Functionally graded porous structures (FGPSs) are attracting increasing interest in the manufacture of prostheses that benefit from lower stiffness and optimized pore size for osseointegration. In ...this work, we explore the possibility of employing FGPSs with auxetic unit cells. Their negative Poisson's ratio was exploited to reduce the loss of connection between prosthesis and bone usually occurring in standard implant loaded under tension and therefore undergoing lateral shrinking. In addition, to further improve osseointegration and mitigate stress shielding effects, auxetic FGPSs were fabricated in this work using a novel β-Ti21S alloy characterized by a lower Young's modulus compared to traditional α + β Ti alloys. Specifically, two different auxetic FGPSs with aspect ratio equal to 1.5 and angle θ of 15° and 25° with a relative density (ρ
) gradient of 0.34, 0.49, 0.66 and of 0.40, 0.58, 0.75 were designed and printed by laser powder bed fusion. The 2D and 3D metrological characterization of the as-manufactured structures was compared with the design. 2D metrological characterization was carried out using scanning electron microscopy analysis, while for the 3D characterization, X-ray micro-CT imaging was used. An undersizing of the pore size and strut thickness in the as-manufactured sample was observed in both auxetic FGPSs. A maximum difference in the strut thickness of -14 and -22% was obtained in the auxetic structure with θ = 15° and 25°, respectively. On the contrary, a pore undersizing of -19% and -15% was evaluated in auxetic FGPS with θ = 15° and 25°, respectively. Compression mechanical tests allowed to determine stabilized elastic modulus of around 4 GPa for both FGPSs. Homogenization method and analytical equation were used and the comparison with experimental data highlights a good agreement of around 4% and 24% for θ = 15° and 25°, respectively.
This research study involves designing and optimizing a sandwich structure based on an auxetic structure to protect the pouch battery system for electric vehicles undergoing ground impact load. The ...core of the sandwich structure is filled with the auxetic structure that has gone through optimization to maximize the specific energy absorbed. Its performance is analyzed with the non-linear finite element method. Five geometrical variables of the auxetic structures are analyzed using the analysis of variance and optimized using Taguchi’s method. The optimum control variables are double-U hierarchal (DUH), the cross-section’s thickness = 2 mm, the length of the cell = 10 mm, the width of the cell = 17 mm, and the bending height = 3 mm. The optimized geometries are then arranged into three different sandwich structure configurations. The core is filled with optimized DUH cells that have been enlarged to 200% in length, arranged in 11 × 11 × 1 cells, resulting in a total dimension and mass of 189 × 189 × 12 mm and 0.75 Kg. The optimized sandwich structure shows that the pouch battery cells can be protected very well from ground impact load with a maximum deformation of 1.92 mm, below the deformation threshold for battery failure.
Mass production of re-entrant cubic auxetic structure Balaji, B; Gupta Burela, Ramesh; Ponniah, Ganeshthangaraj
IOP conference series. Materials Science and Engineering,
02/2021, Letnik:
1070, Številka:
1
Journal Article
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The unique characteristics of auxetic materials has attracted significant attention in the research domain. Its distinguishing negative Poisson's ratio has found its way to targeted applications ...where normal materials fail to deliver the expected results. The manufacturing of macro sized auxetic structures has always been a challenge. This paper proposes the possibility for mass production of auxetic structures. Materials available in standard shapes are utilized to manufacture the auxetic structure. A new re-entrant cubic auxetic structure is developed, analysed and fabricated. This auxetic structure provides a better scope for mass production.
Meta-sandwich composites with three-dimensional (3D) printed architecture structure are characterized by their high ability to absorb energy. In this paper, static and fatigue 3-point bending tests ...are implemented on a 3D printed sandwich composites with a re-entrant honeycomb core.
The skins, core and whole sandwich are manufactured using the same bio-based material which is polylactic acid with flax fiber reinforcement. Experimental tests are performed in order to evaluate the durability and the ability of this material to dissipate energy. First, static tests are conducted
to study the bending behaviour of the sandwich beams, as well as to determine the failure parameters and the characteristic used in fatigue tests. Then, fatigue analyses were carried out to determine the fatigue resistance of these structures. The effects of the core density on the stiffness,
hysteresis loop, energy absorption and loss factor, for two loading level, are determined. Moreover, the behaviour of this sandwich composite with re-entrant honeycomb core is compared with that of sandwiches with different core topologies. The results show that sandwich with high core density
dissipate more energy, which results higher loss factors. The determined properties offer the most sensitive indicators of sandwich composite damage during its lifetime. This work aims to determine the static and fatigue properties of this material, thus, study its potential applications in
industry.
Traditional approaches used for analyzing the mechanical properties of auxetic structures are commonly based on deterministic techniques, where the effects of uncertainties are neglected. However, ...uncertainty is widely presented in auxetic structures, which may affect their mechanical properties greatly. The evidence theory has a strong ability to deal with uncertainties; thus, it is introduced for the modelling of epistemic uncertainties in auxetic structures. For the response analysis of a typical double-V negative Poisson’s ratio (NPR) structure with epistemic uncertainty, a new sequence-sampling-based arbitrary orthogonal polynomial (SS-AOP) expansion is proposed by introducing arbitrary orthogonal polynomial theory and the sequential sampling strategy. In SS-AOP, a sampling technique is developed to calculate the coefficient of AOP expansion. In particular, the candidate points for sampling are generated using the Gauss points associated with the optimal Gauss weight function for each evidence variable, and the sequential-sampling technique is introduced to select the sampling points from candidate points. By using the SS-AOP, the number of sampling points needed for establishing AOP expansion can be effectively reduced; thus, the efficiency of the AOP expansion method can be improved without sacrificing accuracy. The proposed SS-AOP is thoroughly investigated through comparison to the Gaussian quadrature-based AOP method, the Latin-hypercube-sampling-based AOP (LHS-AOP) method and the optimal Latin-hypercube-sampling-based AOP (OLHS-AOP) method.
Materials with a negative Poisson's Ratio (PR), known as auxetics, exhibit the counterintuitive behavior of becoming wider when uniaxially stretched and thinner when compressed. Though negative PR is ...characteristic of polymer foams or cellular solids, tight as well as highly porous rocks have also been reported to exhibit negative PR. The paper proposes a novel auxetic structure based on pore-space configuration observed in rocks. We developed a theoretical auxetic 3D model consisting of rotating rigid bodies. To alleviate the mechanical assumption of rotating bodies, the theoretical model was modified to include crack-like features being represented by intersecting, elliptic cylinders. We then used a 3D printer to create a physical version of the modified model, whose PR was tested. We also numerically explored how the compressibility of fluids located in the pore-space of the modified model as well as how the elastic properties of the material from which the model is made of affect its auxetic behavior. We conclude that for a porous medium composed of a single material saturated with a single fluid (a) the more compliant the fluid is and (b) the lower the PR of the solid material, the lower the PR value of the composite material.
In this paper, the wave propagation in phononic crystal composed of auxetic star-shaped honeycomb matrix with negative Poisson’s ratio is presented. Two types of inclusions with circular and ...rectangular cross sections are considered and the band structures of the phononic crystals are also obtained by the finite element method. The band structure of the phononic crystal is affected significantly by the auxeticity of the star-shaped honeycomb. Some other interesting findings are also presented, such as the negative refraction and the self-collimation. The present study demonstrates the potential applications of the star-shaped honeycomb in phononic crystals, such as vibration isolation and the elastic waveguide.
Abstract
In this study, the bending response of 2D-lattice plates having auxetic unit cells is determined by using the homogenization method based on equivalent strain energy. The auxetic unit cells ...in this study are different topologies of re-entrant hexagonal unit cells. In the homogenization method, the effective out-of-plane elastic properties of an auxetic 2D-lattice plate are obtained from the strain energy values of its unit cell under different curvature modes, determined by the finite element method. In the analysis, the auxetic unit cells are considered as frames whose struts are modeled as Euler beams. The effective elastic properties, i.e., the effective bending moduli, Poisson’s ratios, and shear modulus, are described as the bending response of a 2D-lattice plate subjected to out-of-plane bending. In the validation, the effective elastic properties of some auxetic 2D-lattice plates obtained from their unit cells by the homogenization method are numerically compared with those obtained from direct structural analysis of the plates. Besides, the obtained results show how the bending response of the auxetic 2D-lattice plates can be adjusted by varying their unit-cell geometries, especially the internal cell angle.
The growing demand for tyres in transport industry provides a negative impact on the environment, due to the problems related to recycling and disposal of the rubber waste. However, adding waste ...tyres as a dispersive phase in polymeric composites has shown high toughness and moderate increase of mechanical strength within the composites. In this work, we investigate the use of recycled rubber core to manufacture auxetic (re‐entrant) honeycomb structures. The re‐entarnt honeycombs produced are evaluated using finite element analysis and tensile in‐plane to assess the sensitivity of the mechanical properties versus the cell geometric parameters. A full factorial design (33) was performed to investigate the effects of the factors width (10 and 20 mm), thickness (2 and 4 mm) and angle (−10 and −20°) on Poisson's ratio of honeycomb structures. The results give further evidence that the Poisson's ratio is affected not only by individual factors but also by interaction originated by the other geometric parameters.
Materials and microstructures with specific configurations are able to have negative Poisson’s ratio. This paper proposes a topology optimization methodology of frame structures to design a planar ...periodic structure that exhibits negative Poisson’s ratio. Provided that beam section of each existing member is chosen from a set of some given candidates, we can reduce the topology optimization problem to a mixed-integer linear programming problem. Since the proposed approach treats frame structures and stress constraints are rigorously addressed, the optimal solution contains no hinge region. A heuristic method with local search is used to solve large-scale problems. Numerical examples and fabrication test demonstrate that planar periodic frame structures exhibiting negative Poisson’s ratio can be successfully obtained by the proposed method.
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