Auxetic Material

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Auxetic Materials

Auxetic Materials

1) Indentation Hardness: When a non-conventional material is hit by an object, the material at the site of action flows away from that point by making the material less denser. In case of auxetic materials, when it is hit by any type impact load the material tends to flow towards the site of load acting and makes the material more denser. This phenomenon in auxetic material makes to find application in manufacturing of army jackets, which resists penetration of foreign bodies like bullets.

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Analysis of 2D Auxetic Metamaterial as a Variable Macro and Micro Structural Filter

Analysis of 2D Auxetic Metamaterial as a Variable Macro and Micro Structural Filter

Next, the auxetic material having re-entrant honeycomb structure was manufactured by casting of liquid silicon. The mold cavity was produced on 20mm thickness wooden plank. Potter’s clay was plastered inside the cavity to make the mold block. It took 12 hours for the block to dry up at room temperature. The mold block was extracted from the wooden planks by separating the two wooden pieces. The procedure was repeated again to make 10 such blocks. The top view of the auxetic structure array shown in Fig. 1 was printed on a paper using 1:1 scale. After that, the 10 blocks were stuck to the paper using Fevicol adhesive. The walls of the mold blocks were coated with talcum powder. Talcum powder helps the hot glue to flow and reach greater depths than without. The outer walls were made using talcum coated cardboard. Hot glue was poured inside the cavity. Cooling process got completed in 10 minutes. Cooling was done at room temperature. The clay blocks were removed from the cast. The casted product was cleaned and the entrapped air bubbles were removed using the hot lead of the glue gun. Touch up was done using hot glue. The dimensions used for this casting process using hot glue were h=26mm, l=13mm, ϴ=30°.

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Auxetic Behaviour of Re-entrant Cellular Structured Kirigami at The Nanoscale

Auxetic Behaviour of Re-entrant Cellular Structured Kirigami at The Nanoscale

Due to its specific characteristics, the auxetic behaviour in a re-entrant cellular structured kirigami at the nanoscale is very important to the application in nano-devices such as NEMS. It is well known that Poisson’s ratio and modulus are the most representative mechanical properties showing auxetic behaviour. In case that an auxetic material is under tensile loading, the value of its Poisson’s ratio could govern the extent of its expansion. And the modulus ratio, which is the ratio of equivalent modulus to matrix modulus, could determine the tensile strength of auxetic materials. Therefore, studies on effects of Poisson’s ratio or the modulus ratio on materials’ mechanical behaviour may provide a guidance to the design of auxetic nano-device in NEMS. Regarding the specific microstructure of kirigami, following three major factors could affect aforementioned two mechanical properties:

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Dynamic Tests for Energy Absorption by Selected Auxetic Fabrics

Dynamic Tests for Energy Absorption by Selected Auxetic Fabrics

Until now, research on auxetic fabrics of the above- mentioned structure has related to experimental and numerical studies of a single auxetic fiber (micro scale), a cell of the material (meso scale) and a fabric sample (macro scale) [1, 2, 3, 4]. The impact of the geometry and material parameters on the mechanical properties of a single auxetic fiber or an auxetic fabric was investigated. A characteristic feature was observed in the form of a strong non-linear dependence of effective Poisson’s ratios on the geometry of an auxetic material, including a sign change. Significantly high negative values of the effective Poisson’s ratio up to (–8) can be achieved by changing the fabric geometry.

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Analytic analysis of auxetic metamaterials through analogy with rigid link systems

Analytic analysis of auxetic metamaterials through analogy with rigid link systems

In recent years many structural motifs have been designed with the aim of creating auxetic metamaterials. One area of particular interest in this subject is the creation of auxetic material properties through elastic instability. Such metamaterials switch from conventional behaviour to an auxetic response for loads greater than some threshold value. This paper develops a novel methodology in the analysis of auxetic metamaterials which exhibit elastic instability through analogy with rigid link lattice systems. The results of our analytic approach are confirmed by finite element simulations for both the onset of elastic instability and post-buckling behaviour including Poisson’s ratio. The method gives insight into the relationships between mechanisms within lattices and their mechanical behaviour; as such, it has the potential to allow existing knowledge of rigid link lattices with auxetic paths to be used in the design of future buckling induced auxetic metamaterials.

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Analysis of 2D Auxetic Metamaterial as a Variable Macro and Micro Structural Filter

Analysis of 2D Auxetic Metamaterial as a Variable Macro and Micro Structural Filter

Next, the auxetic material having re-entrant honeycomb structure was manufactured by casting of liquid silicon. The mold cavity was produced on 20mm thickness wooden plank. Potter’s clay was plastered inside the cavity to make the mold block. It took 12 hours for the block to dry up at room temperature. The mold block was extracted from the wooden planks by separating the two wooden pieces. The procedure was repeated again to make 10 such blocks. The top view of the auxetic structure array shown in Fig. 1 was printed on a paper using 1:1 scale. After that, the 10 blocks were stuck to the paper using Fevicol adhesive. The walls of the mold blocks were coated with talcum powder. Talcum powder helps the hot glue to flow and reach greater depths than without. The outer walls were made using talcum coated cardboard. Hot glue was poured inside the cavity. Cooling process got completed in 10 minutes. Cooling was done at room temperature. The clay blocks were removed from the cast. The casted product was cleaned and the entrapped air bubbles were removed using the hot lead of the glue gun. Touch up was done using hot glue. The dimensions used for this casting process using hot glue were h=26mm, l=13mm, ϴ=30°.

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Quasi static characterisation and impact testing of auxetic
foam for sports safety applications

Quasi static characterisation and impact testing of auxetic foam for sports safety applications

Through testing higher energy impacts on larger samples, the work presented here has shown further potential for auxetic foam to be applied to protective sports equipment. Auxetic foam considerably outperformed its conventional counterpart, in agreement with previous work [16-17]. Future work needs to focus on comparing auxetic foam with current materials and products, utilising an improved conversion process and a range of candidate materials. A consistent process for producing homogeneous samples of sufficient size for developing prototypes is needed, so these can be benchmarked against current products and relevant standards. Testing of auxetic foam should also extend to include tissues surrogates [e.g. 31] to provide impact scenarios which are more representative of those experienced by the human body. Finite element analysis has been applied to protective sports equipment [2-3]. Material models of auxetic foam under compression have been created [32], and future work will apply and implement this technique to further our understanding of how best to utilise auxetic foam. The Poisson’s ratio and Young’s modulus responses as a function of strain specific to the test specimen materials and dimensions employed in the impact tests reported here (e.g. Figure 3b for tangent modulus) will be utilised in these modelling investigations.

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A Comparison of Bending Properties for Cellular Core Sandwich Panels

A Comparison of Bending Properties for Cellular Core Sandwich Panels

In the current work, sandwich panels with different cellu- lar cores were designed and compared. Although the 3D reticulate cellular core structures were not optimized for bending, their overall performance showed promising potential as future sandwich cores. During bending, the auxetic sandwich panels exhibited homogeneous distri- bution of stress and deformation. Failure by fracture of the vertical struts located roughly at the middle section between the loading and support rollers was seen in all cases. Future studies should therefore focus on optimiza- tion of the cellular structure based on the expected load- ing patterns. Thickening of the critical vertical struts would potentially lead to significant enhancements in material properties with relatively little increase in mass. The other sandwich structures showed significant stress concentration at the loading area, and failed by face yield.

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Computational Modeling of Honeycomb Structures with Shape Memory Alloys

Computational Modeling of Honeycomb Structures with Shape Memory Alloys

In compressive behavior simulation, the influences of structural imperfection on structure stiffness and stability have been considered. Based on random dislocation of cell joints, the following three variants of the auxetic type structure have been generated: perfect, 1% imperfect, and 10% imperfect. The initial shapes and boundary conditions of the auxetic type honeycomb structures are shown in Fig. 9 [2]. The bottom layer of the structure is fixed in all directions. Compression forces are applied on the top layer. The whole process involves a full loading and unloading cycle.

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Modeling of negative Poisson’s ratio (auxetic) crystalline cellulose Iβ

Modeling of negative Poisson’s ratio (auxetic) crystalline cellulose Iβ

Auxetic materials are attracting interest for their apparently anomalous behavior, and also because the ability to tailor materials to display negative Poisson’s ratio response can open up routes to extreme values of other properties not available for non-auxetic materi- als (Evans 1990; Greaves et al. 2011). Examples of enhanced properties as a consequence of the auxetic effect include fracture toughness (Lakes 1987), double (synclastic) curvature under pure bending (Lakes 1987; Evans 1991), indentation resistance (Alderson et al. 1994), shear rigidity, (Choi and Lakes 1992) and ultrasonic (Alderson et al. 1997) and vibration (How- ell et al. 1991; Scarpa et al. 2005) damping.

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Fabrication, characterisation and modelling of uniform and gradient auxetic foam sheets

Fabrication, characterisation and modelling of uniform and gradient auxetic foam sheets

For large area foams having thin through-thickness dimension (relative to in-plane dimensions), it becomes difficult to achieve the required in-plane compression without creasing or even folding of the foam during insertion into the compression mould. A vacuum bag and ‘half mould’ process has produced 10 mm thick foam sheets of arbitrary curvature displaying anisotropic auxetic behaviour in the plane of the sample. Negative PRs of -0.15 through the thickness and below -1 in some in-plane directions were reported for the ‘half mould’ samples [17]. Uniaxial compression between flat or curved plates, rather than in a fixed compression mould, has been used to produce surface crease-free flat and curved samples, respectively, with thickness as low as 2-3mm [31]. In this case the auxetic effect is evident through the thickness, with negative PR values as low as -3 reported, but auxetic behaviour was not observed in the plane of the converted foam.

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A comparison of novel and conventional fabrication methods for auxetic foams for sports safety applications

A comparison of novel and conventional fabrication methods for auxetic foams for sports safety applications

Poisson’s ratios were closer to zero than expected for all samples, although converted samples appear to be auxetic (Figures 2a & 2b). Samples cut from the cubic monolith exhibit high variation in VCR between samples (Figure 2c): those cut from the surface exhibit higher VCR than those cut from the centre, agreeing with previous work [6]. The surfaces of cuboidal samples fabricated without pins have a large number of prominent folds (Figure 2d). The use of pins used to control lateral compression produced samples with reduced fold area, although the pins left through-thickness holes (Figure 1d).

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Hybrid auxetic foam and perforated plate composites for human body support

Hybrid auxetic foam and perforated plate composites for human body support

The paper describes the design and testing of a hybrid auxetic foam/perforated plate structure for human body support. The foam lining is constituted by auxetic foam produced according the manufacturing parameters described in [39][41]. The negative Poisson’s ratio foam is applied to a curved flexible support characterised by perforations of rhomboidal shape (see Figure 1 and reference [18]). The use of perforation patterns allows the production of plate-like structures with in-plane auxetic behaviour, and constitutes a relatively novel and inexpensive technique to induce negative Poisson’s ratio behaviour in solids [42][43][44]. The hybrid auxetic support is subjected to bending tests through an analysis of variance (ANOVA) to explore the design space of the component, and a Finite Element simulation is carried out to identify a map relating the dimensions of the rhomboidal perforation versus the optimized stiffness of the plate.

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Controlling density and modulus in auxetic foam fabrications— implications for impact and indentation testing

Controlling density and modulus in auxetic foam fabrications— implications for impact and indentation testing

Sporting PPE is a healthy, growing market that can respond to evidence from the scientific community, seen in the previous example (where the increase in helmet sales comes from increased awareness of head injury) [11]. Design and innovation can also increase market share for manufacturers. Innovations include non-Newtonian fluids for PPE (which can pass certification tests without a stiff shell [13]) and slip plane technology in helmets [14], intended to decrease rotational acceleration and concussion risk. The case for improving protection through product and material innovation is ethically and financially viable and justified, publically (to reduce costs and burdens) and commercially (to increase market share and prepare for possible changes in standards).

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Structural Design, Optimization and Application of 3D Re-entrant Auxetic Structures.

Structural Design, Optimization and Application of 3D Re-entrant Auxetic Structures.

Scarpa et al. tested auxetic foams made from PU under harmonic cyclic loading, and discovered that the storage modulus of an auxetic foam was significantly lower than that of the conventional foams for pure compressive-tensile cycles. On the other hand, when pre- deformation was applied, the storage modulus of auxetic PU foam significantly increases whereas with the regular PU foam the storage modulus did not vary much. This result indicates that the energy absorption of auxetic PU foam can be tailored in a very wide range up to several magnitudes of the regular structures. In addition, the storage modulus value is also stable over the testing frequency for auxetic foams, which indicates a higher fatigue strength and a stable energy dissipation rate over the frequencies [96, 97]. The storage modulus is a direct indicator of the energy dissipation for polymer materials, therefore the study showed that the energy absorption for polymer auxetic structures could be controlled through the manner of pre- deformation. Upon dynamic crushing at high strain rate, the auxetic structures exhibit much higher resilience compared with the regular structures. This was thought to be contributed by the fact that during the manufacturing process, the auxetic foam struts were bent and therefore could be readily deformed and lead to the formation of localized crushing bands [97, 98].

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A systematic typology for negative Poisson's ratio materials and the prediction of complete auxeticity in pure silica zeolite JST

A systematic typology for negative Poisson's ratio materials and the prediction of complete auxeticity in pure silica zeolite JST

Class A and class C can be determined unambiguously by the minimum or maximum ν around a single axis (if maximum ν is negative then class C, if minimum ν is negative then class A). Class B relies on ν being averaged, and different options for this are possible. The average could be considered as a direct mean (if the mean ν is negative then class B is satisfied), or a simpler median average (if more than half of the ν values are negative then class B is satisfied). When many single crystals are arranged in a random orientation, it is the average of the Poisson's ratio values that will affect the polycrystalline properties 20 rather than the total number of direction exhibiting a particular Poisson's ratio in each single crystal. Therefore, in this paper the mean Poisson's ratio is used to ascertain if a material satisfies class B, as this will give a greater idea as to which are likely to exhibit auxetic behaviour in a polycrystalline structure.

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Auxetic foam for snowsport safety devices

Auxetic foam for snowsport safety devices

Composites pads - consisting of a 10 x 90 x 90 mm or 20 x 90 x 90 mm foam sample covered with a 4 x 90 x 90 mm polypropylene (PP) sheet (Direct Plastics, PPH/PP- DWST-Homopolymer) - were investigated as a simple model of body armour. The larger pads had similar thickness to commercially available back protectors (Schmitt et al., 2010). To help ensure uniform compression and temperature and the production of homogenous samples, auxetic foam sheets were converted individually at the required thickness, rather than converting and then slicing a cube (Duncan et al. under review). Foam (R30RF and R60RF) samples (15 x 143 x 143 mm and 30 x 143 x 143 mm) were compressed to 70% of their original size along each dimension in a mould, resulting in a VCR of 3. The heating phases at 180°C were 25 minutes long. After one week, a 5 mm wide strip was cut from each side of the foam cuboids to leave 10 x 90 x 90 mm or 20 x 90 x 90 mm samples. Thirteen samples were fabricated, six 10 mm thick and seven 20 mm thick. Two 10 x 90 x 90 mm and two 20 x 90 x 90 mm samples of unconverted foam (R60RF) were cut from a monolith for comparative testing.

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Large scale extrusion of auxetic polypropylene fibre

Large scale extrusion of auxetic polypropylene fibre

The causal mechanism for this was based on its complex microstructure, which consisted of nodules interconnected by fibrils. Depending on the connectivity of such micro- structures, deformation can predominantly be either by ro- tation of the fibrils (or ligaments) [7] or rotation of the nodules [7, 8] in response to a mechanical load. In the case of microporous PTFE, fibril rotation (‘hinging’) causing the nodules to translate was shown to describe the auxetic effect observed experimentally at low and intermediate tensile strain very well [7]. A secondary simultaneous mechanism of fibril stretching in the same microstructural connectivity accounted for the transition to positive Pois- son’s ratio at high tensile strain [9]. This combined mecha- nism was also found to describe the full strain-dependent Young’s modulus behaviour very well. Nodule rotation was additionally shown to lead to a reduction in magnitude of negative Poisson’s ratio at higher strain [7] – but this re- quired an alternative connectivity to the low/intermediate strain microstructure and did not produce the observed positive Poisson’s ratio at high strain.

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Auxetic Foams for Sport Safety Applications

Auxetic Foams for Sport Safety Applications

Auxetic foam - fabricated by converting open-cell foam with a compression and heat treatment process [2-4] – was investigated alongside its conventional counterpart. Quasi-static compression was performed in a uniaxial test machine and Poisson's ratios were obtained by filming marks applied to the front face of the sample and then measuring their positions in the footage. A drop rig was used for impact testing, with performance based on the ability of the samples to attenuate impact acceleration. The adopted methodology was similar to that used by Allen et al. [3].

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DESIGN, MECHANICAL EXPERIMENTS AND MODELING ON A NEW FAMILY OF 3D PRINTED HYBRID CHIRAL
        MECHANICAL METAMATERIALS WITH NEGATIVE POISSON’S RATIO, Yunyao Jiang

DESIGN, MECHANICAL EXPERIMENTS AND MODELING ON A NEW FAMILY OF 3D PRINTED HYBRID CHIRAL MECHANICAL METAMATERIALS WITH NEGATIVE POISSON’S RATIO, Yunyao Jiang

Cephalopods including squid, cuttlefish, and octopus, have amazing camouflaging capability due to responsive chromatophore organs on the skins [82-88]. The major mechanical components of a chromatophore organ is a single pigment-containing chromatophore cell and four to twenty-four radially arranged muscle fibers. Under muscular fiber contraction, the chromatophore cells experience dramatic and rapid change in area during deformation and therefore the pigment translocation. More interestingly, by selectively and sequentially expanding and retracting distinct groups of chromatophores, skin color can vary in a large range. Inspired by this mechanism, artificial chromatophores were designed by utilizing different material systems such as electroactive polymer [89, 90], soft synthetic system [91] and capillary origami [92]. In this paper, inspired by chromatophore organs, to achieve dramatic volume change, the concept of auxetic effect (i.e. negative Poisson’s ratio effect) was used to design innovative auxetic soft meta-materials, in which chirality-induced rotation was used to generate sequential cell opening mechanism upon a single far-field uniaxial loading. The unique chirality-induced sequential cell opening mechanism, together with the auxetic effects can be used to design smart metamaterials for actuation, drug delivery and camouflage.

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