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2 Computational Methods in Science and Technology

2 2004

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Negative Poisson's ratios from rotating rectangles

Grima J. N., Alderson A., Evans K. E.

Computational Methods in Science and Technology

2004

Vol. 10, nr 2

137-145

Materials with a negative Poisson's ratio exhibit the unexpected property of becoming fatter when stretched and narrower when compressed. This counter-intuitive behaviour is known as 'auxetic behaviour' and imparts many beneficial effects on the material’s macroscopic properties. This paper discusses the potential of systems composed of rigid rectangles connected together through flexib1e hinges at their vertices. It will be shown that, on application of uniaxial loads, these rigid rectangles will rotate with respect to each other to form, in some cases, a more open structure hence giving rise to a negative Poisson's ratio.

Molecular modelling of the deformation mechanisms acting in auxetic silica

Alderson A., Alderson K. L., Evans K. E., Grima J. N., Williams M. R., Davies P. J.

Computational Methods in Science and Technology

2004

Vol. 10, nr 2

117-126

Molecular mechanics simulations have been performed to undertake a systematic investigation into the structure and mechanical properties of alfa-cristobalite undergoing uniaxial loading along each of the 3 mutually orthogonal principal directions and also hydrostatic pressure loading. Simulations were performed using both the BKS and Burchart force-fields. The simulations indicate that pressure loading and uniaxial loading along the x3 direction leads to uniform variation of the four independent Si-O-Si intertetrahedral angles, indicative of cooperative tetrahedral rotation about tetrahedral axes which transform the alfa-cristobalite structure into the 'idealised' beat-cristobalite structure. Uniaxial loading along either of the transverse directions (x1; and x2) leads to a divergence of the intertetrahedral angles, consistent with tetrahedral rotation about the tetrahedral axes which transform the idealised beat-cristobalite structure into the "ordered' beta-cristobalite structure. The data also indicate that a phase transition to one of the proposed beta phases may be induced by a negative hydrostatic pressure or tensile stress along x3. The phase transition is accompanied by a change in sign of some of the Poisson's ratios (i.e. from positive to negative). A negative hydrostatic pressure is also predicted to lead to conversion of initially positive to negative Poisson's ratio values (within the same phase).