Compressive Response and Crushing of Cellular Solids

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Abstract Cellular solids are a class of light-weight materials with unique properties such as high stiffness- and strength-to-weight ratios and excellent energy absorption characteristics. Modern foams are made from polymers, metals, ceramics and other materials. They are used as structural components as cores in sandwich structures and in impact mitigation, cushioning and other energy absorption applications in atmospheric and space vehicles, automotive components, ship structures, civil engineering structures, mass transit vehicles, sporting goods, etc. Advances in foaming processes enable their manufacture to prescribed cell sizes and densities. For this technology to reach its full potential, the cell size, density and properties of the base material must be related to the foam properties of interest. The main objective of this project is to use experiment and analysis to understand how the microstructure governs all relevant mechanical properties of open cell foams. Polyurethane and aluminum foams of various densities and cell sizes will be used as representative material systems. A typical foam compressive response consists of a nearly linear elastic regime terminating into a limit load which is followed by an extensive load plateau. The limit load represents the onset of instability and localization. For polyurethane foams the instability is elastic buckling of the microstructure while for aluminum foams it is due to plastic collapse. The plateau, which represents the energy absorbing capacity, is related to progressive spreading of the crushing through the material. The project aims to understand the micromechanisms governing these behaviors and to develop models which are able to reproduce them. The problem will be tackled through experiments coupled with several levels of modeling. The experiments will involve: (a) Measurement of the elastic and "inelastic" mechanical properties of the foams; (b) characterization of the foam microstructure; and (c) measurement of the mechanical properties of the foam struts in situ. The modeling will involve: (a) beam-type models for the elastic properties; (b) numerical characteristic cell-type models for the elastic properties and the onset of instability; and (c) large-scale models which can reproduce all aspects of the compressive response including the large deformation crushing. At first the microstructure will be represented as regular Kelvin cells but with realistic strut geometric characteristics. Other more representative microstructures will be considered as deemed required.