Experimental study of magnetorheological fluids under large periodic deformations

Abstract Magnetorheological (MR) fluids are smart materials whose properties can be controlled by a magnetic field. They are used in a variety of applications with tailored operating parameters, facilitating broad and relatively straightforward implementation and control of the functional properties of these systems. The aim of the study was to determine the variation in shear stress of MR fluids under varying shear deformation directions and different magnetic field induction levels. The experiments were carried out with the use of three commercially available fluids: MRF-122EG, MRF-132DG, and MRF-140CG, which primarily differ in the volumetric fraction of ferromagnetic particles. These fluids are designed for use in mechanical energy dissipation devices, clutches, and brakes. They are also characterized by high stability and a broad controllability range. The experiments were performed using a rotational rheometer with a parallel plate measuring system to examine shear stress variability during both the deformation and rest phases. The study applied relatively large deformations, exceeding the linear viscoelastic range and the deformation corresponding to the yield point. The test results revealed variations in the shear stress values across successive loading cycles, as well as fluctuations during the rest phase. It was also found that, depending on magnetic field induction level (i.e. the level of fluid magnetization), the rest phase could result in either a decrease (relaxation) or an increase (hardening) in shear stress values. These findings have practical significance, particularly for MR fluid devices operating in shear mode.