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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.
•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.
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.
•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.
An auxetic metamaterial is a type of mechanical metamaterial that has a negative Poisson's ratio. Most auxetic metamaterials are truss-based or originate from Boolean operations of simple geometries. ...Herein, we introduce a new 3D auxetic metamaterial that is mathematically generated from an implicit expression. Further, this metamaterial is fabricated by 3D printing using a flexible material, which allows it to recover from large deformations. The buckling-induced auxetic behavior of the metamaterial was first evaluated via compression tests and finite element analyses. A nickel layer was then plated onto the surface to enhance its stiffness, strength, and conductivity without loss of auxeticity and resilience. The integration of 3D printing and electroless plating enabled accurate control over the mechanical and conduction properties of the auxetic metamaterial; these properties are presented as contour maps for guidance in functional applications.
We propose a novel 3D auxetic metamaterial derived from a mathematically defined triply periodic minimal surface. The stiffness, strength, and conductivity of the metamaterial are enhanced by nickel plating without loss of auxeticity and resilience. The effective mechanical and conduction properties were mapped against geometric parameters, including relative density and nickel layer thickness. These data maps provide insight for tuning its performance over a broad range.
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•We propose a novel 3D auxetic metamaterial derived from a mathematically defined triply periodic minimal surface.•Nickel plating enhanced the stiffness, strength, and conductivity of the metamaterial without the auxeticity and resilience.•The relationships between geometric parameters and material properties of the metamaterial were discussed.•These relationships provide insight into tuning its performance over a broad range.
It is generally acknowledged that the indentation resistance or hardness of auxetic materials is higher than that of their conventional counterparts under elastic deformation. However, this property ...of the auxetic material may not always be superior to that of the non-auxetic materials when the deformation is relatively large with plasticity considered. In this study, we come up with an index to quantitatively depict the indentation resistance of the hexagonal honeycombs under large deformation. The indentation resistance of both the auxetic and non-auxetic hexagonal honeycombs is compared and discussed. Results show that in the premise of honeycombs possessing the same relative density, the indentation resistance of auxetic hexagonal honeycombs is not always higher than that of the non-auxetic honeycombs. This phenomenon is verified by the numerical simulations. Further analysis shows that there is a critical value of the absolute value of Poisson’s ratio, which is determined by the cell-wall length ratio, to estimate the higher indentation resistance between the auxetic and non-auxetic hexagonal honeycombs. The influence of indentation velocity is also analyzed based on numerical simulations. This present work is supposed to shed light on the design and evaluation of the indentation resistance for both auxetic and conventional honeycombs.
A hierarchical octet-truss lattice material was proposed by replacing the solid strut of the octet-truss structure with a tubular re-entrant structure. The tubular re-entrant structure was ...constructed via a sheet rolling operation on a planar hexagonal re-entrant structure. Hierarchical octet-truss lattice materials with different relative densities were fabricated via additive manufacturing by adjusting the re-entrant structure rib thickness, and their mechanical properties were studied and compared with the finite element model predictions and shown to be in excellent agreement with each other. The effects of hierarchical ranks on the mechanical properties of structures were also discussed. Results showed that the Poisson’s ratio of the hierarchical octet-truss structure is approximately 0.3 without being affected by the introduction of structural hierarchy. The stress-strain relationships of the hierarchical octet-truss structures exhibit dual-peak characteristic. The lattice stiffness, compressive strength, and energy absorption capacity of the models increased with an increasing rib thickness of the re-entrant structure of the first-order hierarchical octet-truss structure, and the maximum value of specific energy absorption (SEA) was achieved when the relative density is 30.2%. The second-order hierarchical octet-truss structure exhibited lower collapse stress value, and a shorter stress plateau region than those of the zeroth and first-order hierarchical octet-truss structures.
•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.
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.
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•Deflection behaviour of existing auxetic re-entrant beams compared with traditional beam designs.•Overlooked factors of placement and orientation of auxetic clusters utilized for ...enhancement.•Two new improved auxetic beam designs proposed with minimal deflection and 64% reduction in material.•Usefulness of the proposed designs brought out by application into lightweight foot bridge design.
Auxetic materials have gained popularity in engineering applications owing to their unique deformation response mechanism. However, they have not been exploited to their full potential in engineering load bearing applications. The current paper, therefore, is focused on exploring and improving the deflection behaviour of auxetic beam structures. Initially, a single re-entrant unit cell and an array of auxetic clusters are modelled using Finite Element Method (FEM). These numerical models are then verified for its Poisson’s ratio and deflection behaviour using theoretical formulations. Subsequently, in the next phase, a comparison of the deflection characteristics of the in-use common beams with that of the conventional auxetic beam design is carried out. Much overlooked factors such as orientation and placements of auxetic clusters are introduced in beam designs and are exploited to improve the deflection characteristics of conventional auxetic beams. Through this assessment, the paper proposes two novel design concepts of Oriented Re-entrant Structures (ORS) and Assorted Re-entrant Structures (ARS) for improved load bearing response. Novel designs of ORS and ARS beams are observed to perform significantly better than the conventional auxetic and honeycomb beams. The newly proposed beam designs exhibit a 64% reduction in mass in comparison to the homogeneous beam. The usefulness of these designs are brought out by introducing the ARS auxetic beams into a real-world lightweight foot bridge design. The bridge designs with ARS cross beams demonstrates a better behaviour in comparison to the bridge designs with conventional cross beams in terms of both deformation and material usage. This work highlights the potential use of unconventional mechanical metamaterial structures in engineering load bearing problems to address the demands of green engineering and sustainability without compromising on its structural integrity.
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