Evaluation of the dynamic plastic behavior of open-celled foams by merely using their nonuniform deformation images under crushing

Abstract

Metal foams are extensively utilized in engineering due to their excellent energy-absorption capacity. The plateau stress and the densification strain of foams are the two key parameters for the energy absorbing capacity, which are conventionally gained by drawing data from the foams’ stress-strain curves, while they have been found to vary with the foam’s initial density and crushing speed. However, traditional force measurement methods require additional sensors that are often impractical in real applications. In our previous study (Hu et al., 2019), quasi-static experiments on open-celled metal foams demonstrated that instantaneous density—rather than initial density—governs the current stress, enlightening an innovative way to understand the foams’ mechanical behaviors. Under high-speed dynamic crushing, the shock-wave propagation dominates the deformation process with both stress and deformation distributed nonuniformly within the foam. In this study, based on the images obtained from numerical simulations and/or experiments, we examine whether the nonuniform stress distribution is correlated with the nonuniform deformation of the crushed foam by the relation between the current stress and the instantaneous density, which can be regarded as an inherent property of the foam as revealed in our previous study. Then we will show that both experiments and numerical simulations verified this hypothesis. According to the local density variation, the deformed foam is divided into three zones: densified zone, transition zone, and undeformed zone. Plateau stress at the impact-end and the support-end can be evaluated by the density of transition and undeformed zone, respectively, while the densification strain can be evaluated by the average density of the deformed zones (including densified and transition zones) during crushing. The evaluated results agree closely with experimental and numerical data, demonstrating that the dynamic mechanical properties of metal foams can be accurately evaluated using image-based density analysis, reducing reliance on direct force measurements.

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