Mechanical behavior of closed-cell and hollow-sphere metallic foams

Abstract

(cont.) The elastic anisotropy and yield surfaces are fully characterized, and numerical equations are developed to allow the simple evaluation of the effect of geometric and material properties on the mechanical behavior of hollow-sphere foams. The analysis indicates that at relative densities of 10%, hollow-sphere foams have theoretical moduli and strengths that are three times those of existing metallic foams, and this increases to a factor of ten at relative densities below 5%. Several concepts are presented to allow the incorporation of defects into the model, including random packing, variations in bond size, and variations in sphere relative thickness. Finally, the performance of hollow-sphere foams is compared to other low-density engineering materials on a structural basis; hollow-sphere foams offer a beneficial alternative.

Metal foams are low-density materials with multifunctional attributes that make them appealing for numerous uses, including thermal insulation, heat sinks, acoustic insulation, energy absorption devices (crash protection), lightweight structural sandwich panels (as the core material), and vibration damping devices. Metallic foams are commercially available as closed-cell and open-cell foams. Unfortunately, the mechanical behavior of closed-cell metallic foams is far below that which the theory suggests; at low relative densities, the mechanical properties of closed-cell foams are an order of magnitude less than expected. It is shown that defects such as cell wall curvature, cell wall corrugation, and density variations account for a large fraction of the degradation in properties. Hollow-sphere foams offer a solution to the problem of degraded performance in closed-cell foams because ideal spheres can be bonded into a relatively defect-free structure. This thesis focuses on the development of constitutive models to describe the mechanical behavior of this new class of materials; such models are critical in determining whether or not hollow-sphere metallic foams provide an alternative to existing closed-cell metallic foam materials. The uniaxial compression behavior of single hollow spheres is first studied to determine the significant geometric and material parameters of hollow-sphere foams. Detailed constitutive models of the behavior of hollow-sphere foams are developed using finite element simulations of simple cubic, body-centered cubic, and face-centered cubic sphere packings.