Mechanics of Semi-Expanded Hollow Sphere Foams

Confined expansion of polymer spheres coated with a metal slurry is a way of producing cellular microstructures that range from sintered hollow spheres in the unexpanded case to sintered polyhedra with nearly flat cell walls in the fully expanded case. In this paper, the expansion process is simulated by the finite element method for a face-centered cubic arrangement of spheres. Unit cell models for structural analysis are obtained from different stages of the expansion simulation. On the basis of these models, Young's modulus and Poisson's ratio along the axes of cubic symmetry are predicted for different relative densities and different degrees of expansion. It is shown that the elastic stiffness is increasing monotonically with increasing confined expansion of the spheres. Poisson's ratio is predicted as being negative for low degrees of expansion and positive for high degrees of expansion.