Roderic S. Lakes

Foam Structures with a Negative Poisson's Ratio

A novel foam structure is presented, which exhibits a negative Poissons ratio. Such a material expands laterally when stretched, in contrast to ordinary materials.

Poisson's ratio and modern materials

In comparing a materials resistance to distort under mechanical load rather than to alter in volume, Poissons ratio offers the fundamental metric by which to compare the performance of any material when strained elastically. The numerical limits are set by ½ and -1, between which all stable isotropic materials are found. With new experiments, computational methods and routes to materials synthesis, we assess what Poissons ratio means in the contemporary understanding of the mechanical characteristics of modern materials. Central to these recent advances, we emphasize the significance of relationships outside the elastic limit between Poissons ratio and densification, connectivity, ductility and the toughness of solids; and their association with the dynamic properties of the liquids from which they were condensed and into which they melt.

Properties of a chiral honeycomb with a poisson's ratio of — 1

Abstract A theoretical and experimental investigation is conducted of a two-dimensionally chiral honeycomb. The honeycomb exhibits a Poissons ratio of —1 for deformations in-plane. This Poissons ratio is maintained over a significant range of strain, in contrast to the variation with strain seen in known negative Poissons ratio materials.

Extreme damping in composite materials with negative-stiffness inclusions

When a force deforms an elastic object, practical experience suggests that the resulting displacement will be in the same direction as the force. This property is known as positive stiffness. Less familiar is the concept of negative stiffness, where the deforming force and the resulting displacement are in opposite directions. (Negative stiffness is distinct from negative Poissons ratio, which refers to the occurrence of lateral expansion upon stretching an object.) Negative stiffness can occur, for example, when the deforming object has stored (or is supplied with) energy. This property is usually unstable, but it has been shown theoretically that inclusions of negative stiffness can be stabilized within a positive-stiffness matrix. Here we describe the experimental realization of this composite approach by embedding negative-stiffness inclusions of ferroelastic vanadium dioxide in a pure tin matrix. The resulting composites exhibit extreme mechanical damping and large anomalies in stiffness, as a consequence of the high local strains that result from the inclusions deforming more than the composite as a whole. Moreover, for certain temperature ranges, the negative-stiffness inclusions are more effective than diamond inclusions for increasing the overall composite stiffness. We expect that such composites could be useful as high damping materials, as stiff structural elements or for actuator-type applications.

Experimental microelasticity of two porous solids

Abstract Experiments are performed to determine the dependence of torsional and bending rigidity upon diameter for rod-shaped specimens of dense polyurethane foam and syntactic foam. Results show an effect due to-the microstructure. Results are describable by a Cosserat elastic model. The six Cosserat elastic constants are determined.

Negative Poisson's ratio polymeric and metallic foams

Foam materials based on metal and several polymers were transformed so that their cellular architecture became re-entrant, i.e. with inwardly protruding cell ribs. Foams with re-entrant structures exhibited negative Poissons ratios as well as greater resilience than conventional foams. Foams with negative Poissons ratios were prepared using different techniques and materials and their mechanical behaviour and structure evaluated.

Deformation mechanisms in negative Poisson's ratio materials - Structural aspects

Poissons ratio in materials is governed by the following aspects of the microstructure: the presence of rotational degrees of freedom, non-affine deformation kinematics, or anisotropic structure. Several structural models are examined. The non-affine kinematics are seen to be essential for the production of negative Poissons ratios for isotropic materials containing central force linkages of positive stiffness. Non-central forces combined with pre-load can also give rise to a negative Poissons ratio in isotropic materials. A chiral microstructure with noncentral force interaction or non-affine deformation can also exhibit a negative Poissons ratio. Toughness and damage resistance in these materials may be affected by the Poissons ratio itself, as well as by generalized continuum aspects associated with the microstructure.

Nonlinear Ligament Viscoelasticity

AbstractLigaments display time-dependent behavior, characteristic of a viscoelastic solid, and are nonlinear in their stress–strain response. Recent experiments25 reveal that stress relaxation proceeds more rapidly than creep in medial collateral ligaments, a fact not explained by linear viscoelastic theory but shown by Lakes and Vanderby17 to be consistent with non-linear theory. This study tests the following hypothesis: non-linear viscoelasticity of ligament requires a description more general than the separable quasilinear viscoelasticity (QLV) formulation commonly used. The experimental test for this hypothesis involves performing both creep and relaxation studies at various loads and deformations below the damage threshold. Freshly harvested, rat medial collateral ligaments (MCLs) were used as a model. Results consistently show a nonlinear behavior in which the rate of creep is dependent upon stress level and the rate of relaxation is dependent upon strain level. Furthermore, relaxation proceeds faster than creep; consistent with the experimental observations of Thornton et al.25 The above results from rat MCLs are not consistent with a separable QLV theory. Inclusion of these nonlinearities would require a more general formulation.

Indentability of Conventional and Negative Poisson's Ratio Foams

The indentation resistance of foams, both of conventional structure and of a novel re-entrant structure giving rise to negative Poissons ratio, was studied using holo graphic interferometry. In holographic indentation tests, re-entrant foams had higher yield strengths σy and lower stiffness E than conventional foams of the same original relative density. Damage in both kinds of foam occurred primarily directly under the indenter. Calculated energy absorption for dynamic impact is considerably higher for re-entrant foam than conventional foam

Non-linear properties of metallic cellular materials with a negative Poisson's ratio

Negative Poissons ratio polymeric cellular solids (re-entrant foams) were studied to ascertain the optimal processing procedures which give rise to the smallest value of Poissons ratio. The non-linear stress-strain relationship was determined for both conventional and re-entrant foams; it depended upon the permanent volumetric compression achieved during the processing procedure. Poissons ratio of re-entrant foam measured as a function of strain was found to have a relative minimum at small strains. The toughness of re-entrant foam increased with permanent volumetric compression, and hence with density.