Magnetorheological Fluids: Preparation, Property And Modeling

Magnetorheological fluids (MRFs) are dispersions composed of carrier liquid, ferromagnetic particles and additives. The apparent yield strength of MRFs can be changed significantly within milliseconds by the application of an external magnetic field. As a kind of designable and property-controllable smart materials, MRFs have been extensively used for different purposes in various fields and shown brilliant prospects.It is important to provide high-performance MRFs and establish reliable theoretical models for the design and engineering application of MRFs and MRF based devices. In this dissertation, consistent with the research of the project“Coupled-multi-field properties of magnetorheological fluids: microstructural mechanism and trans-scale analysis”, which was financially supported by the Natural Science Foundation of China, the properties of MRFs were systematically studied through experimental investigation, theoretical modeling and numerical simulation.In order to evaluate the mechanical properties of MRFs, the rheological properties of some MRFs were tested with a specially designed testing system, with which the effects of various influencing factors, such as the intensity of the applied magnetic field, the shearing strain rate, and the temperature, etc, can be investigated. On the other hand, the evolution of microstructures of MRFs subjected to static magnetic fields and shearing strain was also observed instantaneously with a specially developed device and a stereomicroscopy, as well as the image acquisition technology.Making use of the knowledge of physical chemistry and material design, some high-performance MRFs of good sedimentation stability were studied, the effects of the influencing parameters, such as ferromagnetic particles, the carrier liquids and additives, are investigated systematically and optimized for the purpose of achieving better mechanical properties and sedimentation stability.The interaction between two dipolar particles based on the exact dipolar model was formulated, under the consideration that the conventional simplified dipolar model may involve remarkable error when the spacing between two dipolar particles is much less than the sizes of the particles. However, the comparison between the results obtained with the exact dipolar model, the simplified dipolar model and that obtained with FE approach based on the Maxwell theory showed that the simplified dipolar model may provide better approximation. Then, considering the microstructures under magnetic field fields and shearing deformation, a micro-macro model was developed for the constitutive behavior of MRFs and the effects of the main influencing factors, such as the intensity of magnetic induction, the size and the volume fraction of dispersed particles, shearing strain rate and saturation magnetization on the macroscopic behavior of MRFs can be considered.The mechanism of chain formation was analyzed based on the simulation of the movement of two dipolar particles. Then the chain formation process and the variation of the magnetic energy in a 600-particle system under an applied magnetic field were simulated. The magnetic energy of some typical microstructures of dipolar particles in a static magnetic field was analyzed. It shows that, among the microstructures composed respectively of single-column, compact two-column, compact three-column, cubic-lattice, BCT and FCC cells, the average magnetic potential energy of the microstructure with cubic-lattice cells is the largest and that with BCT cells is the lowest. It indicates that the microstructures composed of BCT cells and compact multi-column cells are the most possible microstructures in MRFs under static magnetic fields. Through the analysis of the chain formation program, a parallel algorithm was developed based on the OpenMP method. With the developed approach, the response of MRF samples with a large number of particles subjected to applied magnetic field and shear deformation were numerically simulated on a Dawning 4000A cluster. The obtained overall?–?relationship and the microstructures are in reasonable agreement with that obtained in experiment and other analyrical approach, demonstrating the validity of the proposed approach.