The negative Poisson's ratio (NPR) structures exhibit some unusual but highly useful mechanical properties. In comparison to two-dimensional (2D) NPR structures, less attention has been paid to the ...studies on three-dimensional structures due to the complex manufacturing process. In this paper, based on the two-dimensional nonconvex hexagonal cell, a novel three-dimensional (3D) NPR energy-absorbing structure is proposed. According to the beam theory, the equivalent Young's modulus and Poisson's ratio are deduced. Subsequently, numerous finite element (FE) simulations and compression tests were performed and a good agreement was observed among the theoretical results, the numerical simulations, and the experimental results. Moreover, a typical fuselage section of aircraft during a crash landing was considered and the fuselage section with the 3D NPR structure presented in this work showed better energy absorption capacity than the one without the 3D NPR structure.
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Mechanical metamaterials are man-made structures with counterintuitive mechanical properties that originate in the geometry of their unit cell instead of the properties of each ...component. The typical mechanical metamaterials are generally associated with the four elastic constants, the Young's modulus E, shear modulus G, bulk modulus K and Poisson's ratio υ, the former three of which correspond to the stiffness, rigidity, and compressibility of a material from an engineering point of view. Here we review the important advancements in structural topology optimisation of the underlying design principles, coupled with experimental fabrication, thereby to obtain various counterintuitive mechanical properties. Further, a clear classification of mechanical metamaterials have been established based on the fundamental material mechanics. Consequently, mechanical metamaterials can be divide into strong-lightweight (E/ρ), pattern transformation with tunable stiffness, negative compressibility (−4G/3 < K < 0), Pentamode metamaterials (G ≪ K) and auxetic metamaterials (G ≫ K), simultaneously using topology optimisation to share various fancy but feasible mechanical properties, ultralight, ultra-stiffness, well-controllable stiffness, vanishing shear modulus, negative compressibility and negative Poisson’s ratio. We provide here a broad overview of significant potential mechanical metamaterials together with the upcoming challenges in the intriguing and promising research field.
•A class of novel 3D auxetic lattice structures are developed based on the rotating rigid mechanism.•Analytical models for predicting the elastic properties of the new structure are developed.•The ...structures have been shown excellent design flexibility with respect to Poisson’s ratio.
This paper introduces a new methodology for generating three-dimensional (3D) negative Poisson’s ratio behavior with lattice representations of the rotating rigid mechanism as a starting point. Based on it, a class of new 3D auxetic lattice structures is proposed. The elastic properties of a representative 3D auxetic lattice structure, including the homogenized Young’s modulus and Poisson’s ratio along the three principal axes, are systematically investigated in a combination of analytical predictions, numerical simulations and experimental tests. Moreover, effects of the structural geometrical parameters and specimen size on the elastic properties as well as structural nonlinear mechanical responses along the principal axes are carefully discussed. Different from most traditional 3D auxetic materials that are only capable of achieving negative or positive Poisson's ratio, our results suggest that the homogenized Poisson’s ratio effect of the proposed 3D lattice structure along all principal axes can be tuned from positive to negative in a wide range. The excellent design flexibility would help to expand and hasten the adoption of the new 3D auxetic lattice structures in engineering applications.
Auxetic materials with negative Poisson’s ratios unusually exhibit intuitive mechanical behaviors, such as cross-section expansion instead of contraction during tension. Such behaviors are ...interesting because they may enhance unusual mechanical properties. However, controllable preparation of materials with negative Poisson’s ratio is still a major challenge. In this study, we report the synthesis of a flexible auxetic graphene assembled macrofilm (GAMF) from graphene oxide nanosheets by a thermal annealing and press assistant method. The obtained materials exhibit good flexibility and significantly wide tunable negative Poisson’s ratios ranging from −0.11 to −0.53. We also develop a reconstruction model for characterization the uniaxial tension of GAMF based on X-ray tomographic images. The tensile simulation result predicts the function relationship between Poisson’s ratio and critical thickness of pore channels, which is in good agreement with the experimental data. As a result, an effective tunable way is proposed for customizable fabrication of GAMF with tunable negative Poisson’s ratios, and the GAMF materials with good flexibility, high electrical conductivity and superior auxetic behavior looks promising for future development of wearable electronics.
Microporous structure was constructed as determining factor in auxetic performance of fabricated GAMF with high conductivity and outstanding flexibility through graphitization of treated graphene oxide precursor. The simulation results showed that critical thickness had an impact on NPR performance. GAMFs with different critical thicknesses were fabricated and Poisson’s ratios ranging from −0.11 to −0.53 were recorded, which aligned well with the predictions and suggesting promising applications in flexible sensor devices and wearable electronic engineering. Display omitted
This paper reports a new annular honeycomb that shows tunable Poisson's ratio. The presented honeycombs can provide both the in-plane negative Poisson's ratio (NPR) and in-plane zero Poisson's ratio ...(ZPR) while the shapes of the honeycomb core change. The developed honeycomb core is constructed by an annulus with four ligaments distributed bi-axial symmetrically, which shows great diversity of mechanical properties. The developed theoretical models of the in-plane elasticity are based on Castigliano's Second Theorem and energy principle. Two parameters were used to describe the deformation mechanisms of the unit cell concisely. The effect of two parameters on the Poisson's ratio and Young's modulus was investigated. The Young's modulus derived from the Finite Element (FE) analysis all have an good agreement with that derived from theoretical results, and the experiments were carried out to confirm the validity of them convincingly.
•A novel perforated negative Poisson’s ratio core buckling-restrained brace (NP-BRB) was designed and manufactured.•Seismic performance of the proposed buckling-restrained brace (BRB) was ...investigated.•The proposed BRB with auxetic core has a desirable seismic property under cyclic loading.•These findings have values in guiding the investigation and design of other metallic dampers regarding auxetics.
Auxetic materials could exhibit desirable mechanical properties, e.g., fracture resistance, shear resistance and energy dissipation due to their unique deformation characteristics. However, the seismic performance of auxetics in disaster prevention and mitigation of building structures is rarely studied. In this study, a novel perforated negative Poisson’s ratio core buckling-restrained brace (NP-BRB) was designed and manufactured for exploring the hysteretic performance of auxetic metamaterials under cyclic load. Experiments and verified numerical simulations were conducted to investigate the effects of porosity and section weakening rate on the seismic performance. The results show that the NP-BRB has stable hysteretic curves and low compression strength adjustment factor. In addition, the parametric analysis indicated that the energy dissipation capacity of NP-BRB with relatively large section weakening rate could be improved due to auxetic behavior when the average strain exceeds 1%. These findings are beneficial to the applications of auxetic metamaterials in damping devices for mitigating seismic effects.
•A simple seismic metamaterial with ultra-low frequency wide bandgap based on auxetic foam is proposed for the first time.•Band structures, vibration modes and transmission spectrum of the seismic ...metamaterial are calculated and analyzed.•The influences of geometrical parameters, material parameters value on the bandgap width and location are discussed.
Seismic metamaterial (SM) has lately received significant attention in the field of vibration isolation and damping due to its wave manipulation and bandgap properties. However, the limitations of unit cell size and the difference of the material parameters of each component have made it challenging to generate an ultra-low frequency bandgap. In order to overcome this challenge, a novel type of 2D SM composed of auxetic foam and steel is proposed to attenuate seismic waves at ultra-low frequencies. Firstly, the band structure of the SMs and the vibration modes of the upper and lower bounds of the first complete bandgap are calculated and analyzed by using the finite element method, and the mechanism of the bandgap generation is clarified. The transmission spectrum under the Lamb wave incident on the SM is investigated. It is validated that Lamb waves have a good attenuation effect in the frequency range below 10 Hz. Secondly, a parametric study of the SM with auxetic foam-coated hollow steel columns is carried out. The numerical results show that the shape and height of the unit cell, the elastic modulus, density and Poisson’s ratio of the auxetic foam play important roles in the formation of the bandgap. Finally, the numerical simulation verifies that adding through holes in the matrix which reduces the equivalent mass density of the matrix could widen the bandgap and enhance the effective attenuation of seismic waves. Based on theoretical model and combined with the exceptional material characteristics of auxetic foam, the focus of the study is to achieve a wide bandgap coverage for the seismic peak spectrum of 2 Hz which causes the principal damage of surface buildings.
Equivalent sandwich panels composed of auxetic and conventional honeycomb cores and metal facets are analysed and compared for their resistance performances against impulsive loadings. The dynamic ...behaviours of these structures are numerically investigated, taking into account the rate-dependent effects. The Johnson-Cook model is employed to describe the dynamic responses of the composite sandwiches subjected to high strain-rate loadings. Analytical models are derived correlating unit cell geometrical parameters and crushing strengths of the representative panels at different impact velocities. Parametric studies are conducted to evaluate the performances of different sandwich panel designs under impulsive loadings. In particular, transmitted reaction forces and maximum stresses on the protected structure are quantified for various design parameters including the geometrical factors and the effective Poisson’s ratios. A quarter of the panel is symmetrically modelled with shell elements and the CONWEP model is used to simulate the blast loading. Auxetic panels demonstrate interesting crushing behaviour, effectively adapting to the dynamic loading by progressively drawing material into the locally loaded zone to thereby enhance the impact resistance. Meanwhile, conventional honeycomb panels deform plastically without localised stiffness enhancement.
Four metallic metamaterials with tailorable mechanical properties are designed using bi-material star-shaped re-entrant planar lattice structures, which do not involve pins, adhesive, welding or ...pressure-fit joints and can be fabricated through laser-based additive manufacturing. Three length parameters, one angle parameter and three material combinations are used as adjustable design parameters to explore structure-property relations. For each of the four designed metamaterials, the effects of the design parameters on the Poisson’s ratio (PR), coefficient of thermal expansion (CTE), Young’s modulus and relative density are systematically investigated using unit cell-based finite element simulations that incorporate periodic boundary conditions. It is found that the bi-material lattice structures can be tailored to obtain 3-D printable metallic metamaterials with positive, near-zero or negative PR and CTE together with an uncompromised Young’s modulus. In particular, it is shown that metamaterial # 1 can exhibit both a negative PR and a non-positive CTE simultaneously. These metallic metamaterials can find applications in structures or devices such as antennas and precision instruments to reduce thermomechanical stresses and extend service lives.
