Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/8828
The durability of a magnetorheological (MR) fluid is a problem in a commercially available MR fluid device. This device is a prosthetic knee joint equipped with an MR rotary brake actuator that utilizes the MR fluid in direct-shear mode. After long periods of usage, the actuator becomes stiff in off-state mode and eventually needs to be replaced by a new actuator. The reasons behind the increased off-state rotary stiffness and the degradation of the MR fluid are investigated. MR fluids from used actuators are analysed, after an actual usage by an amputee or after an excitation in a test-rig. Energy dissipated by the
fluid in the actuator over its life time is calculated and compared to recognized durability threshold for the in-use-thickening of MR fluids. The energy does not reach this threshold
but the durability is still considered relatively good. The powders are extracted from the base fluid. Their morphology, size distribution, chemical composition and magnetic characteristics are analysed and compared to that of unused powder. The chemical composition of used base fluids are analysed and compared to that of an unused base fluid.
Results indicate that the iron particles tend to fracture and the surface tends to spall. Thin flakes peel away from the surface and are believed to cause agglomeration resulting in thickening of the fluid. Oxidation or formation of nano-sized particles is not observed in the fluid. Alternative fluid compositions are prepared and tested but do not turn out to be superior with respect to durability.