•An analytical model was established and analyzed for the first time.•The design was verified with experiments and finite element analysis.•The tailorability of mechanical properties was demonstrated ...for the structure.•Manufacturing induced mechanical property issues were discussed.
In this work, an analytical model of a 3D re-entrant honeycomb auxetic cellular structure has been established based on both a large deflection beam model and a Timoshenko beam model. Analytical solutions for the modulus, Poisson’s ratios and yield strength of the cellular structure in all principal directions were obtained, which indicate a wide range of mechanical property control via geometrical designs. The results were compared with experimentation and finite element analysis, and it was verified that the analytical model provides a convenient and relatively accurate method in the prediction of the performance for the auxetic cellular structures once the manufacturing related factors are adequately incorporated into the model. It was also found that the model provides less accurate predictions when higher-order coupling effects such as warp locking becomes significant under lower structural symmetry.
•Both piece-wise FG configurations and negative Poisson’s ratios are considered.•The (20/-20/20)S CNT/PmPV laminated beam has the maximum NPR ν13e.•The (70/-70/70)S CNT/PmPV laminated plate has the ...maximum NPR ν23e.•NPR has a significant effect on the nonlinear bending responses of CNTRC laminated beams.
Nanocomposite materials, such as carbon nanotube-reinforced composites (CNTRCs), have emerged as a novel engineering material. They have received growing attentions in various engineering sectors. The fabrication process has also offered the possibility to design and make this type of material to have desired features, such as being functionally graded (FG) or/and having negative Poison’s ratio. This paper reports an investigation on the nonlinear flexural behavior of auxetic laminated beams with each layer is made of CNTRC. Each layer may have different CNT volume fractions and the functional grading occurs in the thickness direction of the beam in the piece-wise pattern. The extended rule of mixture model is used to evaluate the temperature-dependent material properties of CNTRCs. The governing equations for the nonlinear bending of FG-CNTRC laminated beams are derived based on the high order shear deformation beam theory. These equations include the geometrical nonlinearity in the von Kármán sense and take into account the thermal effect and the beam-foundation interaction. The nonlinear bending solutions can be obtained by employing a two-step perturbation approach. The nonlinear flexural responses of FG-CNTRC laminated beams under a uniform pressure in thermal environments are revealed and examined in details through a parametric study. Results showed that the negative Poisson’s ratio has a significant impact on the nonlinear flexural behavior of CNTRC laminated beams.
A novel computational framework for designing metamaterials with negative Poisson’s ratio over a large strain range is presented in this work by combining the density-based topology optimization ...together with a mixed stress/deformation driven nonlinear homogenization method. A measure of Poisson’s ratio based on the macro deformations is proposed, which is further validated through direct numerical simulations. With the consistent optimization formulations based on nonlinear homogenization, auxetic metamaterial designs with respect to different loading orientations and with different unit cell domains are systematically explored. In addition, the extension to multimaterial auxetic metamaterial designs is also considered, and stable optimization formulations are presented to obtain discrete metamaterial topologies under finite strains. Various new auxetic designs are obtained based on the proposed framework. To validate the performance of optimized designs, a multiscale stability analysis is carried out using the Bloch analysis and rank-one convexity check. As demonstrated, short and/or long wavelength instabilities can occur during the loading process, leading to a change of periodicity of the microstructure, which can affect the performance of an optimized design.
•Novel computational framework for design of nonlinear auxetic metamaterials at finite strains is presented.•Nonlinear homogenization at finite strains is consistently incorporated in the density-based topology optimization.•Optimization formulations with single and multiple hyperelastic phases are considered.•New single and multimaterial auxetic metamaterials designs are discovered.•Multiscale micro and macro stability issues are addressed in the context of metamaterials design.
In this paper, rigid polyurethane (PU) foam was filled into hollow auxetic tube for enhancing the energy absorption. The rigid PU foams and stainless-steel auxetic tube with different geometrical ...parameters were manufactured, respectively. They were then assembled to form composite foam-filled auxetic tube (FFAT). Four auxetic tubes and four FFATs were respectively tested to examine the enhancement of energy absorption. Tubular types and the effects of parameters including wall thickness, and ellipticity, on FFATs were analyzed numerically by using the validated models. The results show that the overall absorbed energy of FFAT is larger than the sum of single foams and hollow auxetic tube under compression. The geometrical parameters of wall thickness, ellipticity, have a considerable effect on the structural deformation mode and energy absorption. These findings could promote the applications of auxetic composite structures in protective engineering.
•The first polyurethane foam-filled auxetic tubes (FFAT) are designed, fabricated and investigated.•The proposed FFATs could exhibit superior mechanical properties under axial compression.•The findings are useful to guide the design of advanced tubular structures for energy absorption.
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Artificial auxetic materials 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.
Auxetic textiles are intriguing materials with unusual capabilities. These materials exhibit negative Poisson’s ratio with extraordinary performance in toughness, resilience, shear resistance, and ...acoustic properties mainly due to their special structure and associated deformation mechanics. Auxetic materials can also have applications around energy absorption, and they can effectively be used for vibration damping and shock absorbency. The exceptional behavior of auxetic textiles can be obtained by utilizing specific fibrous materials and introducing auxetic geometry and structures differently during weaving, knitting, and nonwoven manufacturing. This review highlights the fundamental aspects of various auxetic structures and their properties in general and a detailed analysis of auxetic textile structures and composites, their manufacturing processes, characterization, and applications. These materials have the potential to revolutionize their applications in sports, automotive, construction industry, biomedical engineering, aerospace, marine, and defense personal protective equipment. Fundamental understanding of auxetic geometry, followed by developing and analyzing these geometries by analytical and computational modeling, translating the geometry into appropriate textile structures for actual fabric production, characterization of auxetic fabrics and their composites, and finally some innovative applications in technical textiles are some of the fascinating issues addressed in this review.
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.
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.
In order to comprehensively understand the dynamic response of auxetic honeycombs, theoretical analysis are conducted to predict the NPR effect and the crushing stress of the re-entrant hexagonal ...honeycomb. The honeycomb’s crushing stress is a function of the cell’s geometric parameters, crushing velocity and the mechanical property of the cell-wall material. Results show that the crushing stress enhances with the increasing crushing velocity. A dynamic sensitivity index is employed to quantitatively evaluate this enhancement. It is shown that small cell-wall angle, low relative density or high cell-wall length ratio of the honeycomb attribute high velocity-sensitivity to the crushing stress. The Poisson’s ratio of the re-entrant honeycomb is also expressed as a function of the cell’s geometric parameters. It is revealed that the NPR effect enhances with the increasing cell-wall angle and the decreasing cell-wall length ratio. All the theoretical predictions are verified by numerical simulations. Besides, an interesting phenomenon is noticed that the crushing velocity has significant influence on the honeycomb’s NPR effect at the early stage of crushing. However, this influence almost vanishes when the overall strain is larger than about 0.2. This present work is supposed to shed light on the design of the auxetic honeycomb.
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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.
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