Abstract

Auxetics are a broad class of material and structures mainly exhibiting negative Poisson’s ratio (NPR) behaviour. NPR indicates than a solid expand in all directions when pulled along one, behaving therefore in a counterintuitive manner compared to “classical” solids. In this seminar I will present the description of NPR open cell foams produced using the standard manufacturing techniques and modeled via a novel hyperelastic material formulation based on an Ogden-type strain energy approach. When used as load bearing or energy absorption elements, the foams are subjected to large deformations and nonlinear behaviour becomes important. From the modeling perspective several nonlinear elastic models have been formulated in open literature, some of them based on statistical distributions of the density and mechanical properties of the unit cells along the compressive direction. These models are based on a kinematical map between the deformation of the cell ribs and the macroscopic strain. As such, they have the major drawback of being dependent upon the particular foam structure used for their development. Another approach is here adopted allowing the auxetic foam to be modeled as a continuum solid. The evolution of the microstructure of the foam is investigated and it is shown that the main feature observed in the experimental data can be accurately described by a isotropic hyperelastic model with an Ogden-type strain energy function. The model can be easily implemented in a finite element code hus allowing more complex deformation patterns to be simulated.

Jacopo Ciambella is Lecturer in Composites Engineering at the Advanced Composites Centre for Innovation and Science of the University of Bristol. His interest on nonlinear viscoelasticity of nano­ composites has origins from his PhD work undertaken at the University of Rome “La Sapienza” in cooperation with Bridgestone Technical Center Europe (2006-2010). His research also deals with micro-macro modelling of multifunctional composites and energy absorbing composite materials.