Phonons in a two-dimensional auxetic lattice

Discussion of an auxetic model possessing a complete bandgap at Auxetics 2007 @ Malta, Mon 24 - Wed 26 September 200

Phonons in a two-dimensional auxetic lattice

Discussion of an auxetic model possessing a complete bandgap at Auxetics 2007 @ Malta, Mon 24 - Wed 26 September 200

Elastic constants of 3-, 4- and 6-connected chiral and anti-chiral honeycombs subject to uniaxial in-plane loading

Finite Element models are developed for the in-plane linear elastic constants of a family of honeycombs comprising arrays of cylinders connected by ligaments. Honeycombs having cylinders with 3, 4 and 6 ligaments attached to them are considered, with two possible configurations explored for each of the 3- (trichiral and anti-trichiral) and 4- (tetrachiral and anti-tetrachiral) connected systems. Honeycombs for each configuration have been manufactured using rapid prototyping and subsequently characterised for mechanical properties through in-plane uniaxial loading to verify the models. An interesting consequence of the family of 'chiral' honeycombs presented here is the ability to produce negative Poisson's ratio (auxetic) response. The deformation mechanisms responsible for auxetic functionality in such honeycombs are discussed

New auxetic materials

Materials with negative Poisson's ratios (auxetic) exhibit the very unusual property of becoming wider when stretched and narrower when squashed. This thesis presents a global classification system for auxetics based on the geometry and deformation mechanisms. Analytical expressions for the mechanical properties of several new two and three dimensional auxetic structures are derived. These structures involve rotating polygons, chiral units, re-entrant units, sliding wedges and helices. It is observed that the auxetic behaviour is scale independent and hence the structures may be implemented at either the macro, micro or nano level. These auxetic structures are used to explain the negative Poisson's ratios in a number of auxetic materials. A chiral structure is used to model auxetic polyurethane foams having broken ribs. A three dimensional re-entrant hinging/stretching structure is used to model auxetic ex-PTFE. Both models offer significant improvements when compared to previous work. Auxetic structures are also used as models for designing novel nanostructural molecular auxetics. In most cases, the auxeticity of these materials is confirmed through detailed molecular modelling. For example, crystalline molecular polyphenylacetylene networks based on the 'rotating triangles structure' are predicted to exhibit Poisson's ratios as low as -1, in accordance with their analytical models. These molecular networks are particularly interesting as they have all the six on-axis Poisson's ratios being concurrently negative. Their polycrystalline aggregates are also likely to be auxetic. The actual values of the mechanical properties are found to be dependent on the details of the molecular formulae (e.g. predicted single crystalline +0.25>V_z_y>-0.97, 250GPa>E_z>2.4GPa). Some of these specific properties may be explained through the analytical models and offer the possibility of being able to design materials with tailor-made mechanical properties. Molecular systems based on other auxetic structures are also found to exhibit similar properties. A systematic study on zeolites also shows that several examples exhibit auxetic behaviour and in most cases, their auxeticity may be explained via the analytical models. For example, negative Poisson's ratios are predicted for THO, NAT, EDI, APD and ATT, five zeolites whose nanostructure may be described in terms of 'rotating squares' model. The auxetic behaviour in #alpha#-cristobalite was also explained through this model. (author)2 volsAvailable from British Library Document Supply Centre-DSC:DXN033780 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo