Materials Science

Adapting to the Blow

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Science  12 Jan 2007:
Vol. 315, Issue 5809, pp. 162
DOI: 10.1126/science.315.5809.162b

Designing equipment to protect an individual from a collision or impact often requires compromises between safety and comfort. For example, seat cushions, armrests, or headrests need to be fairly soft and compliant to be comfortable, but under these conditions they fail to absorb much energy in a collision. Deshmukh and McKinley have designed a series of adaptive energyabsorbing materials using polyurethane foams impregnated with a magnetorheological fluid (MRF). An MRF consists of a suspension of micometer-sized magnetizable particles, which flow like water under normal conditions. When subjected to a magnetic field, however, the particles align with the field to form columns or aggregates that must be deformed or broken under flow; thus the field confers considerable stiffness. This adaptability is in turn transferred to the foam when an MRF coats the struts of its open cells, offering a means of stiffening upon stress. Application of magnetic fields in the 0-to-0.2 tesla range effectively modulated the energy absorbed by these composite foams by up to a factor of 50. A scaling model allowed the authors to express all of the response data on a single curve governed by only three parameters, a convenient framework for tuning the properties of the composite. Furthermore, they envision making similar composites using a shear-thickening fluid, which responds in accordance with its rate of deformation and so would not require a magnetic field to adaptively alter its properties. — MSL

Smart Mater. Struct. 16, 106 (2007).

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