Query: mechanical metamaterial

title: Observation of non Hermitian topology and its bulk edge correspondence in an active mechanical metamaterial
abstract: In recent years, the mathematical concept of topology has been used to predict and harness the propagation of waves such as light or sound in materials. However, these advances have so far been realized in idealized scenarios, where waves do not attenuate. In this research, we demonstrate that topological properties of a mechanical system can predict the localization of waves in realistic settings where the energy can grow and/or decay. These findings may lead to strategies to manipulate waves in unprecedented fashions, for applications in vibration damping, energy harvesting, and sensing technologies. Topological edge modes are excitations that are localized at the materials' edges and yet are characterized by a topological invariant defined in the bulk. Such bulk edge correspondence has enabled the creation of robust electronic, electromagnetic, and mechanical transport properties across a wide range of systems, from cold atoms to metamaterials, active matter, and geophysical flows. Recently, the advent of non Hermitian topological systems wherein energy is not conserved has sparked considerable theoretical advances. In particular, novel topological phases that can only exist in non Hermitian systems have been introduced. However, whether such phases can be experimentally observed, and what their properties are, have remained open questions. Here, we identify and observe a form of bulk edge correspondence for a particular non Hermitian topological phase. We find that a change in the bulk non Hermitian topological invariant leads to a change of topological edge mode localization together with peculiar purely non Hermitian properties. Using a quantum to classical analogy, we create a mechanical metamaterial with nonreciprocal interactions, in which we observe experimentally the predicted bulk edge correspondence, demonstrating its robustness. Our results open avenues for the field of non Hermitian topology and for manipulating waves in unprecedented fashions.

title: A design framework for gradually stiffer mechanical metamaterial induced by negative Poisson's ratio property
abstract: Gradually stiffer (GS) mechanical metamaterial is a novel kind of adaptive structures under nonlinear varying loads for cushioning and vibration damping in engineering. These man made metamaterials in applications are usually tailored by trial and error. This study develops a design framework that integrates the topology optimization, parametric design and compression experiment for the GS mechanical metamaterials induced by the negative Poisson's ratio (NPR) property. Firstly, a parametric level set method is incorporated with the numerical homogenization method to topologically optimize a series of microstructural unit cells with different NPRs. Then, the nonlinear static analysis is respectively applied to different unit cells to check their GS behavior. Secondly, typical microstructures with GS property are selected and simplified to perform the subsequent parametric design to analyze the expected property. Thirdly, the obtained mechanical metamaterials are fabricated by using the additive manufacturing (AM) GS properties of the obtained designs are analyzed and verified by both the simulation and compression experiments. Results show that we could obtain the GS mechanical metamaterial based on NPR properties for their similar characteristics. This study demonstrates that the proposed design framework is effective to tailor innovative layouts of the GS mechanical metamaterials.

title: A novel two dimensional mechanical metamaterial with negative Poisson's ratio
abstract: with negative Poisson's ratio enable distinctive elastic response in the direction orthogonal to the loaded direction, i.e. shrinking when compressed and expanding when stretched, compared to conventional materials. Such distinctive mechanical characteristic makes auxetic materials unique in practice. Current studies in this aspect focus mainly on the realization of beam dominated microstructures such as re entrant and chiral lattices and of cellular microstructures with orthogonal elliptical hole pattern. In this study, a novel two dimensional auxetic microstructure is designed by introducing peanut shaped holes in solid bulk matrix. Compared to the microstructure with elliptical hole pattern, the present design can produce slightly larger negative Poisson's ratio and achieve significantly lower stress level. The samples consisting of a number of centimeter scale unit cells with the peanut shaped holes are fabricated efficiently via additive manufacturing technique. Experiment and finite element simulation of tensile test are carried out on the specific sample to demonstrate the auxetic effect of the present design and simultaneously verify the computational model. Finally, effects of some parameters on Poisson's ratio, which may control the auxetic behavior of the present microstructure, are discussed for better understanding deformation mechanism of the proposed auxetic material.

title: 3D compression torsion cubic mechanical metamaterial with double inclined rods
abstract: A novel 3D compression torsion cubic mechanical metamaterial (CTCMM) with double inclined rods is proposed converting axial compression into a torsion. The CTCMM under uniaxial compression is studied by theoretical analysis, experiments and numerical simulations. The relationships between relative densities and geometric parameters are analyzed to meet the needs of lightweight. Based on the derivation of the over deformation mechanism, the torsion angle is derived by the Timoshenko beam theory. The arrangements of gradient models are designed and fabricated by 3D printing, as the specimens for static compression experiments. Results show that the torsion angles of the numerical and experimental results agree well with the theoretical solution. Moreover, the proposed CTCMM possesses a similar compression torsion effect compared to previously reported 3D CTCMM. Meanwhile, the improved 3D CTCMM with the better compression torsion effect is proposed, compared with the original structure, the different phenomenon wherein the torsion angle first increases and then declines. Finally, cell number, variable cross section and perforated plates all affect the compression torsion effects of 3D CTCMM. The above research provides a new idea for improving the performance of CTCMM, no longer limited to the deformation mode of a single inclined rod.

title: Parameters analysis and optimization of a typical multistable mechanical metamaterial
abstract: In this work, mechanical properties of a typical multistable mechanical metamaterial were analyzed in detail. When the maximum strain of the structure was kept constant during deflection, it was found that the largest peak force and best energy absorption efficiency can be obtained by adjusting its geometrical parameters. The influence of parameters on the force displacement curve of the unit cell under large deformation is also discussed. Moreover, to further verify the conclusions from theoretical and finite element analysis (FEA) we experimentally investigated the mechanical performances of two different multilayer structures fabricated by a high resolution 3D printer It is noteworthy that both the FEA and experiments reveal the structure with Q 3 6 has significantly better performance than the structure with Q 6 both in terms of the properties of vibration isolation and energy absorption. The remarkable result of this work shows its potential in choosing the layouts of multistable mechanical metamaterials.

title: Mechanical metamaterial piezoelectric nanogenerator (MM PENG) Design principle, modeling and performance
abstract: Piezoelectric nanogenerator (PENG) has been reported as a green energy solution that generates electrical power from mechanical deformations resulted in the environment. Here, a novel design concept of PENG is reported based on mechanical metamaterials (MM) Plate like MM with piezoelectric materials is designed in a sliding cell such that low frequency excitations causes structural instability (i.e. postbuckling) of the piezo MM and thus, the piezo layer is triggered to generate electrical power. In this process, the environmental input

title: Real time tunable negative stiffness mechanical metamaterial
abstract: Real time tunable mechanical metamaterial is an emerging field attracting great attention. It enables a wide range of mechanical properties but is hard to design and fabricate due to its complexity. Here, a pneumatically actuated tunable negative stiffness mechanical metamaterial with real time tunable properties is presented. Numerical simulations and experiments were both employed to investigate the proposed real time tunable negative stiffness mechanical metamaterial. Research results demonstrate that multistage pattern transformation can be realized through the pneumatic actuation. Uniaxial compression and vibration tests reveal the influence of the inner air pressure on the metamaterial's basic mechanical properties and the vibration isolation performance, respectively. Additionally, real negative stiffness behavior and high energy absorption efficiency are achievable for the proposed metamaterial. This study can be a reference to relevant researches on tunable mechanical metamaterials, and presents a new path for designing real time tunable negative stiffness mechanical metamaterials.

title: Truncated regular octahedral tensegrity based mechanical metamaterial with tunable and programmable Poisson's ratio
abstract: Tensegrities consisting of axially loaded strings and bars have various technologically important applications. In this paper, we design a mechanical metamaterial with tunable and programmable Poisson's ratio on the basis of truncated regular octahedral tensegrity (TROT) A theoretical model for evaluating the deformation behaviors of TROT is established by involving the principle of virtual work. Our experimental data obtained from the tests of 3D printed TROT sculptures agree well with theoretical predictions. By systemically investigating the mechanical properties of the TROT metamaterial, it is found that the Poisson's ratio can be programmed to a desired value in a wide positive/negative range through the structural prestress, initial twist angle, and the stiffnesses of strings/bars. This mechanical metamaterial can be utilized not only as elementary cells to assemble other larger scale of metamaterials, and also as prototypes with tunable properties in designing advanced machineries for applications such as robotics and aerospace engineering.

title: The Multidirectional Auxeticity and Negative Linear Compressibility of a 3D Mechanical Metamaterial
abstract: In this work, through the use of a theoretical model, we analyse the potential of a specific three dimensional mechanical metamaterial composed of arrowhead like structural units to exhibit a negative Poisson's ratio for an arbitrary loading direction. Said analysis allows us to assess its suitability for use in applications where materials must be able to respond in a desired manner to a stimulus applied in multiple directions. As a result of our studies, we show that the analysed system is capable of exhibiting auxetic behaviour for a broad range of loading directions, with isotropic behaviour being shown in some planes. In addition to that, we show that there are also certain loading directions in which the system manifests negative linear compressibility. This enhances its versatility and suitability for a number of applications where materials exhibiting auxetic behaviour or negative linear compressibility are normally implemented.

title: Automatic design of mechanical metamaterial actuators
abstract: Mechanical metamaterial actuators achieve pre determined input output operations exploiting architectural features encoded within a single 3D printed element, thus removing the need for assembling different structural components. Despite the rapid progress in the field, there is still a need for efficient strategies to optimize metamaterial design for a variety of functions. We present a computational method for the automatic design of mechanical metamaterial actuators that combines a reinforced Monte Carlo method with discrete element simulations. 3D printing of selected mechanical metamaterial actuators shows that the machine generated structures can reach high efficiency, exceeding human designed structures. We also show that it is possible to design efficient actuators by training a deep neural network which is then able to predict the efficiency from the image of a structure and to identify its functional regions. The elementary actuators devised here can be combined to produce metamaterial machines of arbitrary complexity for countless engineering applications. Efficient strategies to optimize metamaterial design for specific applications are urgently needed despite the rapid progress in this area. Here the authors propose a computational method combining an optimization algorithm with discrete element simulations for the automatic design of mechanical metamaterial actuators.