Study On The Mechanical Properties And Applications Of Magnetorheological Fluid

As a kind of smart materials,magnetorheological fluid(MRF)is mainly composed of magnetic particles and liquid base.Usually,some additives are added into the suspension to improve the sedimentary stability.When an external magnetic field is applied,the MRF can transform from liquid state to solid-like state within a few milliseconds and return to liquid state when the external magnetic field is removed.Because of the controllable properties,MRF has attracted the attentions of many researchers.Many works about improving the magnetorheological(MR)properties,researching the mechanism and applications of the MRF have been taken out.Although many researches about the MRF have been taken out,there are still many problems to be solved.For example,the sedimentation still hiders the applications of MRF,there are too few studies on the conductivity of MRF,the mechanical properties of MR instruments need to be improved and so on.In this dissertation,researches about the influence of the hollow structure particles on the sedimentary stability and the rheological properties of the magnetic fluid are firstly taken out.Then,the influence of the magnetic field,oscillatory shear and squeeze on the electrical conductivity and the relationship between the normal force and the electrical conductivity of the MRF are researched.At last,two kinds of magnetorheological damper are designed and fabricated.The dynamic mechanical properties of the two kinds of MR damper are tested and analyzed.The detailed works are as follows:1.The influence of hollow structure particles on the properties of magnetic fluidA kind of Fe3O4 nanoparticles with hollow structure is synthesized for studying the influence of hollow structure nanoparticles on the properties of magnetic fluid.The characterized results show that the nanoparticles are with hollow structure and high purity.The magnetism performances of the nanoparticles are excellent.Magnetic fluid based on Fe3O4 hollow nanoparticles is prepared.The sedimentary stability and rheological properties of the magnetic fluid are tested.A particle dynamic simulation is conducted to analyze the shear stress and the structure evolution of the magnetic fluid under the effect of external magnetic field.The simulated results match well with the experimental results.Both experimental and simulated results indicate that the hollow structure can improve the MR properties of the magnetic fluid.2.Research on magneto-electro-mechanical coupling responses of MRFThe magneto-electro-mechanical coupling responses of MRF under oscillatory shear and squeeze are studied with the help of a self-built testing system.When an external magnetic field is applied,the electrical conductivity of the MRF can increase by several thousands of times.It is also possible to improve the electrical conductivity of the MRF by applying an oscillatory shear and the resistance of the sample changes with the oscillation.A particle-particle model is proposed to analyze the resistance of magnetorheological fluid under different conditions.When the MRF is squeezed,the resistance of the samples changes with the testing conditions,such as the squeeze speed,squeeze strain,the viscosity of the MRF.The relationship between the normal force and the resistance of MRF is also researched.It is found that the normal force and the resistance of the MRF both depends on the evolution of the particles structures inside the MRF.The above studies provide the necessary theoretical basis for the applications of MRF in the field of sensor,non-contact measurement and control.3.Study on the MR damper working in squeeze modeA MR damper working in squeeze mode of MRF is designed and the dynamic mechanical properties are tested.The testing results show that the mechanical properties of the MR damper are excellent.The maximum damping force can reach almost 6 kN,higher than that offered by the traditional MR dampers with the same size.The working principle and the composition of the damping force are analyzed detailedly.Another novel damper working in the squeeze-valve bi-mode is designed.The highlight of the bi-mode damper is that there is a channel inside the piston for the flow of the MRF.The channel can promote the flow of the MRF in the damper and prevent the MRF from settling.The maximum damping force offered by the bi-mode damper can reach as large as 6.5 kN and the force-displacement curves are plump.These indicate that the channel inside the damper can not only enhance the damping force,but also effectively prevent the sedimentation of MRF.Finally,the Back-Propagation Neural Network is used for analyzing the damping force of the bi-mode damper.The results show that the predicted results matches well with the experimental results under different conditions,which indicates that the damper can be widely used in semi-active control field.