Anisotropic mechanical behavior of magnetically oriented iron particle reinforced foams

Reinforced foams were prepared by exposing a polyurethane matrix filled with iron particles to a magnetic field during the foaming process. The magnetic field induced an alignment of the iron particles along the field direction, giving rise to columnar structures similar to fibrils, as observed by SEM and microtomographic 3D reconstructions. The anisotropic reinforcement induced by the fibrils improved the mechanical performances, yielding a threefold increase of both elastic modulus and yield stress in the alignment direction, whereas minor effects were observed in the transversal direction. In this case, the mechanical properties were comparable with those of randomly filled foams or, in some cases, of unfilled foam. The reinforcing efficiency of fibrils was evaluated through a theoretical model, based on the combination of the mechanics of foams with two micromechanical models for aligned short fibers composites (Halpin-Tsai and Cox-Krenchel). The theoretical predictions based on the Halpin-Tsai equations showed a good agreement with the experimental data, whereas the model derived from Cox-Krenchel equations overestimated data. © 2010 Wiley Periodicals, Inc.