We carry out punching experiments on low-density elastic polyether polyurethane (EPP) foams. The punch is rigid and wedge-shaped, and the specimens are either cubic or brick-shaped. The experimental results display some striking features, most notably a mechanical response that remains linear up to a penetration of the punch of about 40% of the height of the specimen. At higher penetrations, the mechanical response turns abruptly nonlinear: for the cubic specimens, there is a momentary loss of stiffness; for the brick-shaped specimens, there is a sizable increase in stiffness. We formulate a simple theoretical model of the punching of low-density EPP foams. In this model, a cell of the microstruture of an EPP foam behaves as a bistable elastic structure under compression, so that there are two preferred values of strain corresponding to two configurational phases of the foam. We use the model to explain the most salient experimental observations. We also formulate a micromechanical, mean-field model of EPP foams, implement the model in a general-purpose finite element code, and carry out computational simulations of the punching experiments. The computational results are in good accord with the experimental results, and they allow us to verify some of the assumptions on which the theoretical model is predicated.

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