Measurement of longitudinal and transverse strain in an aluminium foam

Abstract

This paper presents an experimental study on a closed-cell aluminium foam (ALPORASm), carried out with the final goal of obtaining data required to identify the parameters of a constitutive model of such material. The work consists mainly of compression tests on prismatic specimens, from which the load-displacement curves are obtained. Several tests have been carried out to assess the influence of strain rate, in the low-value range 10 -3 -10 -1 s -1, and foam density variation, due to the slight difference between samples taken from the inner or the outside of a foam block. The results show that the effect of the strain rate, although small, is noticeable, and becomes more relevant when the material has higher density, in accordance with previous studies. Measuring the strain in the foam requires special procedures because of its heterogeneity; the typical choice is in favour of non-contact methods. In this work, the Digital Image Correlation has been used to obtain the displacement field on the specimen surface: the track of each marker point in a chosen set has been determined, then the strain field has been assessed with an interpolation based on a strain-displacement matrix like in the finite element method. The obtained relationship between longitudinal and transverse average strain is non-linear and influenced by the strain rate, which corresponds to a non-constant apparent Poisson's ratio.

Poisson‘s Ratio Induced Radial Inertia Confinement During Dynamic Compression of Hyperelastic Foams

“…As an important material parameter, Poisson's ratio of foam materials has been observed to be positive, zero, or even negative depending on the cell structure, density, and matrix material [1]. Due to high nonlinearity at large deformation and drastic hardening behavior after densification, Poisson's ratio of foam materials has been found to vary with specimen strain [2][3][4][5][6]. Treated as compressible solids with a constant Poisson's ratio at small deformations, foam materials may approach a nearly incompressible rubbery state when subjected to large deformation into densification.…”