Determination of the Local Stress-Strain Response of Foams

Abstract

A new specimen for determining the local stress-strain response of foams under uniaxial compression is presented along with the corresponding theory. The drawback of using conventional cubic specimens is that average stresses and strains are calculated, which introduces size effects due to structural and material inhomogeneities of commercial foams. Under compression, foam cubes develop unstable regimes, which involve localized deformation. The instabilities cause difficulties in establishing the correct stress-strain response of the material. Tapering specimens can enable controlled motion of the boundary separating the unstable and stable regimes. This concept is exploited in the present paper in experiments on closed-cell aluminum foam trapezoids. A crushing front propagates down the tapered specimen during compression, while the deformed region develops a new lateral shape. The experimental results are used along with several assumptions to extract a more representative stress-strain response of foam. The response is characterized by the initial plateau stress, shape exponent and densification strain. The significant effect of the variable Poisson’s ratio during crushing is also introduced. The results provide a basis for developing local constitutive behavior of foams.