Nowadays, sandwich plates with cellular core structures are gaining the attention of researchers owing to their outstanding features. For a better insight into auxetic structures, in this study, we ...propose an excellent computational approach based on polygonal meshes to comprehensively examine the free vibration, buckling and dynamic instability behaviors of the auxetic honeycomb sandwich plate structures. A generalized C0-type higher-order shear deformation theory (C0-HSDT) in conjunction with both Laplace and quadratic serendipity shape functions is employed to approximate the strain fields for polygonal plate elements. The sandwich plate structures are constituted by an auxetic honeycomb core layer with negative Poisson’s ratio and two skin layers reinforced by graphene nanoplatelets (GNPs). Ultra-light features of the plate structures can be obtained by using the auxetic honeycomb cells with negative Poisson’s ratio while the GNPs are embedded into skin layers to enhance the structural stiffness. In order to determine the dynamic instability region of the sandwich plate, Bolotin’s approach is utilized in the current research. Several numerical examples are carried out to investigate the influences of geometrical parameters of auxetic cell and GNPs reinforcement on the structural behaviors. The results obtained in the current research can be considered as benchmark ones to investigate auxetic sandwich plate structures.
Auxetic materials with negative Poisson’s ratio have potential applications across a broad range of engineering fields. Several FE based design techniques have been developed to achieve auxetic ...materials with targeted effective properties, mostly in linear deformation regime. In this paper, an isogeometric shape optimization framework for designing 2D auxetic materials with prescribed deformation over large strain intervals is presented. Taking into account practical manufacturing considerations, a minimum thickness for each member is imposed via a spline-based geometric constraint. The capability of the framework is demonstrated through two examples. First, the paper considers shape optimization of smoothed hexa-petals in plane strain condition to achieve constant Poisson’s ratios ranging from a null value to -0.5 to an effective tensile strain of 50%. The second example showcases the shape optimization of smoothed tri- and hexa-petals in plane stress condition for targeted nonlinear deformation behaviour of cat’s skin up to 90% tensile strain.
•Shape optimization with IGA for smoothed petals with targeted deformation.•Minimum member thickness enforced with a spline based constraint.•Constant negative Poisson’s ratio achieved for up till 50% effective tensile strain in plane strain.•Replicated nonlinear deformation of cat’s skin up to 90% tensile strain.
Negative Poisson’s ratio structure is one of the international research hotspots, but there are still few reports on negative Poisson’s ratio combination structure. In this paper, two single cell ...structures (concave hexagonal structure and star shape structure) with connection beam and three nine-grid combination structures (X-type, H-type, Sd-type) were designed. To obtain the negative Poisson’s ratio mechanics, based on the Euler beam theory, the theoretical calculation method of single cell structure was established. By comparing the critical force, the sequence of bending and stretching/compression deformation was discussed. The mechanical properties of multi-cell structures were predicted by adopting the mixed law. The results show that the main deformation of the designed structures are bending, and the instability buckling occurs during compression. It is proved that the mechanical properties of multi-cell structures can be characterized by its single cell structure. The combination structure integrates the negative Poisson’s ratio mechanical properties of different single cell structures. An important application is that the mechanical properties of combination structures can be obtained by changing the proportion of single cell structures without changing the structure size.
•In-plane elastic mechanics of negative Poisson’s ratio structure.•Concave hexagon, star shape single-cell and multi-cell structure.•Nine-grid multi-cell combination structure.•Deformation modes in compression and stretching.•Derivation of equivalent elastic mechanical parameters.
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•We propose an inverse design method for auxetic metamaterials using deep learning.•We designed novel 2D auxetic metamaterials based on Voronoi tessellation for the training ...dataset.•The trained neural network can generate 2D auxetic metamaterials with user-desired Young’s moduli and Poisson’s ratios.•The proposed method can easily be extended to the inverse design of other architected materials.
As typical mechanical metamaterials with negative Poisson’s ratios, auxetic metamaterials exhibit counterintuitive auxetic behaviors that are highly dependent on their geometric arrangements. The realization of the geometric arrangement required to achieve a negative Poisson’s ratio relies considerably on the experience of designers and trial-and-error approaches. This report proposes an inverse design method for auxetic metamaterials using deep learning, in which a batch of auxetic metamaterials with a user-defined Poisson’s ratio and Young’s modulus can be generated by a conditional generative adversarial network without prior knowledge. The network was trained based on supervised learning using a large number of geometrical patterns generated by Voronoi tessellation. The performance of the network was demonstrated by verifying the mechanical properties of the generated patterns using finite element method simulations and uniaxial compression tests. The successful realization of user-desired properties can potentially accelerate the inverse design and development of mechanical metamaterials.
•A simple methodology for generating auxetic metamaterials with tuneable stiffness is proposed and verified.•The stiffness of the metamaterial could be tuned by using one single parameter of VSF ...(Variable Stiffness Factor).•The proposed auxetic metamaterials have considerable strength, stability and auxetic behavior.•The potential applications and opportunities of the novel auxetic metamaterial are discussed.
Auxetic materials have attracted a considerable attention due to their excellent properties, e.g., fracture resistance, shear resistance, energy dissipation, etc. However, the stiffness of auxetics tends to be much weaker than solid structure because of the existence of internal holes. Inspired by tuning the compacted point of auxetic structures to enhance their stiffness, a systematic methodology for defining a single parametric of variable stiffness scale factor (VSF) to generate auxetic unit cell with variable stiffness has been proposed and verified in this study. Two models with different VSF proportions were investigated experimentally. Different centres of rotation, heights of deformation area, and VSF percentages were analyzed to prove the effectiveness of the method numerically. The results indicate that the compacted strain can be tuned effectively using the designed VSF proportions, and the difference between the designed VSF and real VSF could be reduced by slightly changing the height of deformation area. These desirable characteristics provide a new idea for the optimal design of auxetics, with potential application in protective structures.
•A metastructure is devised to obtain tailorable thermal expansion and tunable Poisson's ratio.•Analytical expressions for thermal expansion and Poisson's ratio are established and numerically ...validated.•The metastructure can give paired negative thermal expansion and negative Poisson's ratio.•Tailorable thermal expansion and Poisson's ratio are highly coupled with common parameters.
Current reported cellular metastructures can either achieve only tailorable thermal expansion or obtain only tunable Poisson's ratio. By contrast, here, we develop a kind of lightweight cellular metastructure which incorporates coupled tailorable thermal expansion and tunable Poisson's ratio. That is a wide range of positive, zero and especially negative values of both thermal expansion and Poisson's ratio can be simultaneously obtained. The ranges and constraints of the geometrical parameters are revealed, and the relative densities are only about 2%, indicating excellent lightweight character. Besides, analytical expressions for coefficient of thermal expansion (CTE) and Poisson's ratio (PR) are theoretically established and numerically simulated. Parameter analysis confirms that the range of tailorable CTE can be enhanced through rationally selecting large values of CTE ratio, first geometrical angle and height ratio. By adjusting the second and third geometrical angles, Poisson's ratio also can be tuned to be large negative, near zero and positive values. Particularly, the metastructure can give paired negative CTE and negative PR. Different combinations of paired characteristics including positive CTE + negative PR, positive CTE + positive PR and negative CTE + positive PR are also flexibly available. Moreover, CTE and PR are found to be highly coupled. To simultaneously obtain specific CTE and PR, design parameters should be selected with consideration of the coupling effect. The results here are expected to contribute feasibility to structures with both temperature and mechanical sensitivities.
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•An assembled auxetic chiral honeycomb (AACH) is designed and manufactured by 3D printing.•The AACH could exhibit lower peak force and high stress plateau.•The external energy is ...mainly absorbed by the vertical wave plate.•The AACH provides a new idea for the industrial manufacturing of auxetics.
Auxetic metamaterials tend to be fabricated using additive manufacturing and laser cutting due to their special porous microstructures. Although the auxetic metamaterials have many applications currently, the high cost and the low efficiency of available manufacturing are adverse to expend their application range. In this work, the auxetic chiral honeycomb is assembled crosswise using the slotted wave plate. Effects of wave radius, plate thickness, slot percentage, and base material on the Poisson’s ratio and mechanical performance are explored experimentally and numerically. The results show that assembled auxetic chiral honeycomb (AACH) exhibits lower peak force and high plateau stress than the conventional assembled one. With the increase of wave radius and plate thickness, the energy absorption (EA) and specific energy absorption (SEA) would increase. As for the different material combinations, when the base materials in vertical and horizontal wave plates adopt stainless steel and aluminum, respectively, the AACH would exhibit desirable EA, SEA, and auxetic behavior. These findings provide a new approach to the manufacture of auxetics at a low cost, which is beneficial for potential applications.
•Auxetic metallic metamaterial is designed by variable stiffness factor (VSF) method.•The variable energy factor (VEF) method is proposed.•The accuracies of actual VSF and VEF are discussed under ...different loading speed.•Layered gradient design can improve mechanical properties of the metamaterials.
Auxetic metamaterials have attracted increasing attention due to their exceptional mechanical properties. However, the critical parameters of mechanical response and Poisson's ratio would be changed simultaneously when a geometrical parameter is tuned, which is adverse to achieving the quantitative design of energy absorption by tuning a single geometrical parameter. Thus, the methodology based on tuning densification strain is proposed to design auxetic unit cells with tunable stiffness. In this study, the static performance of 2D metallic auxetic metamaterials designed by the variable stiffness factor (VSF) method is examined experimentally and numerically. To further achieve tunable energy absorption under crushing load, the concept of VSF is extended to variable energy factor (VEF). The dynamic response of verified numerical models is investigated subjected to low-, medium-, and high-velocity crushing. Finally, a functionally graded auxetic structure with different design layers is proposed to effectively solve the issues of high stiffness ratio and initial peak force. These results show that the designed structure has the actual VSF and VEF percentages close to the designed value under low- and medium-velocity crushing. The findings from this study are useful for wider applications of auxetics in protective engineering.
Cellular auxetic materials possess a negative Poisson’s ratio (NPR) and attractive mechanical properties; however, these materials are limited by their low stiffness and strength and strong ...anisotropy. To address these issues, we developed a novel auxetic honeycomb with a fully triangular architecture inspired by the anti-tetra chiral configuration. The new stretch-dominant structure was fabricated using a three-dimensional printing process. The resulting deformation behavior of the triangular auxetic honeycomb structure was compared with that predicted from finite element (FE) simulations; these revealed an NPR and high stiffness, in which the ratio of specific stiffness to specific density was 0.12. The simulations allowed for an NPR in both the longitudinal and transverse directions concurrently. Based on the mechanism revealed by the FE simulations, the novel triangular honeycomb can be tuned to have the same behavior in two orthogonal directions, while retaining high stiffness and an NPR.
•We propose the design of an isotropic porous structure with negative Poisson’s ratio.•Isotropy is due to the threefold symmetry of the arrangement of the voids.•Poisson’s ratio is determined ...experimentally through Digital Image Correlation.•The experimental findings match very well with the results of numerical simulations.•Periodic analysis is employed to determine the effective properties of the structure.
This paper proposes the design of a two-dimensional porous solid with omni-directional negative Poisson’s ratio. The hexagonal periodic distribution of the pores makes the effective behavior isotropic. Both experimental tests and numerical simulations have been performed to determine the effective properties of the porous solid. A parametric study on the effect of the geometrical microstructural parameters is also presented. This auxetic structure is easy to fabricate and can be very useful in several engineering applications.
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