Research Of Mechanics Behavior Of MRFs Based On Microstructure

Magnetorheological fluid (MRF) is a new type of intelligent material consisting ofmicron-sized ferromagnetic particles suspended in nonmagnetic carrier fluid. A MRF issensitive to magnetic field and the mechanical property can be controlled by the appliedmagnetic field. It exhibits as a Newtonian fluid without applying a magnetic field,which could be changed to a solid-like material with high and adjustable yield shearstress once a proper magnetic field is applied. MRFs have been extensively used fordifferent purposes, such as vibration control, shock absorbers, buffing attachment,braking devices, damper, etc. Since the performance of MRFs depends strongly on themicrostructures of MRFs, in order to develop high-performance MRFs, it is importantto understand comprehensively the microstructures and their evolution under of MRFsunder various conditions. In this dissertation, the mechanical properties of MRF arestudied based on the microstructures and their evolution of MRFs, which includes thefollowing parts:(1) The formation and evolution of the microstructures in MRFs subjected todifferent magnetic fields and shear deformation are observed using a stereo microscope.The effects of particle volume fraction and the intensity of magnetic induction areinvestigated. The results show that under the same intensity of magnetic induction, thenumber of the ferromagnetic particle chains increases with the increase of the particlevolume fraction, for constant particle volume fraction, the microstructure changes froma single-chain structure to a glomeroplasmatic structure; during shear deformation, theevolution of the microstructure, including chain formation, inclination and elongation,fracture and re-formation, are observed. It provides the experimental evidences forstudying the deformation mechanism of MRFs.(2) Making use of the commercially available finite element (FE) code ANSYS,the magnetization state and the magnetic force between two particles are analysed. Theresults are then used as the benchmark to evaluate the results obtained with the twoexisting magnetic dipole models, with and without adopting the assumption that the sizeof the particles is much smaller than the distance between particles, respectively. It isfound that although the simplified dipole model can match better the result by FEcomputation, but there is still a marked difference. An enhanced dipole model is thensuggested, taking into account the contribution of the magnetized particles to magnetic field. Using the magnetic potential between particles, the the magnetic force is obtained,and the result agrees well with that obtained by FE computation.(3) Based on the micromechanics and a statistical approach and making use of theenhanced dipole model, a micro–macro description is established for the mechanicalproperty of MRFs. It can take into account the effects of magnetic induction intensity,volume fraction of particles and shear strain rate, etc, on the critical shear stress ofMRFs. The mechanical property of MRFs in the compression mode is also analysed.(4) The numerical simulation is developed based on particle dynamics, where threemain forces–magnetic force, viscous force and repelling force, are considered. Theformation of particle chains and the evolution of the microstructure in MRFs underdifferent applied magnetic fields are simulated. The effects of shear strain rate, particlessize, magnetic field intensity and volume fraction on the MRF microstructures are alsoinvestigated. The evolution of the microstructure in MRF during compress is alsoinvestigated.