Beth C. Munoz

A model of the behaviour of magnetorheological materials

Magnetorheological materials are a class of smart materials whose rheological properties may be rapidly varied by application of a magnetic field. These materials typically consist of micron-sized ferrous particles dispersed in a fluid or an elastomer. A quasi-static, one-dimensional model is developed that examines the mechanical and magnetic properties of magnetorheological materials. This model attempts to account for magnetic non-linearities and saturation by establishing a mechanism by which magnetic flux density is distributed within the composite material. Experimental evidence of the viscoelastic behaviour and magnetic properties of magnetorheological fluids and elastomers suggests that the assumptions made in the model development are reasonable. It is shown that the model is semi-empirical in that it must be fit to the experimental data by adjusting a parameter that accounts for unmodelled magnetic interactions.

The Magnetoviscoelastic Response of Elastomer Composites Consisting of Ferrous Particles Embedded in a Polymer Matrix

The mechanical response of elastomer composites to applied magnetic fields is examined. These elastomer composites consist of carbonyl iron particles embedded within a molded elastomer matrix. The composite is subjected to a strong magnetic field during curing, which causes the iron particles to form columnar structures that are parallel to the applied field. This special composite geometry is known to enhance the mechanical response to the application of post-cured magnetic fields. Experimental data is presented that shows that up to a 0.6 MPa change in mechanical shear modulus (which represents 30-40% change in modulus for the materials tested) is possible in response to an applied magnetic field for a composite containing 30% (V/V) iron particles. A simple quasi-static dipole model is presented to examine the magnetoviscoelastic effect of these elastomer composites. The model is semi-empirical in that it may be fit to experimental data over a broad range of applied fields by adjusting a parameter that accounts for unmodeled multipolar magnetic interactions between particles within the composite. Such elastomer composites hold promise in enabling variable stiffness devices and adaptive structures.