperature as determined by micro-hardness testing. However, high temperature tensile elongation is not benefited.