Lamb wave propagation is evaluated for cross-ply laminate composites exhibiting through-the-thickness negative
Poisson's ratio. The laminates are mechanically modeled using the Classical Laminate Theory, while the propagation of
Lamb waves is investigated using a combination of semi analytical models and Finite Element time-stepping
techniques. The auxetic laminates exhibit well spaced bending, shear and symmetric fundamental modes, while
featuring normal stresses for A0 mode 3 times lower than composite laminates with positive Poisson's ratio.
Auxetic (negative Poisson's ratio) cellular materials expand in all direction when pulled in only one, thus behaving in an unusual manner compared to 'classical' materials. Negative Poisson's ratio honeycombs and open cell foams have shown increased shear modulus, indentation resistance and low cut- off frequency acoustic properties. In this paper FEM microstructure models are used to compute the static and viscoelastic properties of closed-cell and two-phase foam composites. The complex modulus of the materials is calculated making use of the correspondence principal and evaluating the strain energy distributions for the different phases. The results are compared to the ones given by models representing a global in-plane uniaxial loading. The static and storage modulus values of two-phase composite foam are significantly enhanced by the presence of a re-entrant (auxetic) skeleton layout. The loss factor shows also a significant sensitivity on the volume fraction and strain energy distribution on the microstructure unit cells. Static and free-vibration simulations on sandwich beams with different core cellular materials show that it is possible to obtain both enhanced stiffness per unit weight values and modal loss factors using two-phase cellular solids with a re-entrant skeleton.
In this paper a theoretical and numerical study on the viscoelastic behavior of auxetic polymers and cellular materials is presented. Negative Poisson's ratio materials ((alpha) (upsilon) (eta) (xi) (epsilon) (omicron) (sigma) in Greek) expand in all directions when pulled in only one, and contract when compressed in one direction. This behavior is due to the special geometrical layout of their unit cells. A theoretical model including viscoelastic and inertia effects on the unit cell has been prepared in order to compute the equivalent in- plane dynamic storage modulus and loss factor of the cellular material. The calculations show how inertia effects and geometric layout of the unit cell affect the viscoelastic behavior of the material over the frequency domain. The results show a very good agreement with the ones from analogous FEM models. Auxetic honeycombs are a good example of cellular materials with negative Poisson's ratio behavior. A Finite Element model has been elaborated to model also the viscoelastic response of the transverse shear modulus of this kind of honeycombs and compared with analytical results.