Measurement of Magnetorheological Fluid Properties at Shear Rates of up to 25 000 s $^{-1}$

Magnetorheological energy absorbers (MREAs) have been successfully deployed in occupant protection systems to protect against potentially injurious shock, crash and blast loads. These MREAs operate at shear rates upwards of 25 000 s^-1, but magnetorheological fluids (MRFs) are typically characterized for shear rates up to 1000 s^-1 in commercially available parallel counter-rotating disk rheometers. Because of the lack of availability of data at the required high shear rates, a Searle-type magnetorheometer (essentially a concentric cylinder rotating in a cup) was designed and fabricated at the University of Maryland. Using this magnetorheometer, two commercial MRFs were characterized over the shear rate range of 0-25 000 s^-1 . It is shown that the rheometer was successful in replicating available characterization data at low shear rate, as well as quantifying high shear rate behavior as a function of applied field. In addition, it was shown that the Herschel-Bulkley constitutive model is appropriate and successfully characterized the apparent viscosity vs. shear rate behavior of the MRFs over this shear rate range. Experimental data demonstrate that an increase in field dependent yield stress can be realized over this entire shear rate range, so that MREAs can be designed using data taken with the magnetorheometer. Finally, the Mason number, which has been shown to be a useful non-dimensional number at low shear rates, also provides a useful physical interpretation at high shear rates.